physics experiments based on light

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Explore optics: visible, ultraviolet, and infrared light . Create your own light-up device (like an infinity mirror or color mixer), learn how to measure the colors of visible light in a solution, or change the way a camera or kaleidoscope works.

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physics experiments based on light

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3 Super Simple Light Experiments for Kids to Do

Literacy & ABCs Science Toddlers Grade School Kindergartners Preschoolers Experiment Paper Plates 12 Comments

Science experiments are always a big hit in my house and this light experiment for kids will brighten everyone’s day – literally!

Learn about the properties of light with a quick, simple set of light experiments for kids to do at home. You already have all the supplies!

3 Super Simple Light Experiments for Kids

What three things can light do? This is the guiding question for this simple and fun light experiment for kids.

To Set up Your Own Simple Light Science Experiment, You’ll Need:

Try our favorite 50 simple science experiments .

We love a good science activity that uses supplies we already have at home like this one!

Talking About Science Basics with Kids

Science activities are always a great time to practice using fun science terms. This simple light science experiment introduces three new ones:

It can help if you write down these words and their meanings on a piece of paper or flashcards.

You could use actual words or draw a picture.

For older kids, you could also dive a little bit deeper. I love this quick explanation about the properties of light from Ducksters .

Before Your Light Experiments for Kids

This simple science experiment includes an opportunity for making predictions and recording observations.

Predicting is just making a guess based on what you already know.

You could get started by asking your kids: “What do you know about light?”

Create a quick and simple legend for the light experiment.

Write down your children’s predictions and make a quick chart. One column is for the prediction and the other is for the observation, plus some rows for the variables.

Label the rows with the names of your three objects, or variables (what’s changing each time). Hint: mirror, magnifying glass, plate, etc.

At the top of one column write: “What will the light do?” . (Prediction)

And then above the other column, write: “What does the light do?” . (Observations)

Record your predictions and observations for your simple light experiments for kids!

As you experiment, you’ll also jot down what happens with the light, or what you observe. Observe and observation in science is just a fancy way to explain telling what you saw happening during the experiment.

Ask these helpful questions as you predict what happens:

Take time to look at each object, discuss the three terms associated with light (penetrate, reflect, stop).

Make predictions, or guesses, about what the light will do with each object.

Write your predictions in the first column of the chart.

Predict what you think light will do in this easy science experiment activity for kids!

Now Experiment with Light Together

Once your predictions are made and the properties of light have been discussed, it’s time to do the experiment.

Choose the first object and have your kids shine the flashlight at the object.

Watch how the light reacts with the object. Does it shine through, shine back at you, or stop completely?

Record on your observation chart what the light did with that object. Check to see if your predictions were correct.

Keep going with the rest of the objects, making sure to observe and record your findings.

Our Easy Light Experiments for Kids

We chose the mirror first. My son held the mirror and my daughter used the flashlight.

Check to see what objects reflect with easy light experiments for kids

I encouraged them to explain what they noticed about the light. Both recognized that the light was shining back at us, or reflecting.

We talked for a minute about using “refect” to describe what the light was doing.

Keep shining with a simple indoor reflection activity !

My daughter wrote “reflect” in our observation column on our chart. I helped her with the spelling, but only a little.

The Paper Plate

Our second variable for the light experiment was the paper plate. This time my kids switched roles with my daughter holding the plate and my son shining the flashlight at the object.

Check to see how light acts with a plate in this easy experiment for kids.

My kids quickly noticed that the light didn’t go anywhere except for on the plate.

We discussed together how this showed that the light stopped because the plate blocks or stops the light. I also added in the word “opaque,” which means that light does not pass through.

My son recorded “stop” for the plate.

You can also introduce the word “absorb” to your kids at this point in the experiment, as that is another term for stopping the light.

Originally, the kids had thought that the plate might reflect the light. Our prediction was incorrect and we talked about that for a minute or so.

Chart your light experiments for kids results

Learn more about opaque objects with a fun shadow play activity !

The Magnifying Glass

Our final object was the magnifying glass. It was my turn to shine the light as both my kids held the object.

This time the light went through the magnifying glass, shining onto the floor below. I shared the term “transparent,” meaning that light passes completely through, as we talked about this part of the experiment.

See how light acts with a fun science experiment for kids

I recorded our findings on the chart. We reviewed each object and outcome together while comparing our observations to our predictions.

Chart the activity and results of your science activity with kids

Keep Playing with Light!

Even though we had finished the “formal” experiment, my kids kept the learning going! They ran through the house, shining the flashlight on all sorts of objects and saying whether the light reflected, stopped, or penetrated.

I love how much ownership they took of their learning!

Learn about the properties of light with a quick, simple set of light experiments for kids to do at home. You already have all the supplies!

We love playing with a fun flashlight scavenger hunt for kids !

This fun extension activity went on for quite a while. And it’s something that I know I can keep returning to again and again, adding more challenging terminology as they grow.

What are some other fun science experiments for kids you have done? We’d love to check-out your creative learning ideas!

About alisha warth.

I have raised my children doing activities with them. As a homeschool mom, I am always looking for ways to make our learning fun. I'm honored to be able to contribute my ideas to the awesome site that is Hands On As We Grow.

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physics experiments based on light

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Stacey A Johnson says

November 24, 2020 at 8:46 pm

This is fantastic! Thank you for sharing! I have been putting science bags together to send home for my kinders because we are doing online school….I was looking for some light activities because we are going to tie them into the holidays we study in December. (The idea that most celebrations, customs, rituals, use some sort of light) I can’t wait to do this with them!

MaleSensePro says

February 10, 2020 at 11:29 pm

Its a great learning experience.. its indeed the best kind of way kids should learn, thanks for sharing :)

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Double-slit experiment . “Slit experiment” redirects here. For other uses, see Diffraction.

In 2005, E. R. Eliel presented an experimental and theoretical study of the optical transmission of a thin metal screen perforated by two subwavelength slits, separated by many optical wavelengths. The total intensity of the far-field double-slit pattern is shown to be reduced or enhanced as a function of the wavelength of the incident light beam.

Video advice: The Original Double Slit Experiment

Light is so common that we rarely think about what it really is. But just over two hundred years ago, a groundbreaking experiment answered the question that had occupied physicists for centuries. Is light made up of waves or particles?

Light Wave Experiments

In modern physics, the double-slit experiment is a demonstration that light and matter can display characteristics of both classically defined waves and particles; moreover, it displays the fundamentally probabilistic nature of quantum mechanical phenomena. This type of experiment was first performed, using light, by Thomas Young in 1801, as a demonstration of the wave behavior of light. At that time it was thought that light consisted of either waves or particles. With the beginning of modern physics, about a hundred years later, it was realized that light could in fact show behavior characteristic of both waves and particles. In 1927, Davisson and Germer demonstrated that electrons show the same behavior, which was later extended to atoms and molecules. (3) Thomas Young’s experiment with light was part of classical physics long before the development of quantum mechanics and the concept of wave-particle duality. He believed it demonstrated that the wave theory of light was correct, and his experiment is sometimes referred to as Young’s experiment(4) or Young’s slits.

Bending Light – Light waves, which have been found to exhibit characteristics of particles, behave in certain ways that we can observe by experimentation. Light waves diffract in the same manner that waves diffract when they collide with an object. They also undergo interference when passing through or reflecting against objects of different mediums. Bending Light Remove the sharp end of a blue tack and glue the top to a penny. Place a solid-colored ceramic bowl on a tabletop, and then place the penny in the bowl, with the tack side facing up. Back away from the bowl until you cannot see the penny. Fill a large glass with water and pour it slowly into the ceramic bowl. From a distance, watch the penny begin to appear as you fill the bowl with water. This demonstrates the ability to bend light over a top or corner, where an object was not visible before. Sunlight Waves and Particles Fill one clear plastic cup with tonic water and another clear plastic cup with tap water. Use a felt pen to mark the tonic cup with a “T.

Light Waves Project & Worksheets

This Department for Education clip illustrates the major role that science, technology, engineering and mathematics subjects play in the creative industries. The video follows Will a lighting software designer who designs the lighting for concerts, plays, films, pop videos and other entertainment events. Will describes the need for a good grounding in physics and mathematics to do his job.

The game on pages 13-16 could be transported out like a demonstration which students will discover highly amusing. For any variation of the activity, you are able to draw an easy maize and ask students to absorb it turns to test to draw a line from the beginning the the conclusion by only searching within the mirror, however they will not have the ability to get it done! Use a visualiser to project their efforts to the board.

Produced for Future Morph, this short video looks at several students following ophthalmic dispensing and contact lens courses. The students describe the work they are doing with lenses and how this relates to the science they learnt at GCSE level. It illustrates some of the career opportunities available in the eye care industry. Also included are some simple student activities, with accompanying teacher guidance, that illustrate how images are formed with lenses.

Home experiments to support remote teaching of light, sound and waves – Welcome to IOPSpark – Unlimited Access to Over 2,000 Physics Teaching Resources| Home-based teaching resources for 11-19 year olds | A selection of home experiments that are suitable to use with your students remotely on light, sound and waves.

Marvin and Milo Eerie Blue Water – for students aged 11-14 this shows how sunlight contains lots of different colours of light. For students aged 14-16 it considers another part of the electromagnetic spectrum. For students aged 16-19 it could be used to consider the excitation and de-excitation of electrons (a nice link to how a fluorescent light works!).

How Light Works

Some of the brightest minds in history have focused their intellects on the subject of light. Einstein even tried to imagine riding on a beam of light. We won’t get that crazy, but we will shine a light on everything scientists have found so far.

Unlike water waves, light waves follow more difficult pathways, plus they have no need for a medium to visit through. Once the 1800s dawned, no real evidence had accrued to demonstrate the wave theory of sunshine. That altered in 1801 when Thomas Youthful, an British physician and physicist, designed and ran probably the most famous experiments within the good reputation for science. It’s known today because the double-slit experiment and needs simple equipment — a source of light, a skinny card with two holes cut alongside along with a screen. To operate the experiment, Youthful permitted a laser beam to feed a pinhole and strike the credit card. If light contained particles or simple straight-line sun rays, he reasoned, light not blocked through the opaque card would go through the slits and travel inside a straight line towards the screen, where it might form two vibrant spots. This is not what Youthful observed. Rather, he saw a barcode pattern of alternating light and dark bands on screen. To describe this unpredicted pattern, he imagined light traveling through space just like a water wave, with crests and troughs.

Thomas Young’s Double Slit Experiment

Explore how light waves diffracted by a twin-slit apparatus can recombine through interference to produce a series of dark and light fringes on a reflective screen. The tutorial enables visitors to adjust the slit distances and alter the resulting interference patterns.

Thomas Young’s Double Slit Experiment – Java TutorialIn 1801, an British physicist named Thomas Youthful performed a test that strongly deduced the wave-like nature of sunshine. While he thought that light was made up of waves, Youthful reasoned that some form of interaction would occur when two light waves met. This interactive tutorial explores how coherent light waves interact when undergone two carefully spaced slits. The tutorial initializes with sun rays in the sun being undergone just one slit inside a screen to create coherent light. This light will be forecasted onto another screen which has twin (or double) slits, which again diffracts the incident illumination because it goes through. The outcomes of interference between your diffracted light beams could be visualized as light intensity distributions around the dark film. The slider labeled Distance Between Slits may be used to alter the space between your slits and convey corresponding variations within the interference intensity distribution patterns.

Wave-Particle Duality

The Wave-Particle Duality Complete Teacher’s Manual.

Within this experiment, students is going to be requested to describe the type of light. Prior to the experiment starts, ask students to calculate the things they anticipate seeing once they shine a laser onto a screen with no obstacle after which using the double slit. Keep these things sketch their predictions and supply a reason.

The concept behind how light travels and behaves has been one of physics’ greatest mysteries. In the world we experience every day, we see that objects, like a chair or a rock, can only be in one place at one time. We can say that these objects behave like a particle, which is a tiny object that is characterized by only being in one place at a time. On the other hand, we have things that can be at different places at the same time, such as waves. Have you noticed what happens when you throw a rock in the water? It makes waves that spread in a ring and grow as they move outward. In physics, waves are described as the spread of the disturbance or perturbation of something, often energy. When you throw a rock in the water, you are transferring some energy of the movement of the rock into the water, which causes the water to move around to disperse that energy.

Quantum Mystery of Light Revealed by New Experiment

While scientists know light can act like both a wave and a particle, they’ve never before seen it behaving like both simultaneously. Now a new experiment has shown light’s wave-particle duality at once.

Video advice: Can You Capture a Light Wave? Mind-Blowing Wave-Particle Duality Experiment!

In this video I show you an easy way to show that light is neither a wave nor a particle (or it is both?) by doing the double slit experiment followed by an analog of the photoelectric effect. This is a crazy experiment that shows how weird quantum mechanics really is. And an added bonus is that you can do these experiments at home! Finally I even show you what an electron orbital really means.

Light Wave Experiments

Is light made of waves, or particles? This fundamental question has dogged scientists for decades, because light seems to be both. However, until now, experiments have revealed light to act either like a particle, or a wave, but never the two at once. Now, for the first time, a new type of experiment has shown light behaving like both a particle and a wave simultaneously, providing a new dimension to the quandary that could help reveal the true nature of light, and of the whole quantum world. The debate goes back at least as far as Isaac Newton, who advocated that light was made of particles, and James Clerk Maxwell, whose successful theory of electromagnetism, unifying the forces of electricity and magnetism into one, relied on a model of light as a wave. Then in 1905, Albert Einstein explained a phenomenon called the photoelectric effect using the idea that light was made of particles called photons (this discovery won him the Nobel Prize in physics). (What’s That? Your Physics Questions Answered)Ultimately, there’s good reason to think that light is both a particle and a wave.

11+ Bright and Shining Light Experiments for Kids

11+ really cool light experiments for kids! Learn about where light comes from, how light travels, ways light can bend, and more!

