Wondrium Science Series Answers: Is Light a Wave or a Particle?

nature of light puzzles scientists for centuries

By Jonny Lupsha, Wondrium Staff Writer

Since the time of Newton, scientists have studied the nature of light. One recurring question is whether light is a wave or a particle. Even Sir Isaac himself got some things wrong.

Portrait of Isaac Newton (1642-1727)
Isaac Newton (1642-1727), an English physicist and mathematician, was a leading figure in the Scientific Revolution of the 17th century. Painting by Godfrey Kneller / Wikimedia Commons / Public Domain

Is light a wave, a particle, neither, or both? Based on our understanding of the electromagnetic spectrum, the answer may seem obvious. However, the question itself is a bit deceptive. Sir Isaac Newton believed that light is a particle, while many of his contemporaries claimed that light is a wave. More than 200 years later, Einstein postulated that light is a wave that retains some qualities of a particle.

So, who was right? In his video series The Theory of Everything: The Quest to Explain All Reality, Dr. Don Lincoln, Senior Scientist at Fermi National Accelerator Laboratory, dives into this question’s history and its complicated answer.

Who Developed the Particle Theory of Light?

One of the chief proponents of the theory that light was a particle was Isaac Newton. The idea is that as a particle, light reflects off surfaces like bouncing a tennis ball on the ground. If we assume light travels faster through solid objects than through air, that can also explain refraction.

“We now know that this assumption is exactly backward—light travels more slowly through solid matter than it does air, but that wasn’t known at the time,” Dr. Lincoln said. “It was Newton’s extraordinary scientific reputation that led to the particle theory of light to be favored for a very long time.”

However, light is not exactly a particle.

Who Developed the Wave Theory of Light?

Two of Newton’s contemporaries, English scientist Robert Hooke and Dutch mathematician Christiaan Huygens, developed the first theory based on the idea that light is a wave, which also explained reflection and refraction. In 1800, English polymath Thomas Young demonstrated that light is in fact a wave. Keep in mind that waves can interfere with one another, and particles cannot.

“Young’s experiment used an opaque barrier with two adjacent slits,” Dr. Lincoln said. “When light illuminated a single slit, it passes through the slit and spreads out. That’s true of the other slit, as well.”

According to Dr. Lincoln, if light is a wave, you could expect to see ripples like those on the surface of a pond. If two light sources shone through the two slits, there would be two sources of waves and the ripples would interfere with one another.

“There would be spots that are peaks of one set of waves and troughs of another set of waves, which would cancel out,” he said. “Cancelling out light means there would be dark spots on a distant screen. Similarly, there would be places where the peaks of both sets of waves would appear at the same time and same location, resulting in bright spots on the distant screen.”

Surely enough, that’s exactly what is seen when Young’s double slit experiment is performed.

However, light is not exactly a wave, either.

The real answer is somewhere in the middle. In the intervening years, science has discovered that photons, which are the elementary particles of electromagnetic radiation that light is made of, are governed by wave physics but detected as particles. They, like electrons, act like both waves and particles.

So is light a wave, a particle, neither, or both? Yes!

The Theory of Everything: The Quest to Explain All Reality is now available to stream on Wondrium.

Edited by Angela Shoemaker, Wondrium Daily