Photons and Wavelength: Is Light a Particle or a Wave?

From the Lecture Series: Understanding the Misconceptions of Science

By Don Lincoln, Ph.D., Fermi National Accelerator Laboratory (Fermilab)

Scientists were studying light for a long time, trying to understand its behavior and essence. After an experiment, it got a bit more confusing to figure out if light is a particle or a wave because it acted like both! It took a little while for them to find photons and untangle many of the mysteries.

Decorative antique edison style light bulbs against brick wall background.
Photons are the particles of light, but light on its own shows wave-like behavior, which explains why it is both a wave and a particle. (Image: Ezume Images/Shutterstock)

Light was a mystery for many scientists, including Isaac Newton. He believed that light was a particle, while the Dutch physicist Christiaan Huygens thought it was a wave. When in 1801, long before photons were discovered, British polymath and physicist Thomas Young concluded that light was a wave after his experiment, scientists thought the mystery was finally solved. They were wrong.

Electrons and Light

In the late 1800s, a new complex question arose on light: does it have electrons? Light carries energy, and that is how it can cause, for example, sunburn. But how does it affect electrons in atoms of matters?

Laser reflects on optic table in quantum laboratory.
Young’s experiment proved light’s wave-like behavior, and when it was repeated with electrons, they also showed the waves’ interference effect. (Image: Mike_shots/Shutterstock)

In an atom, electrons are held tightly to the atomic nucleus. However, they can be blown away from the nucleus with high-enough energy. As light has energy, if a bright light is shone on a material, it must be able to take away some electrons from the atoms.

This is called the photoelectric effect. It took scientists two decades to explain it after learning about the effect. Of course, many experiments tried to answer questions, but there remained two mysteries.

This is a transcript from the video series Understanding the Misconceptions of Science. Watch it now, on The Great Courses Plus.

Colors of Light

Scientists eventually found out that the color of light mattered. It was not the strength of light that led to electron emission, but the color. Back then, scientists already knew that each color has a specific wavelength.

Using very bright red light could not lead to electron emission, while even weak purple light could. Red, orange, and yellow lights have long wavelengths, and the blue, indigo, and violet side have short wavelengths. Thus, a short wavelength could emit electrons, but the long wavelength could not. Light was acting like waves, but something was not right.

Light as Particles: Photons

In 1905, Albert Einstein resolved the situation. He said that light came in discrete lumps, or in particle form. The energy of each lump was proportional to the frequency of light, while the frequency was inversely proportional to wavelength. This explained why blue, indigo, violet light had enough energy to emit electrons.

These particles of light are called photons. Photons with higher energy can take away electrons from atom nuclei. The idea was so powerful that it won Einstein a Noble Prize in physics. So far, light seems to be both wave and particle, which sounds impossible.

Learn more about how relativity is misunderstood.

Waves of Photons

Scientists repeated Thomas Young’s experiment with shining light through two narrow slits on a far-away screen behind the wall with the slits. They used different environments and still concluded that light acted like a wave. But there was Einstein’s logical explanation of photons, too.

It took one century until they slightly changed the experiment. They turned down the intensity of the light so low that only one photon was emitted at a time, and they shot the light at a pair of slits and looked for the photon on the distant screen.

An illustration of a fiber optic.
When photons are shot in high numbers, light shows the wave-like behavior. However, photons themselves are particles and behave accordingly. (Image: asharkyu/Shutterstock)

The expectation was that light should appear as a dot on the screen if it is a particle. If it was a wave, it should appear in the wave pattern, but very faint.

What happened in the experiment was that light first appeared at a single spot, just like a particle should. They repeated the experiment, eventually increasing the number of photons. Up to a point, light was still acting like a particle. However, after millions and billions of photons, what they found was the wave pattern of the double-slit experiment!

Apparently, light acts like a particle at times and like a wave at other times. Researchers conducted the same experiments with electrons as well.

Learn more about the truth about radiation.

Electrons: Wave or Particle?

Electrons also show the two-sided wave/particle behavior. However, unlike photons, electrons can be detected without being destroyed. Researchers tried to detect which slit the electrons went through, and they realized that the locations where the electrons were detected changed.

Electrons were acting like particles, but also like waves on different occasions. If it seems too strange to be true, it means that you have understood the article so far. Niels Bohr once said, “Anyone who is not shocked by quantum theory has not understood a single word.”

Learn more about what’s inside atoms.

Electrons and Photons

Electrons emit photons when they receive enough energy. For example, when metal is heated, it glows red, then yellow, and then white. The point is, any matter emits only some specific colors of light, not all of them. Experiments with sodium and hydrogen gases show that, too.

That, however, is not the only relationship between photons and electrons. Using electrons in Young’s light experiment also showed the interference effects that belong to waves. They are both wave and particle at the same time, and among the most complex things discovered at their time!

Common Questions about Photons

Q: Are photons matter?

A photon particle has no mass, and it is not considered as matter. However, it can carry and pass on energy like matter, and at the same time, acts like a wave.

Q: Are photons electrons?

No. Electrons have a negative charge, which makes them move away from other electrons and be drawn to protons because of their positive charge. However, photons are units of energy with no electric charge.

Q: Is light a wave?

Light is both a wave and particle in nature. Einstein believed that light is a particle, which is called a photon, and the flow of photons is a wave. Thus, it acts like both wave and particle.

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