These light experiments for children are enjoyable and simple to complete. They’re a great way to educate them about how exactly light works as well as proceed further to speak about refraction and reflection too. You may also begin using these activities to speak about circuits and just how energy travels to produce the sunshine that we’ve all arrived at need and love.

Supplies for Light Science Activities

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Curious Kids: is light a wave or a particle? – Einstein was awarded a Nobel prize for his explanation for how light can be described as being made up of individual particles of energy under certain conditions.

This is because light, in this situation, acts like a wave. When we shoot a beam of light through the holes, it breaks into two beams. The two resulting waves then interfere with each other to become either stronger (constructive interference) or weaker (destructive interference).

The Nature of Light

In many cases, the properties of light can be explained as a wave, as was shown in Young’s double-slit experiment.

Wave motion arises whenever a periodic disturbance of some type is propagated with an elastic medium. Pressure variations through air, transverse motions along an instrument string, or variations within the intensities from the local electric and magnetic fields wide, referred to as radio waves, are types of waves.

The Photoelectric Effect

In the early 19th century, English scientist Thomas Young carried out the famous double-slit experiment (also known as Young’s experiment), which demonstrated that a beam of light, when split into two beams and then recombined, will show interference effects that can only be explained by assuming that light is a wavelike disturbance. If light consisted strictly of ordinary or classical particles, and these particles were fired in a straight line through a slit and allowed to strike a screen on the other side, we would expect to see a pattern corresponding to the size and shape of the slit. However, when this single-slit experiment is actually performed, the pattern on the screen is a diffraction pattern in which the light is spread out. The smaller the slit, the greater the angle of spread.

Browse light waves project resources on Teachers Pay Teachers, a marketplace trusted by millions of teachers for original educational resources.

In this project, students use basic content vocabulary related to waves, sound, and light to explain how these concepts work in a poster format. This is supposed to be modeled off of the website howstuffworks. com, but this is very basic, surface level project. This includes a grading rubric for Help students learn important vocabulary when introducing or reviewing a unit on sound and light waves with this word search worksheet. The kids will enjoy themselves searching for the hidden words and will be reviewing meaning and spelling as they have fun. The definitions for the 23 hidden wordsTeachers looking to integrate technology skills and content areas with love this review of PowerPoint/Google Slides skills for students with the topic of Light Waves. While this is a simple research project, the real goal of this activity is for students to put their notes into a Presentation templaby Researchers at the MOMA need a new color to present at their showcase. Impress the donors and design a light wave that will reflect a cool new hue.

Two-Slit Experiment

Recreate the two-slit experiment by shining a laser pointer through two narrow slits and observing the interference pattern on a distant screen.

When light experiences a slit, diffraction causes it to bend and spread in all directions, creating a foreseeable banded pattern. When light experiences two slits, new dark regions appear. The dark and lightweight regions are created by interference from the light passing with the slits.

Light plus light equals dark

As light coming through one slit reaches the screen, it overlaps with light coming through the other slit. When the crest of one wave of light overlaps with the crest of another wave, the two waves combine to make a bigger wave and you see a bright blob of light. When the trough of one wave overlaps with the crest of another wave, the waves cancel each other out and you see a dark band. The appearance of dark bands when two light sources strike a screen shows that light is a wave phenomenon.

Double-Slit Science: How Light Can Be Both a Particle and a Wave

Learn how light can be two things at once with this illuminating experiment.

You might have heard that light includes particles known as photons. How could simple things like light be produced of particles? Physicists describe light as both a particle along with a wave. Actually, light’s wavelike behavior is accountable for several its awesome effects, like the iridescent colors created at first glance of bubbles. To determine an impressive and mind-bending illustration of how light behaves just like a wave, you just need three bits of mechanical pencil lead, a laser pointer along with a dark room.

Molecular Expressions Microscopy Primer: Light and Color

This interactive tutorial explores how coherent light waves interact when passed through two closely spaced slits.

In 1801, an British physicist named Thomas Youthful performed a test that strongly deduced the wave-like nature of sunshine. While he thought that light was made up of waves, Youthful reasoned that some form of interaction would occur when two light waves met. This interactive tutorial explores how coherent light waves interact when undergone two carefully spaced slits.

The tutorial initializes with rays from the sun being passed through a single slit in a screen to produce coherent light. This light is then projected onto another screen that has twin (or double) slits, which again diffracts the incident illumination as it passes through. The results of interference between the diffracted light beams can be visualized as light intensity distributions on the dark film. The slider labeled Distance Between Slits can be utilized to vary the distance between the slits and produce corresponding variations in the interference intensity distribution patterns.

Light as a Wave

Christian Huygens, who was a contemporary of Isaac Newton, suggested that light travels in waves. Isaac Newton, however, thought that light was compsed of particles that were too small to detect individually. In 1801 a physicist in England, Thomas Young, performed an experiment that showed that light behaves as a wave. He passed a beam of light through two thin, parallel slits. Alternating bright and dark bands appeared on a white screen some distance from the slit. Young reasoned that if light were made of particles as Newton suggested, only two bright bands of light would be projected on the white surface. The bright and dark bands demonstrated that the slits were causing light waves to interfere with each other. Sometimes this interference is constructive, and the light waves add together to create a bright patch. Sometimes the intereference is destructive and results in the light waves cancelling each other out creating dark patches on the screen. Electromagnetic waves, including visible light, are made up of oscillating electric and magnetic fields as shown.

The wave-particle duality of photons

Let’s think about the true nature of light. We described that light has the properties of wave and a particle. On this page, we will take a second look at that concept.

You are able to that one of the four forces constituting the world, the photon serves to share electromagnetic pressure. Another three forces are gravitational pressure, strong pressure, and weak pressure. The photon plays a huge role within the structure around the globe where we live and it is deeply associated with causes of matter and existence.

The duality of photons

The photoelectric effect is a phenomenon where irradiating a blue light on metal emits electrons from it. However, red light does not cause electron emission from metal no matter how long or how intense the light is applied. To understand this effect, you should think of photon as (clusters of) particles containing energy. Blue light is particles having high energy capable of emitting electrons. Red light is particles containing low energy not capable of emitting electrons.



Light Wave Experiments

What are examples of light waves?

Examples of Light Waves Sunlight is a source of visible light. It enables humans to perceive things around them. Light bulbs, fireflies, and stars all emit visible light. However, harmful ultraviolet radiation also comes from the sun and may result in skin damage after long exposures.

How do you make light waves?

3:469:45Light Is Waves: Crash Course Physics #39 - YouTubeYouTubeStart of suggested clipEnd of suggested clipSo that only a very narrow stream of sunlight passed through a slit into the room then he positionedMoreSo that only a very narrow stream of sunlight passed through a slit into the room then he positioned a plate with two more tiny slits cut into it spaced very close together.

What experiment proved that light and electrons are waves?

the double-slit experimentIn modern physics, the double-slit experiment is a demonstration that light and matter can display characteristics of both classically defined waves and particles; moreover, it displays the fundamentally probabilistic nature of quantum mechanical phenomena.

What are the 4 reaction of light waves?

When a light wave encounters an object, they are either transmitted, reflected, absorbed, refracted, polarized, diffracted, or scattered depending on the composition of the object and the wavelength of the light.

How are light waves used in everyday life?

Aside from sight, there are other important uses for visible light. We concentrate visible light to make lasers to use in everything from surgery, to CD players to laser pointers. Visible light waves also make our TV, computer and cell phone screens work .

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Stress and anxiety researcher at CHUV2014–present Ph.D. from Radboud University NijmegenGraduated 2002 Lives in Lausanne, Switzerland2013–present

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Try a quick experiment: Take two flashlights into a dark room and shine them so that their light beams cross. Notice anything peculiar? The rather anticlimactic answer is, probably not. That’s because the individual photons that make up light do not interact. Instead, they simply pass each other by, like indifferent spirits in the night.

But what if light particles could be made to interact, attracting and repelling each other like atoms in ordinary matter? One tantalizing, albeit sci-fi possibility: light sabers — beams of light that can pull and push on each other, making for dazzling, epic confrontations. Or, in a more likely scenario, two beams of light could meet and merge into one single, luminous stream.

It may seem like such optical behavior would require bending the rules of physics, but in fact, scientists at MIT, Harvard University, and elsewhere have now demonstrated that photons can indeed be made to interact — an accomplishment that could open a path toward using photons in quantum computing, if not in light sabers.

In a paper published today in the journal Science , the team, led by Vladan Vuletic, the Lester Wolfe Professor of Physics at MIT, and Professor Mikhail Lukin from Harvard University, reports that it has observed groups of three photons interacting and, in effect, sticking together to form a completely new kind of photonic matter.

In controlled experiments, the researchers found that when they shone a very weak laser beam through a dense cloud of ultracold rubidium atoms, rather than exiting the cloud as single, randomly spaced photons, the photons bound together in pairs or triplets, suggesting some kind of interaction — in this case, attraction — taking place among them.

While photons normally have no mass and travel at 300,000 kilometers per second (the speed of light), the researchers found that the bound photons actually acquired a fraction of an electron’s mass. These newly weighed-down light particles were also relatively sluggish, traveling about 100,000 times slower than normal noninteracting photons.

Vuletic says the results demonstrate that photons can indeed attract, or entangle each other. If they can be made to interact in other ways, photons may be harnessed to perform extremely fast, incredibly complex quantum computations.

“The interaction of individual photons has been a very long dream for decades,” Vuletic says.

Vuletic’s co-authors include Qi-Yung Liang, Sergio Cantu, and Travis Nicholson from MIT, Lukin and Aditya Venkatramani of Harvard, Michael Gullans and Alexey Gorshkov of the University of Maryland, Jeff Thompson from Princeton University, and Cheng Ching of the University of Chicago.

Biggering and biggering

Vuletic and Lukin lead the MIT-Harvard Center for Ultracold Atoms, and together they have been looking for ways, both theoretical and experimental, to encourage interactions between photons. In 2013, the effort paid off, as the team observed pairs of photons interacting and binding together for the first time, creating an entirely new state of matter.

In their new work, the researchers wondered whether interactions could take place between not only two photons, but more.

“For example, you can combine oxygen molecules to form O 2 and O 3 (ozone), but not O 4 , and for some molecules you can’t form even a three-particle molecule,” Vuletic says. “So it was an open question: Can you add more photons to a molecule to make bigger and bigger things?”

To find out, the team used the same experimental approach they used to observe two-photon interactions. The process begins with cooling a cloud of rubidium atoms to ultracold temperatures, just a millionth of a degree above absolute zero. Cooling the atoms slows them to a near standstill. Through this cloud of immobilized atoms, the researchers then shine a very weak laser beam — so weak, in fact, that only a handful of photons travel through the cloud at any one time.

The researchers then measure the photons as they come out the other side of the atom cloud. In the new experiment, they found that the photons streamed out as pairs and triplets, rather than exiting the cloud at random intervals, as single photons having nothing to do with each other.

In addition to tracking the number and rate of photons, the team measured the phase of photons, before and after traveling through the atom cloud. A photon’s phase indicates its frequency of oscillation.

“The phase tells you how strongly they’re interacting, and the larger the phase, the stronger they are bound together,” Venkatramani explains. The team observed that as three-photon particles exited the atom cloud simultaneously, their phase was shifted compared to what it was when the photons didn’t interact at all, and was three times larger than the phase shift of two-photon molecules. “This means these photons are not just each of them independently interacting, but they’re all together interacting strongly.”

Memorable encounters

The researchers then developed a hypothesis to explain what might have caused the photons to interact in the first place. Their model, based on physical principles, puts forth the following scenario: As a single photon moves through the cloud of rubidium atoms, it briefly lands on a nearby atom before skipping to another atom, like a bee flitting between flowers, until it reaches the other end.

If another photon is simultaneously traveling through the cloud, it can also spend some time on a rubidium atom, forming a polariton — a hybrid that is part photon, part atom. Then two polaritons can interact with each other via their atomic component. At the edge of the cloud, the atoms remain where they are, while the photons exit, still bound together. The researchers found that this same phenomenon can occur with three photons, forming an even stronger bond than the interactions between two photons.

“What was interesting was that these triplets formed at all,” Vuletic says. “It was also not known whether they would be equally, less, or more strongly bound compared with photon pairs.”

The entire interaction within the atom cloud occurs over a millionth of a second. And it is this interaction that triggers photons to remain bound together, even after they’ve left the cloud.

“What’s neat about this is, when photons go through the medium, anything that happens in the medium, they ‘remember’ when they get out,” Cantu says.

This means that photons that have interacted with each other, in this case through an attraction between them, can be thought of as strongly correlated, or entangled — a key property for any quantum computing bit.

“Photons can travel very fast over long distances, and people have been using light to transmit information, such as in optical fibers,” Vuletic says. “If photons can influence one another, then if you can entangle these photons, and we’ve done that, you can use them to distribute quantum information in an interesting and useful way.”

Going forward, the team will look for ways to coerce other interactions such as repulsion, where photons may scatter off each other like billiard balls.

“It’s completely novel in the sense that we don’t even know sometimes qualitatively what to expect,” Vuletic says. “With repulsion of photons, can they be such that they form a regular pattern, like a crystal of light? Or will something else happen? It’s very uncharted territory.”

This research was supported in part by the National Science Foundation.

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Press mentions, motherboard.

MIT physicists have created a new form of light that allows up to three photons to bind together, writes Daniel Oberhaus for Motherboard . While the research is experimental, Oberhaus writes that the trio of photons “are much more strongly bound together and are, as a result, better carriers of information” than other photonic qubits.

Smithsonian Magazine

Research published in Science demonstrates the ability of photons to bind together in a way previously thought impossible – creating a new form of light. “The photon dance happens in a lab at MIT where the physicists run table-top experiments with lasers,” writes Marissa Fessenden for Smithsonian . “Photons bound together in this way can carry information – a quality that is useful for quantum computing.”

New Scientist

Research by Physics PhD candidate Sergio Cantu has led to the discovery of a new form of light, which happens when photos stick together, as opposed to passing through one another. “’We send the light into the medium, it gets effectively dressed up as if it were atoms, and then when it turns back into photons they remember interactions that happened in the medium,” Cantu explains to Leah Crane at New Scientist . 

Writing for Newsweek, Katherine Hignett reports that for the first time, scientists have observed groups of three photons interacting and effectively producing a new form of light. “Light,” Prof. Vladan Vuletic, who led the research, tells Hignett, “is already used to transmit data very quickly over long distances via fiber optic cables. Being able to manipulate these photons could enable the distribution of data in much more powerful ways.”

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Space-Based Test Proves Light's Quantum Weirdness

Lasers bounced off satellites replicate classic “delayed choice” experiment

Space-Based Test Proves Light's Quantum Weirdness

Physicists sometimes say that a beam of light traveling through space is like a “great smoky dragon.” One can know much about where the light comes from (the dragon’s tail) and where it is seen (the dragon’s head), yet still know precious little about the journey in between (the dragon’s mysterious, nebulous body). As light travels from source to detection, it can behave as either a particle or a wave—or, paradoxically, both states or neither state. Now an experiment using laser beams shot at satellites in low-Earth orbit has confirmed that this bizarre detail about the nature of light holds true across record-breaking distances.

Quantum physics , the best description yet of how all known particles behaves, suggests that reality is fuzzy and uncertain at its most basic levels. For instance, the surreal quantum effect known as superposition essentially allows electrons, atoms and other building blocks of the universe to each exist in two or more places simultaneously.

Another strange quantum phenomenon is particle-wave duality. Whereas Isaac Newton thought light was made up of particles, his contemporary the Dutch scientist Christiaan Huygens argued that it consisted of waves. Eventually, researchers performing the so-called double-slit experiment demonstrated that Newton and Huygens were both right—photons of light could behave as both particles and waves.

The double-slit experiment involves shining a single light source through two adjacent slits in an opaque plate, and onto a detecting screen. If an experimenter closes one slit, the light passing through the other forms a bar on the screen—as if the light were behaving like a stream of particles. But if the experimenter leaves both slits open the light will not form two such bars. Instead it generates a series of bright and dark bands on the screen, as if waves of light scattered through the slits and interfered with each other. The bright bands indicate where light waves reinforced each other, and dark bands where they canceled each other out. Remarkably, this interference pattern will materialize even if photons are projected at the slits one at a time.

The way quantum physics explains these confounding results is that the instruments used to detect light determine its state as a particle or a wave. Describing this situation, in 1983 the American physicist Jonathan Wheeler coined the now-famous “ great smoky dragon ” comparison.

To examine how light “chooses” to become either a particle or wave upon its detection, Wheeler conceived of the delayed-choice experiment . In it, an optical device called a beam-splitter offers a single photon two paths to take. At the end of each path is a detector. If the photon behaves like a particle, it has an equal chance of taking either path and being seen by either detector. If the photon instead behaves like a wave, it will take both paths simultaneously and register in both detectors.

When the experiment only incorporates one beam-splitter, a single photon will take either path and just one detector will see it. This suggests the photon made the “choice” at the beam-splitter to behave like a particle. However, a beam-splitter can also act in reverse to merge two photons into one; if the experiment has the paths converge at a second beam-splitter before channeling them to a detector the result will be the interference pattern from the photons acting as waves and reacting to each other. This holds true even when the second beam-splitter is introduced in the split second after light passes through the first one—but has yet to reach the detectors.

Scientists have successfully carried out both versions of this experiment in the decades since Wheeler proposed it. Its results make sense if the photon “delays” making the choice to become a particle or a wave until it actually gets detected. The alternative would suggest that the photon could somehow decide to become a particle if it encountered one beam-splitter but then change its decision and become a wave if it ran across a second beam-splitter.

Historically, all delayed-choice experiments have been performed on Earth. But now scientists are increasingly conducting quantum experiments involving lasers shot across the vastness of outer space . Quantum physicist Paolo Villoresi at the University of Padua in Italy and his colleagues wanted to verify if the dual nature of light still held true even across the distances between the ground and satellites in low-Earth orbit. “As Galileo—who did most of his work at the University of Padua—said, ‘we have to prove the laws we know in new contexts,’” Villoresi quips.

Using the Matera Laser Ranging Observatory in Italy, Villoresi and his colleagues performed Wheeler’s delayed-choice experiment by firing green laser pulses at the Beacon-C and Starlette satellites, which reflected the photons back at the observatory. At their farthest, the satellites were 1,771 kilometers (1,100 miles) away from the observatory.

“Distance matters,” explains astrophysicist Brian Koberlein at the Rochester Institute of Technology in New York, who did not take part in this research. “In a single lab, you could argue that maybe in some way the experimenters are affecting the outcome. But over larger distances, there isn’t a clear way to affect outcomes.”

Instead of having the photons travel down one of two separate paths of equal lengths, the scientists measured two different aspects of each photon—how each one oscillated in space, and whether it took a shorter or longer path to the detectors. Their results confirmed light’s curious quantum behavior over distances tens to hundreds of times greater than previously shown, Villoresi says. The team’s findings appeared Oct. 25 in the journal Science Advances .

“This work further confirms that quantum mechanics really is the description of the ‘great smoky dragon,’” Koberlein says. “It may be strange, but it is logically and mathematically consistent.”

Aside from testing the quantum qualities of light across unprecedented distances, Villoresi notes that quantum physics experiments conducted across space could help lead to satellite-based telecommunications networks protected by nigh-unhackable quantum cryptography . By clarifying the fundamental properties of photons during such experiments as was done in this study, “there may be direct applications for larger bandwidths in quantum communications,” Villoresi says. Indeed, great smoky dragons may someday carry secrets in their jaws.

physics experiments based on light



Charles Q. Choi is a frequent contributor to Scientific American . His work has also appeared in The New York Times, Science, Nature, Wired, and LiveScience, among others. In his spare time, he has traveled to all seven continents.  Follow Charles Q. Choi on Twitter

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The double-slit experiment: Is light a wave or a particle?

The double-slit experiment is universally weird.

The double-slit experiment shows light waves rippling across between two slits and interfering with each other.

How does the double-slit experiment work?

Interference patterns from waves, particle patterns, double-slit experiment: quantum mechanics, history of the double-slit experiment, additional resources.

The double-slit experiment is one of the most famous experiments in physics and definitely one of the weirdest. It demonstrates that matter and energy (such as light) can exhibit both wave and particle characteristics — known as the particle-wave duality of matter — depending on the scenario, according to the scientific communication site Interesting Engineering (opens in new tab) .

According to the University of Sussex (opens in new tab) , American physicist Richard Feynman referred to this paradox as the central mystery of quantum mechanics. 

We know the quantum world is strange, but the two-slit experiment takes things to a whole new level. The experiment has perplexed scientists for over 200 years, ever since the first version was first performed by British scientist Thomas Young in 1801.

Related: 10 mind-boggling things you should know about quantum physics (opens in new tab)  

Christian Huygens was the first to describe light as traveling in waves whilst Isaac Newton thought light was composed of tiny particles according to Las Cumbres Observatory (opens in new tab) . But who is right? British polymath Thomas Young designed the double-slit experiment to put these theories to the test. 

To appreciate the truly bizarre nature of the double-split experiment we first need to understand how waves and particles act when passing through two slits. 

When Young first carried out the double-split experiment in 1801 he found that light behaved like a wave. 

Firstly, if we were to shine a light on a wall with two parallel slits — and for the sake of simplicity, let's say this light has only one wavelength. 

As the light passes through the slits, each, in turn, becomes almost like a new source of light. On the far side of the divider, the light from each slit diffracts and overlaps with the light from the other slit, interfering with each other. 

According to Stony Brook University (opens in new tab) , any wave can create an interference pattern, whether it be a sound wave, light wave or waves across a body of water. When a wave crest hits a wave trough they cancel each other out — known as destructive interference — and appear as a dark band. When a crest hits a crest they amplify each other — known as constructive interference — and appear as a bright band. The combination of dark and bright bands is known as an interference pattern and can be seen on the sensor screen opposite the slits. 

This interference pattern was the evidence Young needed to determine that light was a wave and not a particle as Newton had suggested. 

But that is not the whole story. Light is a little more complicated than that, and to see how strange it really is we also need to understand what pattern a particle would make on a sensor field. 

If you were to carry out the same experiment and fire grains of sand or other particles through the slits, you would end up with a different pattern on the sensor screen. Each particle would go through a slit end up in a line in roughly the same place (with a little bit of spread depending on the angle the particle passed through the slit).  

Clearly, waves and particles produce a very different pattern, so it should be easy to distinguish between the two right? Well, this is where the double-slit experiment gets a little strange when we try and carry out the same experiment but with tiny particles of light called photons. Enter the realm of quantum mechanics. 

The smallest constituent of light is subatomic particles called photons. By using photons instead of grains of sand we can carry out the double-slit experiment on an atomic scale. 

If you block off one of the slits, so it is just a single-slit experiment, and fire photons through to the sensor screen, the photons will appear as pinprick points on the sensor screen, mimicking the particle patterns produced by sand in the previous example. From this evidence, we could suggest that photons are particles. 

Now, this is where things start to get weird. 

If you unblock the slit and fire photons through both slits, you start to see something very similar to the interference pattern produced by waves in the light example. The photons appear to have gone through the pair of slits acting like waves. 

But what if you launch photons one by one, leaving enough time between them that they don't have a chance of interfering with each other, will they behave like particles or waves? 

At first, the photons appear on the sensor screen in a random scattered manner, but as you fire more and more of them, an interference pattern begins to emerge. Each photon by itself appears to be contributing to the overall wave-like behavior that manifests as an interference pattern on the screen — even though they were launched one at a time so that no interference between them was possible.

It's almost as though each photon is "aware" that there are two slits available. How? Does it split into two and then rejoin after the slit and then hit the sensor? To investigate this, scientists set up a detector that can tell which slit the photon passes through. 

Again, we fire photons one at a time at the slits, as we did in the previous example. The detector finds that about 50% of the photons have passed through the top slit and about 50% through the bottom, and confirms that each photon goes through one slit or the other. Nothing too unusual there. 

But when we look at the sensor screen on this experiment, a different pattern emerges. 

This pattern matches the one we saw when we fired particles through the slits. It appears that monitoring the photons triggers them to switch from the interference pattern produced by waves to that produced by particles. 

If the detection of photons through the slits is apparently affecting the pattern on the sensor screen, what happens if we leave the detector in place but switch it off? (Shh, don't tell the photons we're no longer spying on them!) 

This is where things get really, really weird. 

Same slits, same photons, same detector, just turned off. Will we see the same particle-like pattern? 

No. The particles again make a wave-like interference pattern on the sensor screen. 

The atoms appear to act like waves when you're not watching them, but as particles when you are. How? Well, if you can answer that, a Nobel Prize is waiting for you. 

In the 1930s, scientists proposed that human consciousness might affect quantum mechanics. Mathematician John Von Neumann first postulated this in 1932 in his book " The Mathematical Foundations of Quantum Mechanics ." In the 1960s, theoretical physicist, Eugene Wigner conceived a thought experiment called Wigner's friend — a paradox in quantum physics that describes the states of two people, one conducting the experiment and the observer of the first person, according to science magazine Popular Mechanics . The idea that the consciousness of a person carrying out the experiment can affect the result is knowns as the Von Neumann–Wigner interpretation.

Though a spiritual explanation for quantum mechanic behavior is still believed by a few individuals, including author and alternative medicine advocate Deepak Chopra , a majority of the science community has long disregarded it. 

As for a more plausible theory, scientists are stumped. 

Furthermore —and perhaps even more astonishingly — if you set up the double-slit experiment to detect which slit the photon went through after the photon has already hit the sensor screen, you still end up with a particle-type pattern on the sensor screen, even though the photon hadn't yet been detected when it hit the screen. This result suggests that detecting a photon in the future affects the pattern produced by the photon on the sensor screen in the past. This experiment is known as the quantum eraser experiment and is explained in more detail in this informative video from Fermilab (opens in new tab) . 

We still don't fully understand how exactly the particle-wave duality of matter works, which is why it is regarded as one of the greatest mysteries of quantum mechanics. 

The first version of the double-slit experiment was carried out in 1801 by British polymath Thomas Young, according to the American Physical Society (opens in new tab) (APS). His experiment demonstrated the interference of light waves and provided evidence that light was a wave, not a particle. 

Young also used data from his experiments to calculate the wavelengths of different colors of light and came very close to modern values.

Despite his convincing experiment that light was a wave, those who did not want to accept that Isaac Newton could have been wrong about something criticized Young. (Newton had proposed the corpuscular theory, which posited that light was composed of a stream of tiny particles he called corpuscles.) 

According to APS, Young wrote in response to one of the critics, "Much as I venerate the name of Newton, I am not therefore obliged to believe that he was infallible."

Since the development of quantum mechanics, physicists now acknowledge light to be both a particle and a wave. 

Explore the double-slit experiment in more detail with this article from the University of Cambridge, (opens in new tab) which includes images of electron patterns in a double-slit experiment. Discover the true nature of light with Canon Science Lab (opens in new tab) . Read about fragments of energy that are not waves or particles — but could be the fundamental building blocks of the universe — in this article from The Conversation (opens in new tab) . Dive deeper into the two-slit experiment in this article published in the journal Nature (opens in new tab) . 


Grangier, Philippe, Gerard Roger, and Alain Aspect. " Experimental evidence for a photon anticorrelation effect on a beam splitter: a new light on single-photon interferences. (opens in new tab) " EPL (Europhysics Letters) 1.4 (1986): 173.

Thorn, J. J., et al. "Observing the quantum behavior of light in an undergraduate laboratory. (opens in new tab) " American Journal of Physics 72.9 (2004): 1210-1219.

Ghose, Partha. " The central mystery of quantum mechanics. (opens in new tab) " arXiv preprint arXiv:0906.0898 (2009).

Aharonov, Yakir, et al. " Finally making sense of the double-slit experiment. (opens in new tab) " Proceedings of the National Academy of Sciences 114.25 (2017): 6480-6485.

Peng, Hui. " Observations of Cross-Double-Slit Experiments. (opens in new tab) " International Journal of Physics 8.2 (2020): 39-41. 

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Daisy Dobrijevic joined in February 2022 as a reference writer having previously worked for our sister publication All About Space magazine as a staff writer. Before joining us, Daisy completed an editorial internship with the BBC Sky at Night Magazine and worked at the National Space Centre in Leicester, U.K., where she enjoyed communicating space science to the public. In 2021, Daisy completed a PhD in plant physiology and also holds a Master's in Environmental Science, she is currently based in Nottingham, U.K.

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14.2: Light As a Particle

The only thing that interferes with my learning is my education. -- Albert Einstein

Radioactivity is random, but do the laws of physics exhibit randomness in other contexts besides radioactivity? Yes. Radioactive decay was just a good playpen to get us started with concepts of randomness, because all atoms of a given isotope are identical. By stocking the playpen with an unlimited supply of identical atom-toys, nature helped us to realize that their future behavior could be different regardless of their original identicality. We are now ready to leave the playpen, and see how randomness fits into the structure of physics at the most fundamental level.

The laws of physics describe light and matter, and the quantum revolution rewrote both descriptions. Radioactivity was a good example of matter's behaving in a way that was inconsistent with classical physics, but if we want to get under the hood and understand how nonclassical things happen, it will be easier to focus on light rather than matter. A radioactive atom such as uranium-235 is after all an extremely complex system, consisting of 92 protons, 143 neutrons, and 92 electrons. Light, however, can be a simple sine wave.

However successful the classical wave theory of light had been --- allowing the creation of radio and radar, for example --- it still failed to describe many important phenomena. An example that is currently of great interest is the way the ozone layer protects us from the dangerous short-wavelength ultraviolet part of the sun's spectrum. In the classical description, light is a wave. When a wave passes into and back out of a medium, its frequency is unchanged, and although its wavelength is altered while it is in the medium, it returns to its original value when the wave reemerges. Luckily for us, this is not at all what ultraviolet light does when it passes through the ozone layer, or the layer would offer no protection at all!

13.2.1 Evidence for light as a particle

For a long time, physicists tried to explain away the problems with the classical theory of light as arising from an imperfect understanding of atoms and the interaction of light with individual atoms and molecules. The ozone paradox, for example, could have been attributed to the incorrect assumption that one could think of the ozone layer as a smooth, continuous substance, when in reality it was made of individual ozone molecules. It wasn't until 1905 that Albert Einstein threw down the gauntlet, proposing that the problem had nothing to do with the details of light's interaction with atoms and everything to do with the fundamental nature of light itself.


a / Digital camera images of dimmer and dimmer sources of light. The dots are records of individual photons.

In those days the data were sketchy, the ideas vague, and the experiments difficult to interpret; it took a genius like Einstein to cut through the thicket of confusion and find a simple solution. Today, however, we can get right to the heart of the matter with a piece of ordinary consumer electronics, the digital camera. Instead of film, a digital camera has a computer chip with its surface divided up into a grid of light-sensitive squares, called “pixels.” Compared to a grain of the silver compound used to make regular photographic film, a digital camera pixel is activated by an amount of light energy orders of magnitude smaller. We can learn something new about light by using a digital camera to detect smaller and smaller amounts of light, as shown in figure a . Figure a /1 is fake, but a /2 and a /3 are real digital-camera images made by Prof. Lyman Page of Princeton University as a classroom demonstration. Figure a /1 is what we would see if we used the digital camera to take a picture of a fairly dim source of light. In figures a /2 and a /3, the intensity of the light was drastically reduced by inserting semitransparent absorbers like the tinted plastic used in sunglasses. Going from a /1 to a /2 to a /3, more and more light energy is being thrown away by the absorbers.


b / A wave is partially absorbed.

The results are drastically different from what we would expect based on the wave theory of light. If light was a wave and nothing but a wave, b , then the absorbers would simply cut down the wave's amplitude across the whole wavefront. The digital camera's entire chip would be illuminated uniformly, and weakening the wave with an absorber would just mean that every pixel would take a long time to soak up enough energy to register a signal.


c / A stream of particles is partially absorbed.

But figures a /2 and a /3 show that some pixels take strong hits while others pick up no energy at all. Instead of the wave picture, the image that is naturally evoked by the data is something more like a hail of bullets from a machine gun, c . Each “bullet” of light apparently carries only a tiny amount of energy, which is why detecting them individually requires a sensitive digital camera rather than an eye or a piece of film.

Although Einstein was interpreting different observations, this is the conclusion he reached in his 1905 paper: that the pure wave theory of light is an oversimplification, and that the energy of a beam of light comes in finite chunks rather than being spread smoothly throughout a region of space.

We now think of these chunks as particles of light, and call them “photons,” although Einstein avoided the word “particle,” and the word “photon” was invented later. Regardless of words, the trouble was that waves and particles seemed like inconsistent categories. The reaction to Einstein's paper could be kindly described as vigorously skeptical. Even twenty years later, Einstein wrote, “There are therefore now two theories of light, both indispensable, and --- as one must admit today despite twenty years of tremendous effort on the part of theoretical physicists --- without any logical connection.” In the remainder of this chapter we will learn how the seeming paradox was eventually resolved.


d / Einstein and Seurat: twins separated at birth? Seine Grande Jatte by Georges Seurat (19th century).

Discussion Questions

◊ Suppose someone rebuts the digital camera data in figure a , claiming that the random pattern of dots occurs not because of anything fundamental about the nature of light but simply because the camera's pixels are not all exactly the same --- some are just more sensitive than others. How could we test this interpretation?

◊ Discuss how the correspondence principle applies to the observations and concepts discussed in this section.

13.2.2 How much light is one photon?

The photoelectric effect.

We have seen evidence that light energy comes in little chunks, so the next question to be asked is naturally how much energy is in one chunk. The most straightforward experimental avenue for addressing this question is a phenomenon known as the photoelectric effect. The photoelectric effect occurs when a photon strikes the surface of a solid object and knocks out an electron. It occurs continually all around you. It is happening right now at the surface of your skin and on the paper or computer screen from which you are reading these words. It does not ordinarily lead to any observable electrical effect, however, because on the average free electrons are wandering back in just as frequently as they are being ejected. (If an object did somehow lose a significant number of electrons, its growing net positive charge would begin attracting the electrons back more and more strongly.)


e / Apparatus for observing the photoelectric effect. A beam of light strikes a capacitor plate inside a vacuum tube, and electrons are ejected (black arrows).

Figure e shows a practical method for detecting the photoelectric effect. Two very clean parallel metal plates (the electrodes of a capacitor) are sealed inside a vacuum tube, and only one plate is exposed to light. Because there is a good vacuum between the plates, any ejected electron that happens to be headed in the right direction will almost certainly reach the other capacitor plate without colliding with any air molecules.

The illuminated (bottom) plate is left with a net positive charge, and the unilluminated (top) plate acquires a negative charge from the electrons deposited on it. There is thus an electric field between the plates, and it is because of this field that the electrons' paths are curved, as shown in the diagram. However, since vacuum is a good insulator, any electrons that reach the top plate are prevented from responding to the electrical attraction by jumping back across the gap. Instead they are forced to make their way around the circuit, passing through an ammeter. The ammeter allows a measurement of the strength of the photoelectric effect.

An unexpected dependence on frequency

The photoelectric effect was discovered serendipitously by Heinrich Hertz in 1887, as he was experimenting with radio waves. He was not particularly interested in the phenomenon, but he did notice that the effect was produced strongly by ultraviolet light and more weakly by lower frequencies. Light whose frequency was lower than a certain critical value did not eject any electrons at all. (In fact this was all prior to Thomson's discovery of the electron, so Hertz would not have described the effect in terms of electrons --- we are discussing everything with the benefit of hindsight.) This dependence on frequency didn't make any sense in terms of the classical wave theory of light. A light wave consists of electric and magnetic fields. The stronger the fields, i.e., the greater the wave's amplitude, the greater the forces that would be exerted on electrons that found themselves bathed in the light. It should have been amplitude (brightness) that was relevant, not frequency. The dependence on frequency not only proves that the wave model of light needs modifying, but with the proper interpretation it allows us to determine how much energy is in one photon, and it also leads to a connection between the wave and particle models that we need in order to reconcile them.

To make any progress, we need to consider the physical process by which a photon would eject an electron from the metal electrode. A metal contains electrons that are free to move around. Ordinarily, in the interior of the metal, such an electron feels attractive forces from atoms in every direction around it. The forces cancel out. But if the electron happens to find itself at the surface of the metal, the attraction from the interior side is not balanced out by any attraction from outside. In popping out through the surface the electron therefore loses some amount of energy \(E_s\), which depends on the type of metal used.


f / The hamster in her hamster ball is like an electron emerging from the metal (tiled kitchen floor) into the surrounding vacuum (wood floor). The wood floor is higher than the tiled floor, so as she rolls up the step, the hamster will lose a certain amount of kinetic energy, analogous to \(E_s\). If her kinetic energy is too small, she won't even make it up the step.

Suppose a photon strikes an electron, annihilating itself and giving up all its energy to the electron. (We now know that this is what always happens in the photoelectric effect, although it had not yet been established in 1905 whether or not the photon was completely annihilated.) The electron will (1) lose kinetic energy through collisions with other electrons as it plows through the metal on its way to the surface; (2) lose an amount of kinetic energy equal to \(E_s\) as it emerges through the surface; and (3) lose more energy on its way across the gap between the plates, due to the electric field between the plates. Even if the electron happens to be right at the surface of the metal when it absorbs the photon, and even if the electric field between the plates has not yet built up very much, \(E_s\) is the bare minimum amount of energy that it must receive from the photon if it is to contribute to a measurable current. The reason for using very clean electrodes is to minimize \(E_s\) and make it have a definite value characteristic of the metal surface, not a mixture of values due to the various types of dirt and crud that are present in tiny amounts on all surfaces in everyday life.

We can now interpret the frequency dependence of the photoelectric effect in a simple way: apparently the amount of energy possessed by a photon is related to its frequency. A low-frequency red or infrared photon has an energy less than \(E_s\), so a beam of them will not produce any current. A high-frequency blue or violet photon, on the other hand, packs enough of a punch to allow an electron to make it to the other plate. At frequencies higher than the minimum, the photoelectric current continues to increase with the frequency of the light because of effects (1) and (3).

Numerical relationship between energy and frequency

Prompted by Einstein's photon paper, Robert Millikan (whom we first encountered in chapter 8 ) figured out how to use the photoelectric effect to probe precisely the link between frequency and photon energy. Rather than going into the historical details of Millikan's actual experiments (a lengthy experimental program that occupied a large part of his professional career) we will describe a simple version, shown in figure g , that is used sometimes in college laboratory courses. 2 The idea is simply to illuminate one plate of the vacuum tube with light of a single wavelength and monitor the voltage difference between the two plates as they charge up. Since the resistance of a voltmeter is very high (much higher than the resistance of an ammeter), we can assume to a good approximation that electrons reaching the top plate are stuck there permanently, so the voltage will keep on increasing for as long as electrons are making it across the vacuum tube.


g / A different way of studying the photoelectric effect.

At a moment when the voltage difference has a reached a value \(\Delta \)V, the minimum energy required by an electron to make it out of the bottom plate and across the gap to the other plate is \(E_s+e\Delta \)V. As \(\Delta V\) increases, we eventually reach a point at which \(E_s+e\Delta V\) equals the energy of one photon. No more electrons can cross the gap, and the reading on the voltmeter stops rising. The quantity \(E_s+e\Delta V\) now tells us the energy of one photon. If we determine this energy for a variety of wavelengths, h , we find the following simple relationship between the energy of a photon and the frequency of the light:

where \(h\) is a constant with the value \(6.63\times10^{-34}\ \text{J}\cdot\text{s}\). Note how the equation brings the wave and particle models of light under the same roof: the left side is the energy of one particle of light, while the right side is the frequency of the same light, interpreted as a wave . The constant \(h\) is known as Planck's constant, for historical reasons explained in the footnote beginning on the preceding page.


h / The quantity \(E_s+e\Delta V\) indicates the energy of one photon. It is found to be proportional to the frequency of the light.


How would you extract \(h\) from the graph in figure h ? What if you didn't even know \(E_s\) in advance, and could only graph \(e\Delta V\) versus \(f\)?

(answer in the back of the PDF version of the book)

Since the energy of a photon is \(hf\), a beam of light can only have energies of \(hf\), \(2hf\), \(3hf\), etc. Its energy is quantized --- there is no such thing as a fraction of a photon. Quantum physics gets its name from the fact that it quantizes quantities like energy, momentum, and angular momentum that had previously been thought to be smooth, continuous and infinitely divisible.

◊ The photoelectric effect only ever ejects a very tiny percentage of the electrons available near the surface of an object. How well does this agree with the wave model of light, and how well with the particle model? Consider the two different distance scales involved: the wavelength of the light, and the size of an atom, which is on the order of \(10^{-10}\) or \(10^{-9}\) m.

◊ What is the significance of the fact that Planck's constant is numerically very small? How would our everyday experience of light be different if it was not so small?

◊ How would the experiments described above be affected if a single electron was likely to get hit by more than one photon?

◊ Draw some representative trajectories of electrons for \(\Delta V=0\), \(\Delta V\) less than the maximum value, and \(\Delta V\) greater than the maximum value.

◊ Explain based on the photon theory of light why ultraviolet light would be more likely than visible or infrared light to cause cancer by damaging DNA molecules. How does this relate to discussion question C?

◊ Does \(E=hf\) imply that a photon changes its energy when it passes from one transparent material into another substance with a different index of refraction?

13.2.3 Wave-particle duality

How can light be both a particle and a wave? We are now ready to resolve this seeming contradiction. Often in science when something seems paradoxical, it's because we (1) don't define our terms carefully, or (2) don't test our ideas against any specific real-world situation. Let's define particles and waves as follows:

As a real-world check on our philosophizing, there is one particular experiment that works perfectly. We set up a double-slit interference experiment that we know will produce a diffraction pattern if light is an honest-to-goodness wave, but we detect the light with a detector that is capable of sensing individual photons, e.g., a digital camera. To make it possible to pick out individual dots due to individual photons, we must use filters to cut down the intensity of the light to a very low level, just as in the photos by Prof. Page on p. 837. The whole thing is sealed inside a light-tight box. The results are shown in figure i . (In fact, the similar figures in on page 837 are simply cutouts from these figures.)


i / Wave interference patterns photographed by Prof. Lyman Page with a digital camera. Laser light with a single well-defined wavelength passed through a series of absorbers to cut down its intensity, then through a set of slits to produce interference, and finally into a digital camera chip. (A triple slit was actually used, but for conceptual simplicity we discuss the results in the main text as if it was a double slit.) In panel 2 the intensity has been reduced relative to 1, and even more so for panel 3.

Neither the pure wave theory nor the pure particle theory can explain the results. If light was only a particle and not a wave, there would be no interference effect. The result of the experiment would be like firing a hail of bullets through a double slit, j . Only two spots directly behind the slits would be hit.


j / Bullets pass through a double slit.

If, on the other hand, light was only a wave and not a particle, we would get the same kind of diffraction pattern that would happen with a water wave, k . There would be no discrete dots in the photo, only a diffraction pattern that shaded smoothly between light and dark.


k / A water wave passes through a double slit.

Applying the definitions to this experiment, light must be both a particle and a wave. It is a wave because it exhibits interference effects. At the same time, the fact that the photographs contain discrete dots is a direct demonstration that light refuses to be split into units of less than a single photon. There can only be whole numbers of photons: four photons in figure i /3, for example.

A wrong interpretation: photons interfering with each other

One possible interpretation of wave-particle duality that occurred to physicists early in the game was that perhaps the interference effects came from photons interacting with each other. By analogy, a water wave consists of moving water molecules, and interference of water waves results ultimately from all the mutual pushes and pulls of the molecules. This interpretation was conclusively disproved by G.I. Taylor, a student at Cambridge. The demonstration by Prof. Page that we've just been discussing is essentially a modernized version of Taylor's work. Taylor reasoned that if interference effects came from photons interacting with each other, a bare minimum of two photons would have to be present at the same time to produce interference. By making the light source extremely dim, we can be virtually certain that there are never two photons in the box at the same time. In figure i /3, however, the intensity of the light has been cut down so much by the absorbers that if it was in the open, the average separation between photons would be on the order of a kilometer! At any given moment, the number of photons in the box is most likely to be zero. It is virtually certain that there were never two photons in the box at once.


l / A single photon can go through both slits.

The concept of a photon's path is undefined.

If a single photon can demonstrate double-slit interference, then which slit did it pass through? The unavoidable answer must be that it passes through both! This might not seem so strange if we think of the photon as a wave, but it is highly counterintuitive if we try to visualize it as a particle. The moral is that we should not think in terms of the path of a photon. Like the fully human and fully divine Jesus of Christian theology, a photon is supposed to be 100% wave and 100% particle. If a photon had a well defined path, then it would not demonstrate wave superposition and interference effects, contradicting its wave nature. (In subsection 13.3.4 we will discuss the Heisenberg uncertainty principle, which gives a numerical way of approaching this issue.)

Another wrong interpretation: the pilot wave hypothesis

A second possible explanation of wave-particle duality was taken seriously in the early history of quantum mechanics. What if the photon particle is like a surfer riding on top of its accompanying wave ? As the wave travels along, the particle is pushed, or “piloted” by it. Imagining the particle and the wave as two separate entities allows us to avoid the seemingly paradoxical idea that a photon is both at once. The wave happily does its wave tricks, like superposition and interference, and the particle acts like a respectable particle, resolutely refusing to be in two different places at once. If the wave, for instance, undergoes destructive interference, becoming nearly zero in a particular region of space, then the particle simply is not guided into that region.

The problem with the pilot wave interpretation is that the only way it can be experimentally tested or verified is if someone manages to detach the particle from the wave, and show that there really are two entities involved, not just one. Part of the scientific method is that hypotheses are supposed to be experimentally testable. Since nobody has ever managed to separate the wavelike part of a photon from the particle part, the interpretation is not useful or meaningful in a scientific sense.

The probability interpretation

The correct interpretation of wave-particle duality is suggested by the random nature of the experiment we've been discussing: even though every photon wave/particle is prepared and released in the same way, the location at which it is eventually detected by the digital camera is different every time. The idea of the probability interpretation of wave-particle duality is that the location of the photon-particle is random, but the probability that it is in a certain location is higher where the photon-wave's amplitude is greater.

More specifically, the probability distribution of the particle must be proportional to the square of the wave's amplitude,

This follows from the correspondence principle and from the fact that a wave's energy density is proportional to the square of its amplitude. If we run the double-slit experiment for a long enough time, the pattern of dots fills in and becomes very smooth as would have been expected in classical physics. To preserve the correspondence between classical and quantum physics, the amount of energy deposited in a given region of the picture over the long run must be proportional to the square of the wave's amplitude. The amount of energy deposited in a certain area depends on the number of photons picked up, which is proportional to the probability of finding any given photon there.

The probability interpretation was disturbing to physicists who had spent their previous careers working in the deterministic world of classical physics, and ironically the most strenuous objections against it were raised by Einstein, who had invented the photon concept in the first place. The probability interpretation has nevertheless passed every experimental test, and is now as well established as any part of physics.

An aspect of the probability interpretation that has made many people uneasy is that the process of detecting and recording the photon's position seems to have a magical ability to get rid of the wavelike side of the photon's personality and force it to decide for once and for all where it really wants to be. But detection or measurement is after all only a physical process like any other, governed by the same laws of physics. We will postpone a detailed discussion of this issue until p. 864, since a measuring device like a digital camera is made of matter, but we have so far only discussed how quantum mechanics relates to light.

◊ Referring back to the example of the carrot in the microwave oven, show that it would be nonsensical to have probability be proportional to the field itself, rather than the square of the field.

◊ Einstein did not try to reconcile the wave and particle theories of light, and did not say much about their apparent inconsistency. Einstein basically visualized a beam of light as a stream of bullets coming from a machine gun. In the photoelectric effect, a photon “bullet” would only hit one atom, just as a real bullet would only hit one person. Suppose someone reading his 1905 paper wanted to interpret it by saying that Einstein's so-called particles of light are simply short wave-trains that only occupy a small region of space. Comparing the wavelength of visible light (a few hundred nm) to the size of an atom (on the order of 0.1 nm), explain why this poses a difficulty for reconciling the particle and wave theories.

◊ Can a white photon exist?

◊ In double-slit diffraction of photons, would you get the same pattern of dots on the digital camera image if you covered one slit? Why should it matter whether you give the photon two choices or only one?

13.2.4 Photons in three dimensions

Up until now I've been sneaky and avoided a full discussion of the three-dimensional aspects of the probability interpretation. The example of the carrot in the microwave oven, for example, reduced to a one-dimensional situation because we were considering three points along the same line and because we were only comparing ratios of probabilities. The purpose of bringing it up now is to head off any feeling that you've been cheated conceptually rather than to prepare you for mathematical problem solving in three dimensions, which would not be appropriate for the level of this course.

A typical example of a probability distribution in section 13.1 was the distribution of heights of human beings. The thing that varied randomly, height, \(h\), had units of meters, and the probability distribution was a graph of a function \(D(h)\). The units of the probability distribution had to be \(\text{m}^{-1}\) (inverse meters) so that areas under the curve, interpreted as probabilities, would be unitless: \((\text{area})=(\text{height})(\text{width})=\text{m}^{-1}\cdot\text{m}\).

Now suppose we have a two-dimensional problem, e.g., the probability distribution for the place on the surface of a digital camera chip where a photon will be detected. The point where it is detected would be described with two variables, \(x\) and \(y\), each having units of meters. The probability distribution will be a function of both variables, \(D(x,y)\). A probability is now visualized as the volume under the surface described by the function \(D(x,y)\), as shown in figure n . The units of \(D\) must be \(\text{m}^{-2}\) so that probabilities will be unitless: \((\text{probability})=(\text{depth})(\text{length})(\text{width}) =\text{m}^{-2}\cdot\text{m}\cdot\text{m}\). In terms of calculus, we have \(P\:=\:\int Ddx dy\).


n / Probability is the volume under a surface defined by \(D(x,y)\).

Generalizing finally to three dimensions, we find by analogy that the probability distribution will be a function of all three coordinates, \(D(x,y,z)\), and will have units of \(\text{m}^{-3}\). It is unfortunately impossible to visualize the graph unless you are a mutant with a natural feel for life in four dimensions. If the probability distribution is nearly constant within a certain volume of space \(v\), the probability that the photon is in that volume is simply \(vD\). If not, then we can use an integral, \(P\:=\:\int Ddx dydz\).


Benjamin Crowell (Fullerton College).  Conceptual Physics is copyrighted with a CC-BY-SA license.

Ideas, Inspiration, and Giveaways for Teachers

We Are Teachers

55 Best Science Experiments for High School Labs and Science Fairs

Fire up the Bunsen burners!

WeAreTeachers Staff

The cool thing about high school science experiments and projects is that kids are old enough to tackle some pretty amazing concepts. Some science experiments for high school are just advanced versions of simpler projects they did when they were younger, with detailed calculations or fewer instructions. Other projects involve fire, chemicals, or other materials they couldn’t use before.

Many of these science experiments for high school are intended for classroom labs, but most can be adapted to become science fair projects too. Just consider variables that you can change up, like materials or other parameters. That changes a classroom lab into a true scientific method experiment!

(Just a heads up, WeAreTeachers may collect a share of sales from the links on this page. We only recommend items our team loves!)

Biology Experiments for High School

When it comes to biology, science experiments for high school students usually bring dissection to mind. But there are plenty of other useful labs and hands-on projects for teens to try. Here are some of our favorites.

1. Mash potatoes to learn about catalase

Three test tubes in a red holder, filled with a white substance

Catalase is found in nearly all living cells, protecting them from oxidative damage. Try this lab to isolate catalase from potatoes using hydrogen peroxide.

Learn more: Potato Catalase/Practical Biology

2. Extract DNA from a strawberry

Collage of steps to extract DNA from a strawberry (Science Experiments for High School)

You don’t need a lot of supplies to perform this experiment, but it’s impressive nonetheless. Turn this into a science fair project by trying it with other fruits and vegetables too.

Learn more: Strawberry DNA/Numbers to Neurons

3. Re-create Mendel’s pea plant experiment

Pea plants growing in white square containers on a lab table

Gregor Mendel’s pea plant experiments were some of the first to explore inherited traits and genetics. Re-create his cross-pollination experiments with a variety of pea plants you’ve grown yourself.

Learn more: Mendel’s Pea Plants/Love to Know

4. Make plants move with light

Diagram of plant seedlings moving toward light affected by different variables (Science Experiments for High School)

By high school age, kids know that many plants move toward sunlight, a process known as phototropism. So science experiments for high school students on this topic need to introduce variables into the process, like covering seedling parts with different materials to see the effects.

Learn more: Phototropism/Science Buddies

5. Test the five-second rule

We’d all like to know the answer to this one: Is it really safe to eat food you’ve dropped on the floor? Design and conduct an experiment to find out (although we think we might already know the answer).

6. Taste foods to find your threshold for sour, sweet, and bitter

Human tongue with an arrow pointing to the papillae

The sense of taste is fascinating—what some people think is delicious, others just can’t stand. Try this experiment to test subjects’ taste perceptions and thresholds using a series of diluted solutions.

Learn more: Taste Threshold/Science Buddies

7. Complete a field survey

Students examining the water in a ditch in a green field (Science Experiments for High School)

Teaching students to conduct field surveys opens up the possibility of lots of different science experiments for high school. Show them how to observe an area over time, record their findings, and analyze the results.

Learn more: Field Survey/Love to Know

8. See the effects of antibiotics on bacteria

Test tubes containing various bacteria

Bacteria can be divided into two groups: gram-positive and gram-negative. In this experiment, students first determine the two groups, then try the effects of various antibiotics on them. You can get a gram stain kit , bacillus cereus and rodospirillum rubrum cultures, and antibiotic discs from Home Science Tools.

Learn more: Antibiotics Project/Home Science Tools

9. Witness the carbon cycle in action

Test tubes filled with plants and green and blue liquid

We know that plants take in carbon dioxide and give off oxygen, right? Well, this experiment helps you prove that and see the effect light has on the process.

Learn more: Carbon Cycle/Science Lessons That Rock

10. Look for cell mitosis in an onion

Cell mitosis (division) is actually easy to see in action when you look at onion root tips under a microscope. Students will be amazed to see science theory become science reality right before their eyes.

11. Test the effects of disinfectants

Petri dish divided in half with bacteria and paper disks on the surface

Grow bacteria in a petri dish along with paper disks soaked in various antiseptics and disinfectants. You’ll be able to see which ones effectively inhibit bacteria growth.

Learn more: Antiseptics and Disinfectants/Amy Brown Science

12. Investigate the efficacy of types of fertilizer

How to choose the fertilizer that will make plants grow the fastest.

Let’s spice things up in the botanical kitchen! Mix up some “recipes” for your students’ plants by experimenting with different types of fertilizer and see which one they devour the most.

Learn more: Best Fertilizer/

13. Explore the impact of genetic modification on seeds

Competition between crops and weeds and introduction of genetically modified seeds

Let’s go green and see what happens when we pit our crops against some weeds! Will genetically modified plants come out on top or will the weeds reign supreme? Let’s find out in this exciting biotech and plant challenge!

Learn more: Genetically Modified Seeds/Science Buddies

Chemistry Experiments for High School

Perhaps no class is better suited to science experiments for high school kids than chemistry. Bunsen burners, beakers and test tubes, and the possibility of (controlled) explosions? Students will love it!

14. Watch a beating heart made of gallium

Blob of gallium with the image of a beating heart and the periodic table symbol for gallium

This is one of those science demos that’s so cool to see in action. An electrochemical reaction causes a blob of liquid metal to oscillate like a beating heart!

Learn more: Gallium Demo/Science Notes

15. Break apart covalent bonds

Tub of water with battery leads in it

Break the covalent bond of H 2 O into H and O with this simple experiment. You only need simple supplies for this one.

Learn more: Covalent Bonds/Teaching Without Chairs

16. Measure the calories in various foods

Collage of steps for measuring calories with a homemade calorimeter (Science Experiments for High School)

How do scientists determine the number of calories in your favorite foods? Build your own calorimeter and find out! This kit from Home Science Tools has all the supplies you’ll need.

Learn more: DIY Calorimeter/Science Buddies

17. Detect latent fingerprints

Fingerprint divided into two, one half yellow and one half black

Forensic science is engrossing and can lead to important career opportunities too. Explore the chemistry needed to detect latent (invisible) fingerprints, just like they do for crime scenes!

Learn more: Fingerprints/HubPages

18. Use Alka-Seltzer to explore reaction rate

Collage of reaction rate experiment steps (Science Experiments for High School)

Tweak this basic concept to create a variety of science experiments for high school students. Change the temperature, surface area, pressure, and more to see how reaction rates change.

Learn more: Reaction Rate/Numbers to Neurons

19. Determine whether sports drinks provide more electrolytes than OJ

Open circuit equipment for testing for electrolytes (Science Experiments for High School)

Are those pricey sports drinks really worth it? Try this experiment to find out. You’ll need some special equipment for this one; buy a complete kit at Home Science Tools .

Learn more: Electrolytes Experiment/Science Buddies

20. Extract bismuth from Pepto-Bismol

Piece of bismuth extracted from Pepto Bismol

Bismuth is a really cool metal with a rainbow sheen. It’s also an ingredient in Pepto-Bismol, and by carefully following the procedures at the link, you can isolate a chunk of this amazing heavy metal.

Learn more: Extracting Bismuth/Popular Science

21. Turn flames into a rainbow

You’ll need to get your hands on a few different chemicals for this experiment, but the wow factor will make it worth the effort! (Click through to the YouTube link for an explanation of how this one works.)

22. Test and sort elements

Students using electrical circuits to test items in a petri dish (Science Experiments for High School)

Elements in the periodic table are grouped by metals, nonmetals, and metalloids. But how do chemists determine where each element belongs? This ready-to-go science kit contains the materials you need to experiment and find out.

Learn more: Metals, Nonmetals, and Metalloids/Ward’s Science

23. Discover the size of a mole

Supplies needed for mole experiment, included scale, salt, and chalk

The mole is a key concept in chemistry, so it’s important to ensure students really understand it. This experiment uses simple materials like salt and chalk to make an abstract concept more concrete.

Learn more: How Big Is a Mole?/Amy Brown Science

24. Cook up candy to learn mole and molecule calculations

Aluminum foil bowl filled with bubbling liquid over a bunsen burner

This edible experiment lets students make their own peppermint hard candy while they calculate mass, moles, molecules, and formula weights. Sweet!

Learn more: Candy Chemistry/Dunigan Science TpT

25. Make soap to understand saponification

Colorful soaps from saponification science experiments for high school

Take a closer look at an everyday item: soap! Students use oils and other ingredients to make their own soap, learning about esters and saponification.

Learn more: Saponification/Chemistry Solutions TpT

26. Uncover the secrets of evaporation

This systematic and classic example of changing one variable at a time by creating several mini-projects will have your high schoolers engaged in a high-level review of the classic scientific method.

Learn more: Evaporation/Science Projects

27. Investigate the principles of pyrotechnics

Explore how fireworks work - a high school chemistry experiment.

Let’s dive into the explosive world of fireworks and discover the colorful secrets behind these dazzling pyrotechnic displays! Your students will be ecstatic to use party poppers (and sparklers, if you’re feeling really daring) to explore the science behind fireworks.

Learn more: How Fireworks Work/Royal Society of Chemistry

Physics Experiments for High School

When you think of physics science experiments for high school, the first thing that comes to mind is probably the classic build-a-bridge. But there are plenty of other ways for teens to get hands-on with physics concepts. Here are some to try.

28. Remove the air in a DIY vacuum chamber

DIY vacuum chamber made from a jar and large hypodermic needle

You can use a vacuum chamber to do lots of cool experiments, but a ready-made one can be expensive. Try this project to make your own with basic supplies.

Learn more: Vacuum Chamber/Instructables

29. Put together a mini Tesla coil

Looking for a simple but showy high school science fair project? Build your own mini Tesla coil and wow the crowd!

30. Boil water in a paper cup

Logic tells us we shouldn’t set a paper cup over a heat source, right? Yet it’s actually possible to boil water in a paper cup without burning the cup up! Learn about heat transfer and thermal conductivity with this experiment. Go deeper by trying other liquids like honey to see what happens.

31. Blast music using magnets

A paper speaker built from magnets, cardboard, and a paper plate

We spend a lot of time telling teens to turn down their music, so they’ll appreciate the chance to turn it up for once! Using strong magnets and an amplifier (both available on Amazon), plus a few other supplies, they’ll build a speaker and measure how the magnets affect the volume.

Learn more: Paper Speaker/Science Buddies

32. Construct a light bulb

Emulate Edison and build your own simple light bulb! You can turn this into a science fair project by experimenting with different types of materials for filaments.

33. Measure the speed of light—with your microwave

Student measuring the distance between holes in cooked egg whites (High School Science Experiments)

Grab an egg and head to your microwave for this surprisingly simple experiment! By measuring the distance between cooked portions of egg whites, you’ll be able to calculate the wavelength of the microwaves in your oven, and in turn, the speed of light.

Learn more: Microwave Speed of Light/Science Buddies

34. Generate a Lichtenberg figure

Lichtenberg figure generated on a sheet of Plexiglassd in

See electricity in action when you generate and capture a Lichtenberg figure with polyethylene sheets, wood, or even acrylic and toner. Change the electrical intensity and materials to see what types of patterns you can create.

Learn more: Lichtenberg Figure/Science Notes

35. Build your own Newton’s Cradle

Student swinging the right ball on a DIY Newton's Cradle made of popsicle sticks and marbles

Newton’s Cradle demonstrates the concept of momentum—and it’s really fun to play with! Challenge students to design and build their own, experimenting with different materials or changing up the number of balls to see how it affects momentum.

Learn more: How To Make a Simple Newton’s Cradle/Babble Dabble Do

36. Explore the power of friction with sticky note pads

A wood platform holding a weight suspended by chains from two sticky note pads interleaved together (Science Experiments for High School)

Ever try to pull a piece of paper out of the middle of a big stack? It’s harder than you think it would be! That’s due to the power of friction. In this experiment, students interleave the sheets of two sticky note pads, then measure how much weight it takes to pull them apart. The results are astonishing!

Learn more: Sticky Notes Friction/Science Buddies

37. Bounce balls to explore stored energy and energy transfer

Colorful rubber balls bouncing against a white background

Learn about potential and kinetic energy by bouncing balls and measuring their heights on each rebound. This is one of those classic physics science experiments for high school that students are sure to enjoy!

Learn more: Rebound Experiment/Science Buddies

38. Build a cloud chamber to prove background radiation

A cloud chamber constructed of a plastic container, cookie sheet, and dry ice, and

Ready to dip your toe into particle physics? Learn about background radiation and build a cloud chamber to prove the existence of muons.

Learn more: Background Radiation/Science Buddies

39. Slide into kinetic friction

Measure the effect of friction on different surfaces.

Students will investigate kinetic friction and its effects on the speed of a rolling object by giving the objects a little push and watching them fly, on surfaces both smooth and rough. Stay tuned to see which texture wins the race!

Learn more: Effect of Friction on Objects in Motion/Science Buddies

40. Harness the power of air drag

Design and test parachutes to study air drag.

Who can make the slowest descent? Students will use the power of drag to create a design that takes its sweet time falling to the ground. They’ll be encouraged to tinker and tweak until they have the ultimate sky-sailing machine.

Learn more: Science World and Scientific American

41. Magnetize a motor

5 high school physics science projects with magnets.

Magnets lend themselves as a helpful material in many a science experiment. Your students will explore the properties of magnetism with any one of these five experiments using magnets. They’ll even learn the basics of Fleming’s left-hand rule.

Learn more: Simple Electric Motor/School Science Experiments

42. Explore interference and diffraction

Explore interference and diffraction using CDs.

Investigate the physics of light and optics using CDs and DVDs. Though both of these optical objects might be quickly becoming a thing of the past, your students can utilize their diffraction patterns to explore the science behind optics.

Learn more: Science Buddies

Engineering Experiments for High School

Engineering involves the hands-on application of multiple types of science. Teens with an interest in designing and building will especially enjoy these STEM challenge science experiments for high school. They’re all terrific for science fairs too.

43. Re-create Da Vinci’s flying machine

Da Vinci flying machine built from a paper cup and other basic supplies

Da Vinci sketched several models of “flying machines” and hoped to soar through the sky. Do some research into his models and try to reconstruct one of your own.

Learn more: Da Vinci Flying Machine/Student Savvy

44. Peer into an infinity mirror

Rectangular and circular mirrors with lights reflecting into the distance (Science Experiments for High School)

Optical illusions are mesmerizing, but they also help teach kids about a variety of science concepts. Design and build a mirror that seems to reflect lights on and on forever. The supplies are basic, but the impact is major!

Learn more: Infinity Mirror/Science Buddies

45. Design a heart-rate monitor

DIY heart rate monitor made from blue fabric and a red heart

Smartwatches are ubiquitous these days, so pretty much anyone can wear a heart-rate monitor on their wrist. But can you build your own? It takes some specialized supplies, but they’re not hard to track down. You can buy items like an Arduino LilyPad Board on Amazon.

Learn more: Heart Rate Monitor/Science Buddies

46. Race 3D printed cars

Simple 3-D printed race cars with vegetables strapped to them (Science Experiments for High School)

3D printers are a marvel of the modern era, and budding engineers should definitely learn to use them. Use Tinkercad or a similar program to design and print race cars that can support a defined weight, then see which can roll the fastest! (No 3D printer in your STEM lab? Check the local library: Many of them have 3D printers available for patrons to use.)

Learn more: 3D Printed Cars/Instructables

47. Launch a model rocket

Model rockets built from water bottles and other supplies

Bottle rockets are another one of those classic science experiments for high school classes, and for good reason! The engineering involved in designing and launching a rocket capable of carrying a specified payload involves the practical application of all sorts of concepts. Plus, it’s fun!

Learn more: Bottle Rockets/Science Buddies

48. Grow veggies in a hydroponic garden

Vertical hydroponic garden made from PVC pipes and aluminum downspouts

Hydroponics is the gardening wave of the future, making it easy to grow plants anywhere with minimal soil required. For a science fair engineering challenge, design and construct your own hydroponic garden capable of growing vegetables to feed a family. This model is just one possible option.

Learn more: Hydroponics/Instructables

49. Grab items with a mechanical claw

KiwiCo hydraulic claw kit (Science Experiments for High School)

Delve into robotics with this engineering project! This kit includes all the materials you need, with complete video instructions.

Learn more: Hydraulic Claw/KiwiCo

50. Play volleyball with machines

Challenge your students to design and build machines that will volley a Ping-Pong ball back and forth, using only basic materials. They can even compare their results to those from students around the world!

Learn more: Volleyball Challenge/Science Buddies

51. Construct a crystal radio

Homemade crystal radio set (Science Experiments for High School)

Return to the good old days and build a radio from scratch! This makes a cool science fair project if you experiment with different types of materials for the antenna. It takes some specialized equipment, but fortunately, Home Science Tools has an all-in-one kit for this project.

Learn more: Crystal Radio/SciToys

52. Build a burglar alarm

Simple electronic burglar alarm with a cell phone

The challenge? Set up a system to alert you when someone has broken into your house or classroom. This can take any form students can dream up, and you can customize this STEM high school science experiment for multiple skill levels. Keep it simple with an alarm that makes a sound that can be heard from a specified distance. Or kick it up a notch and require the alarm system to send a notification to a cell phone, like the project at the link.

Learn more: Intruder Alarm/Instructables

53. Walk across a plastic bottle bridge

Students sitting on a large bridge made of plastic bottles

Balsa wood bridges are OK, but this plastic bottle bridge is really impressive! In fact, students can build all sorts of structures using the concept detailed at the link. It’s the ultimate upcycled STEM challenge!

Learn more: TrussFab Structures/Instructables

54. Unleash the power of geothermal energy

How to use heat as a source of renewable energy.

This experiment is all about tapping into the fiery fury deep underground within the Earth and harnessing it for clean, renewable power. It will definitely spark your students’ interest and exploration of geothermal energy.

Learn more: Geothermal Energy/Science Buddies

55. Construct a Rube Goldberg machine

In this activity, students will unleash their creativity as they design and build their very own contraptions that perform a simple task in the most complicated way possible. Your students will be using the engineering design process, problem-solving skills, and teamwork to create truly unique machines.

Learn more: Design and Build a Rube Goldberg/Teach Engineering

Looking for more science content? Check out the Best Science Websites for Middle and High School .

Plus, get all the latest teaching tips and tricks when you sign up for our newsletters .

Whether you're a student looking for a science fair idea or a teacher seeking new science experiments for high school labs, find them here!

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physics experiments based on light

Go Science Girls

Physics Experiments for Kids

Learning physics is important to become a science person. Physics is the heart of knowing how everything works in this world. It includes understanding of light, electricity, heat, sound, atoms, magnetism, etc.

Feel free to try any of the below activities at home to give an interesting head start on physics to your kids..

Physics Project Activities for Kids

How to Build a Mini Tesla Coil at Home

How to Build a Mini Tesla Coil at Home

How to Use Iron Filings to See Magnetic Field

How to Use Iron Filings to See Magnetic Field

DIY Potato Battery: Potato Light bulb Science Fair Project

DIY Potato Battery: Potato Light bulb Science Fair Project

How to do an Air Pressure on Water Experiment for Kids

How to do an Air Pressure on Water Experiment for Kids

Dancing Ghosts : Halloween Balloon Static Electricity Activity

Dancing Ghosts : Halloween Balloon Static Electricity Activity

Balloon and Pin Experiment (Air Pressure Experiment for Kids)

Balloon and Pin Experiment (Air Pressure Experiment for Kids)

Balloon Balance Experiment (Air has Weight)

Balloon Balance Experiment (Air has Weight)

How to Make Self Retracting Pinwheel from Popsicle Sticks

How to Make Self Retracting Pinwheel from Popsicle Sticks

How to Make DIY Magnetic Compass

How to Make DIY Magnetic Compass

Lemon Light Experiment (How to Make a Lemon Battery)

Lemon Light Experiment (How to Make a Lemon Battery)

Static Comb Experiment : Explore Static Electricity for Kids

Static Comb Experiment : Explore Static Electricity for Kids

How To Make a Balloon Hovercraft

How To Make a Balloon Hovercraft

Egg in a Bottle – Air Pressure Experiment

Egg in a Bottle – Air Pressure Experiment

How to Make An Electromagnet

How to Put a Skewer Through a Balloon : Science Fair Project

How to Put a Skewer Through a Balloon : Science Fair Project

Why does Water Bend with Static Electricity (Worksheets Included)

Why does Water Bend with Static Electricity (Worksheets Included)

Balloon In Hot and Cold Water – Experiment

Balloon In Hot and Cold Water – Experiment

How to Make a Toy Car Launcher From Popsicle Sticks

How to Make a Toy Car Launcher From Popsicle Sticks

Floating Paper Clip on Water – Science Experiment

Floating Paper Clip on Water – Science Experiment

Standing Hair : Static Electricity Experiment With Kids

Standing Hair : Static Electricity Experiment With Kids

Floating Egg Science Experiment ( Using Salt, Sugar & Saline Water)

Floating Egg Science Experiment ( Using Salt, Sugar & Saline Water)

Tornado in a Bottle : Best Weather Science Activity

Tornado in a Bottle : Best Weather Science Activity

Crushing Can Experiment : Effect of Atmospheric Pressure

Crushing Can Experiment : Effect of Atmospheric Pressure

How to Make a Candle Seesaw? Balancing Act Experiment

How to Make a Candle Seesaw? Balancing Act Experiment

Burning Candle Rising Water Experiment

Burning Candle Rising Water Experiment

Drip Drop Bottle-Water Bottle Pressure Experiment

Drip Drop Bottle-Water Bottle Pressure Experiment

Balloon in a Bottle : Air Pressure Experiment

Balloon in a Bottle : Air Pressure Experiment

How to Make a Square Bubble

How to Make a Square Bubble

How to Build a Balloon Rocket (Balloon Rocket Race)

How to Build a Balloon Rocket (Balloon Rocket Race)

How to Draw on Water Using Dry Erase Markers (Dancing Drawings)

How to Draw on Water Using Dry Erase Markers (Dancing Drawings)

How to Build a Fast Balloon Powered Car (Air Powered Car Project Ideas)

Walking Water Experiment – Teach Capillary Action to Kids

Walking Water Experiment – Teach Capillary Action to Kids

Walking on Eggs : Measure the Strength of Egg Shell

Walking on Eggs : Measure the Strength of Egg Shell

How Strong is an Egg Shell? Strength of Eggshell Bridge

How Strong is an Egg Shell? Strength of Eggshell Bridge

How to Make an Electric Newton’s Disc

4 Fun Ways To Introduce the Planets To Toddlers

Easy Experiments to Introduce Magnetism to Kids

DIY Pully – Physics Fun Experiment for Kids

DIY Pully – Physics Fun Experiment for Kids

Refraction of Light : Play & Learn Activity for Kids

Refraction of Light : Play & Learn Activity for Kids

Geomag Panels – Review

Geomag Panels – Review

DIY Christmas Tree Magnet Maze (Fun Science Game)

DIY Christmas Tree Magnet Maze (Fun Science Game)

DIY Christmas Tree Bubble Wand {Learn bubble physics}

Catapult STEM Project – DIY Catapult for Kids

Upcycled Catapult – STEM go green DIY Challenge

Static Electricity Balloon and Salt and Pepper Experiment

How To Make Balance Scales for Toddlers and Preschoolers

How To Make Balance Scales for Toddlers and Preschoolers

Halley Harper; Science Girl Extraordinare: Summer Set In Motion : Book Review by GoScienceGirls

How to Make Heart Shaped Bubble Wand

Easter Egg Bubble Wands

Magnet Maze Game Designing Activity – Learn Science and Art

Magnet Maze Game Designing Activity – Learn Science and Art

Shape Bubble Wands – DIY in Different Shapes

DIY Fridge Door Marble Run Using Magnets

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Encyclopedia Britannica

Ray theories in the ancient world

Why is light important for life on Earth?

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visible spectrum of light

What is light in physics?

Light is electromagnetic radiation that can be detected by the human eye. Electromagnetic radiation occurs over an extremely wide range of wavelengths, from gamma rays with wavelengths less than about 1 × 10 −11 metres to radio waves measured in metres.

What is the speed of light?

The speed of light in a vacuum is a fundamental physical constant, and the currently accepted value is 299,792,458 metres per second, or about 186,282 miles per second.

What is a rainbow?

A rainbow is formed when sunlight is refracted by spherical water droplets in the atmosphere; two refractions and one reflection, combined with the chromatic dispersion of water, produce the primary arcs of colour.

Light is a primary tool for perceiving the world and interacting with it for many organisms. Light from the Sun warms the Earth, drives global weather patterns, and initiates the life-sustaining process of photosynthesis; about 10 22 joules of solar radiant energy reach Earth each day. Light’s interactions with matter have also helped shape the structure of the universe.

What is colour's relation to light?

In physics colour is associated specifically with electromagnetic radiation of a certain range of wavelengths visible to the human eye. The radiation of such wavelengths constitutes that portion of the electromagnetic spectrum known as the visible spectrum—i.e., light.

Read a brief summary of this topic

light , electromagnetic radiation that can be detected by the human eye . Electromagnetic radiation occurs over an extremely wide range of wavelengths , from gamma rays with wavelengths less than about 1 × 10 −11 metre to radio waves measured in metres. Within that broad spectrum the wavelengths visible to humans occupy a very narrow band, from about 700 nanometres (nm; billionths of a metre) for red light down to about 400 nm for violet light. The spectral regions adjacent to the visible band are often referred to as light also, infrared at the one end and ultraviolet at the other. The speed of light in a vacuum is a fundamental physical constant , the currently accepted value of which is exactly 299,792,458 metres per second, or about 186,282 miles per second.


No single answer to the question “What is light?” satisfies the many contexts in which light is experienced, explored, and exploited. The physicist is interested in the physical properties of light, the artist in an aesthetic appreciation of the visual world. Through the sense of sight, light is a primary tool for perceiving the world and communicating within it. Light from the Sun warms the Earth , drives global weather patterns, and initiates the life-sustaining process of photosynthesis . On the grandest scale, light’s interactions with matter have helped shape the structure of the universe . Indeed, light provides a window on the universe, from cosmological to atomic scales. Almost all of the information about the rest of the universe reaches Earth in the form of electromagnetic radiation. By interpreting that radiation, astronomers can glimpse the earliest epochs of the universe, measure the general expansion of the universe, and determine the chemical composition of stars and the interstellar medium . Just as the invention of the telescope dramatically broadened exploration of the universe, so too the invention of the microscope opened the intricate world of the cell . The analysis of the frequencies of light emitted and absorbed by atoms was a principal impetus for the development of quantum mechanics . Atomic and molecular spectroscopies continue to be primary tools for probing the structure of matter, providing ultrasensitive tests of atomic and molecular models and contributing to studies of fundamental photochemical reactions .

Light transmits spatial and temporal information. This property forms the basis of the fields of optics and optical communications and a myriad of related technologies, both mature and emerging. Technological applications based on the manipulations of light include lasers , holography , and fibre-optic telecommunications systems .

In most everyday circumstances, the properties of light can be derived from the theory of classical electromagnetism , in which light is described as coupled electric and magnetic fields propagating through space as a traveling wave . However, this wave theory, developed in the mid-19th century, is not sufficient to explain the properties of light at very low intensities. At that level a quantum theory is needed to explain the characteristics of light and to explain the interactions of light with atoms and molecules . In its simplest form, quantum theory describes light as consisting of discrete packets of energy , called photons . However, neither a classical wave model nor a classical particle model correctly describes light; light has a dual nature that is revealed only in quantum mechanics. This surprising wave-particle duality is shared by all of the primary constituents of nature (e.g., electrons have both particle-like and wavelike aspects). Since the mid-20th century, a more comprehensive theory of light, known as quantum electrodynamics (QED), has been regarded by physicists as complete. QED combines the ideas of classical electromagnetism, quantum mechanics, and the special theory of relativity .

Italian physicist Guglielmo Marconi at work in the wireless room of his yacht Electra, c. 1920.

This article focuses on the physical characteristics of light and the theoretical models that describe the nature of light. Its major themes include introductions to the fundamentals of geometrical optics, classical electromagnetic waves and the interference effects associated with those waves, and the foundational ideas of the quantum theory of light. More detailed and technical presentations of these topics can be found in the articles optics , electromagnetic radiation , quantum mechanics , and quantum electrodynamics . See also relativity for details of how contemplation of the speed of light as measured in different reference frames was pivotal to the development of Albert Einstein ’s theory of special relativity in 1905.

Theories of light through history


While there is clear evidence that simple optical instruments such as plane and curved mirrors and convex lenses were used by a number of early civilizations, ancient Greek philosophers are generally credited with the first formal speculations about the nature of light. The conceptual hurdle of distinguishing the human perception of visual effects from the physical nature of light hampered the development of theories of light. Contemplation of the mechanism of vision dominated these early studies. Pythagoras ( c. 500 bce ) proposed that sight is caused by visual rays emanating from the eye and striking objects, whereas Empedocles ( c. 450 bce ) seems to have developed a model of vision in which light was emitted both by objects and the eye. Epicurus ( c. 300 bce ) believed that light is emitted by sources other than the eye and that vision is produced when light reflects off objects and enters the eye. Euclid ( c. 300 bce ), in his Optics , presented a law of reflection and discussed the propagation of light rays in straight lines. Ptolemy ( c. 100 ce ) undertook one of the first quantitative studies of the refraction of light as it passes from one transparent medium to another, tabulating pairs of angles of incidence and transmission for combinations of several media.

Roger Bacon

With the decline of the Greco-Roman realm, scientific progress shifted to the Islamic world . In particular, al-Maʾmūn , the seventh ʿAbbāsid caliph of Baghdad, founded the House of Wisdom (Bayt al-Hikma) in 830 ce to translate, study, and improve upon Hellenistic works of science and philosophy. Among the initial scholars were al-Khwārizmī and al-Kindī . Known as the “philosopher of the Arabs,” al-Kindī extended the concept of rectilinearly propagating light rays and discussed the mechanism of vision. By 1000, the Pythagorean model of light had been abandoned, and a ray model, containing the basic conceptual elements of what is now known as geometrical optics, had emerged. In particular, Ibn al-Haytham (Latinized as Alhazen), in Kitab al-manazir ( c. 1038; “Optics”), correctly attributed vision to the passive reception of light rays reflected from objects rather than an active emanation of light rays from the eyes. He also studied the mathematical properties of the reflection of light from spherical and parabolic mirrors and drew detailed pictures of the optical components of the human eye. Ibn al-Haytham’s work was translated into Latin in the 13th century and was a motivating influence on the Franciscan friar and natural philosopher Roger Bacon . Bacon studied the propagation of light through simple lenses and is credited as one of the first to have described the use of lenses to correct vision.

More From Forbes

Observing the universe really does change the outcome, and this experiment shows how.

The wave pattern for electrons passing through a double slit, one-at-a-time. If you measure “which ... [+] slit” the electron goes through, you destroy the quantum interference pattern shown here. However, the wave-like behavior remains so long as the electrons have a de Broglie wavelength that's smaller than the size of the slit they're passing through.

When we divide up matter into the smallest possible chunks that it's made of — into the stuff that can be divided or split no further — those indivisible things we arrive at are known as quanta. But it's a complicated story each time we ask the question: how does each individual quantum behave? Do they behave like particles? Or do they behave like waves?

The most puzzling fact about quantum mechanics is that the answer you get depends on how you look at the individual quanta that are part of the experiment. If you make certain classes of measurements and observations, they behave like particles; if you make other choices, they behave like waves. Whether and how you observe your own experiment really does change the outcome, and the double-slit experiment is the perfect way to show how.

This diagram, dating back to Thomas Young's work in the early 1800s, is one of the oldest pictures ... [+] that demonstrate both constructive and destructive interference as arising from wave sources originating at two points: A and B. This is a physically identical setup to a double slit experiment, even though it applies just as well to water waves propagated through a tank.

More than 200 years ago, the first double-slit experiment was performed by Thomas Young, who was investigating whether light behaved as a wave or a particle. Newton had famously claimed that it must be a particle, or corpuscle, and was able to explain a number of phenomena with this idea. Reflection, transmission, refraction, and any ray-based optical phenomena were perfectly consistent with Newton's view of how light should behave.

But other phenomena seemed to need waves to explain them: interference and diffraction in particular. When you passed light through a double slit, it behaved just the same way that water waves do, producing that familiar interference pattern. The light-and-dark spots that appeared on the screen behind the slit corresponded to constructive-and-destructive interference, indicating that — at least under the right circumstances — light behaves as a wave does.

If you have two slits very close to one another, it stands to reason that any individual quantum of energy will go through either one slit or the other. Like many others, you might think that the reason light produces this interference pattern is because you have lots of different quanta of light — photons — all going through the various slits together, and interfering with one another.

So you take a different set of quantum objects, like electrons, and fire them at the double slit. Sure, you get an interference pattern, but now you come up with a brilliant tweak: you fire the electrons one-at-a-time through the slits. With each new electron, you record a new data point for where it landed. After thousands upon thousands of electrons, you finally look at the pattern that emerges. And what do you see? Interference.

Electrons exhibit wave properties as well as particle properties, and can be used to construct ... [+] images or probe particle sizes just as well as light can. Here, you can see the results of an experiment where electrons are fired one-at-a-time through a double-slit. Once enough electrons are fired, the interference pattern can clearly be seen.

Somehow, each electron must be interfering with itself, acting fundamentally like a wave.

For many decades, physicists have puzzled and argued over what this means must really be going on. Is the electron going through both slits at once, interfering with itself somehow? This seems counterintuitive and physically impossible, but we have a way to tell whether this is true or not: we can measure it.

So we set up the same experiment, but this time, we have a little light we shine across each of the two slits. When the electron goes through, the light is slightly perturbed, so we can "flag" which one of the two slits it passed through. With each electron that goes through, we get a signal coming from one of the two slits. At last, each electron has been counted, and we know which slit every one went through. And now, at the end, when we look at our screen, this is what we see.

If you measure which slit an electron goes through when performing a one-at-a-time double slit ... [+] experiment, you don't get an interference pattern on the screen behind it. Instead, the electrons behave not as waves, but as classical particles.

That interference pattern? It's gone. Instead, it's replaced by just two piles of electrons: the paths you'd expect each electron to take if there were no interference at all.

What's going on here? It's as though the electrons "know" whether you're watching them or not. The very act of observing this setup — of asking "which slit did each electron pass through?" — changes the outcome of the experiment.

If you measure which slit the quantum passes through, it behaves as though it passes through one and only one slit: it acts like a classical particle. If you don't measure which slit the quantum passes through, it behaves as a wave, acting like it passed through both slits simultaneously and producing an interference pattern.

What's actually going on here? To find out, we have to perform more experiments.

By setting up a movable mask, you can choose to either block one or both slits for the double slit ... [+] experiment, seeing what the outcomes are and how they change with the motion of the mask.

One experiment you can set up is to put a movable mask in front of both slits, while still firing electrons through them one-at-a-time. Practically, this has now been accomplished  in the following fashion:

How does the pattern change?

The results of the 'masked' double-slit experiment. Note that when the first slit (P1), the second ... [+] slit (P2), or both slits (P12) are open, the pattern you see is very different depending on whether one or two slits are available.

Exactly like you might expect:

It's as though if both paths are there as available options simultaneously, without restriction, you get interference and wave-like behavior. But if you only have one path available, or if either path is restricted somehow, you won't get interference and will get particle-like behavior.

So we go back to having both slits in the "open" position, and shining light across both of them as you pass electrons one-at-a-time through the double slits.

A tabletop laser experiment is a modern outgrowth of the technology that enabled proving the absurd: ... [+] that light didn't behave like a particle.

If your light is both energetic (high energy per photon) and intense (a large number of total photons), you won't get an interference pattern at all. 100% of your electrons will be measured at the slits themselves, and you'll get the results you'd expect for classical particles alone.

But if you lower the energy-per-photon, you'll discover that when you drop below a certain energy threshold, you don't interact with every electron. Some electrons will pass through the slits without registering which slit they went through, and you'll start to get the interference pattern back as you lower your energy.

Same thing with intensity: as you lower it, the "two pile" pattern will slowly disappear, replaced with the interference pattern, while if you dial up the intensity, all traces of interference disappear.

And then, you get the brilliant idea to use photons to measure which slit each electron goes through, but to destroy that information before looking at the screen.

A quantum eraser experiment setup, where two entangled particles are separated and measured. No ... [+] alterations of one particle at its destination affect the outcome of the other. You can combine principles like the quantum eraser with the double-slit experiment and see what happens if you keep or destroy, or look at or don't look at, the information you create by measuring what occurs at the slits themselves.

This last idea is known as a quantum eraser experiment , and it produces the fascinating result that if you destroy the information sufficiently, even after measuring which slit the particles went through, you'll see an interference pattern on the screen.

Somehow, nature knows whether we have the information that "marks" which slit a quantum particle passed through. If the particle is marked in some fashion, you will not get an interference pattern when you look at the screen; if the particle is not marked (or was measured and then unmarked by destroying its information), you will get an interference pattern.

We've even tried doing the experiment with quantum particles that have had their quantum state "squeezed" to be narrower than normal, and they not only exhibit this same quantum weirdness , but the interference pattern that comes out is also squeezed relative to the standard double slit pattern .

The results of unsqueezed (L, labeled CSS) versus squeezed (R, labeled squeezed CSS) quantum states. ... [+] note the differences in the density-of-states plots, and that this translates into a physically squeezed double slit interference pattern.

It is extremely tempting, in light of all of this information, to ask what thousands upon thousands of scientists and physics students have asked upon learning it: what does it all mean about the nature of reality?

Does it mean that nature is inherently non-deterministic?

Does it mean that what we keep or destroy today can affect the outcomes of events that should already be determined in the past?

That the observer plays a fundamental role in determining what is real?

A variety of quantum interpretations and their differing assignments of a variety of properties. ... [+] Despite their differences, there are no experiments known that can tell these various interpretations apart from one another, although certain interpretations, like those with local, real, deterministic hidden variables, can be ruled out.

The answer, disconcertingly, is that we cannot conclude whether nature is deterministic or not, local or non-local, or whether the wavefunction is real. What the double slit experiment reveals is as complete a description of reality as you're ever going to get. To know the results of any experiment we can perform is as far as physics can take us. The rest is just an interpretation.

If your interpretation of quantum physics can successfully explain what the experiments reveal to us, it is valid; all the ones that cannot are invalid. Everything else is aesthetics, and while people are free to argue over their favorite interpretation, none can lay any more claim to being "real" than any other. But the heart of quantum physics can be found in these experimental results. We impose our preferences on the Universe at our own peril. The only path to understanding is to listen to what the Universe tells us about itself.

Ethan Siegel

APS Physics

physics experiments based on light

Entering a New Era of Dark Energy Cosmology

Figure caption

In 1998, Saul Perlmutter was leading one of the two teams that discovered that the expansion of the Universe is accelerating. The finding stunned the scientific world, as most expectations were that the gravitational attraction of matter to other matter would slow down the expansion. Instead, a repulsive and enigmatic force—later called dark energy—appeared to be pushing the Universe apart.

Perlmutter, an astrophysicist at the Lawrence Berkeley National Lab and the University of California, Berkeley, eventually shared the physics Nobel Prize for the discovery. In response to the results, he turned to theorists in search of explanations. “Very quickly, we realized that to distinguish between competing theories, we would need to measure the expansion history of the Universe with at least 20 times the precision achievable at that time,” said Perlmutter at the Cosmic Controversies conference in Chicago in October.

Despite intense research efforts over the past two decades, cosmologists haven’t made much progress on understanding the nature of dark energy. The lack of progress may seem frustrating, “but it’s the absolute wrong time to feel depressed,” said Perlmutter, because the kinds of observations that he and others wanted 20 years ago are finally becoming possible. “The first experiments achieving the new level of precision are just about to turn on,” he said.

The first of the new generation of experiments to kick off will be the Dark Energy Spectroscopy Instrument (DESI), installed at the Mayall telescope on top of Kitt Peak in Arizona. DESI is currently being tested and is expected to start collecting scientific data next month. Other experiments, some ground-based and some on satellites, will begin in the next few years. The new observations will use several different, complementary techniques that cosmologists hope will provide new clues about the nature of dark energy.

Figure caption

DESI: Industrial-Scale Galaxy Observations

There are several ways to probe the Universe’s expansion. The most established method is the one famously used by Perlmutter and others in the 1990s. It relies on type Ia supernovae, thermonuclear explosions of white dwarfs that are visible even in galaxies ten billion light years away. Since the intrinsic brightness of this type of supernova is known, these events serve as “standard candles”: the brightness observed on Earth can be used to calculate their distances. Astronomers can also determine the speed with which a supernova is moving away from us by measuring its redshift, the change in wavelength of spectral features caused by the Doppler effect. The redshift provides the cosmic time at which we are seeing the explosion, since objects that are receding faster are farther away in both space and time than ones receding more slowly. By measuring the distances of many supernovae as a function of their redshift, researchers can put together the expansion history of the Universe.

DESI will use a different method, based on measuring the large-scale distribution of galaxies. This distribution retains the signatures of a phenomenon known as baryon acoustic oscillations (BAOs). BAOs were produced by acoustic waves in the hot, primordial plasma making up the early Universe. These waves froze into place about 380,000 years after the big bang, when the plasma’s electrons and protons became bound into neutral atoms. This freezing ultimately resulted in subtle, periodic oscillations in the density of galaxies—pick a galaxy, and you are slightly more likely to find other galaxies at a specific distance away from it, a distance called the acoustic scale.

Since the acoustic scale at the freeze-out time can be calculated from first principles, “It’s like having a reference yardstick imprinted in the Universe,” says DESI director Michael Levi. The acoustic scale increased as the Universe expanded, so that it is now almost half a billion light years. Researchers can follow how this yardstick changed with time by studying galaxies at a range of redshifts.

Figure caption

Levi says that using BAOs to determine the Universe’s expansion history is less prone to systematic errors than interpreting supernova measurements, which requires detailed modeling of stellar explosions. Measuring the acoustic scale, however, is anything but straightforward. To measure it precisely, one must observe large numbers of galaxies across billions of light years. Previous BAO surveys mapped about 2.5 million galaxies. DESI will surpass the precision of those surveys by mapping over 35 million galaxies and 2.4 million quasars—extremely bright and distant galactic nuclei. From such maps, DESI will reconstruct the last 11 billion years of the Universe’s expansion. “It’s like taking a 3D MRI scan of the Universe,” says Levi.

DESI will be able to measure such a large number of objects thanks to robotic technology. On the focal plane of the telescope, 5000 optical fibers will each gather light from one galaxy and send the light to a spectrometer, which will provide the galaxy’s redshift. Every 20 minutes, the 5000 robotic positioners will realign the fiber ends, pointing them at a new set of galaxies and cycling through over 100,000 galaxies every night. To prepare the ground for DESI, three previous surveys have observed one third of the sky, collecting images of over one billion galaxies, from which the most promising targets for DESI were selected. These data are collected in a publicly available online Sky Viewer .

Figure caption

DESI opened its eyes to the night sky in October 2019, and its scientists and engineers have been testing and calibrating the instrument. “We are still wrapping our heads around the complexity of this machine,” says Levi. “It’s like using robots to control the most complex Alvin Ailey choreography tens of times every night.”

More Ways to Measure Dark Energy

Beyond supernovae and BAOs, there are two other important techniques for studying dark energy. One is counting galaxy clusters—the clusters’ abundance varies with time depending on the cosmic mix of matter and dark energy. The other is weak lensing measurements, which map out the distribution of dark matter based on its subtle warping of spacetime and the resulting distortions of observed galaxies’ shapes and positions. Cluster counts and weak lensing—both measured as a function of redshift—will reveal how the accelerated expansion has changed the growth of structures of dark and visible matter.

“People used to fight over which of the four techniques is the best one,” says cosmologist Andreas Albrecht of the University of California, Davis. “Today we believe that the combination of different techniques is much more powerful than each individual technique.” Mark Kamionkowski, a theoretical physicist at Johns Hopkins University in Baltimore, agrees and refers to a famous South Asian parable. “If we focused on one observable only, we’d be like one of those six blind men trying to understand what an elephant is by touching just one of its parts.”

Following this philosophy, upcoming facilities include a variety of experiments that will exploit the full spectrum of dark energy observables. The European Space Agency will launch Euclid in 2022, and NASA plans to launch WFIRST in 2025. Between these two satellites, all four techniques will be carried out with high resolution from space. On the ground, the next big thing is the Large-aperture Synoptic Survey Telescope, a 6.5-m optical telescope expected to be operational in 2022. Although it won’t have DESI’s spectroscopic abilities, it will be able to see many more faint objects than any existing surveys, peering deeper into the Universe’s past.

Better Interactions with Data and between Scientists

Perlmutter’s optimism is not only motivated by the new measurement technology but also by dramatic improvements in the way researchers handle data. He cited key progress in several areas: Researchers have gotten better at understanding how random and systematic errors affect experiments; machine-learning tools have been increasingly applied to the analysis of big data; and advances in so-called blinding techniques have allowed researchers to better prevent bias from affecting their data processing.

There have also been important “sociological changes”—collaborations are increasingly adopting open-science philosophies, sharing all of their data publicly in real time. Astrophysicist Wendy Freedman of the University of Chicago said that many young researchers are now entering the dark energy arena, coming from other areas, like particle physics, and bringing new techniques. “These are times when we get a fertile cross-pollination of ideas from different fields,” she said.

Albrecht marveled at the changes over the last few decades. “When I entered the field, there were many big questions and no way to answer them. The transformation of cosmology into a precision science has been truly exhilarating,” he said. While Perlmutter admitted that there is no guarantee that the new level of precision will be sufficient to see something new, “a breakthrough feels like a natural thing to occur soon,” he said.

–Matteo Rini

Matteo Rini is the Deputy Editor of Physics .

Subject Areas

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