1938

Why Light Is a Wave…Sometimes: A Mini-History of the Photoelectric Effect

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When a child cannonballs into a pool, a simple series of waves radiate outward from the point where they plunged in. But if two children jump in at once, they create a more intricate waterscape. Where the two sets of waves meet, they interfere with each other. In some spots, the peak of one wave meets the valley of another, and they cancel each other out. The waves in that location disappear.

Look at the nearest lightbulb. Light waves are zipping toward your eye in simple, parallel paths. Now, hold your two fingers directly in front of your eye. Leave only enough space between them to let through a tiny crack of light. Do you see them? Dark lines appear in the gap.

As the light waves pass the edges of your fingers, they are bent out of parallel. And like the waves from two cannonballing children, the light waves interact. In those spots where peak meets valley, the light waves cancel out each other and our eye sees a sliver of shadow. This proves that light is composed of waves. Right?

Enter Heinrich Hertz. In the 1880s, he was investigating electromagnetic waves, of which light is an example, using a device called a spark generator. By accident, he observed that the two brass nodes of his device spontaneously generated sparks when the apparatus was exposed to certain wavelengths of light (specifically, ultraviolet). Hertz knew that sparks are made when electrons—the tiny negative particles that make up electricity—jumped the gap from one node to another. So he reasoned that the ultraviolet light was providing electrons with the energy to move—a phenomenon known as the “photoelectric effect.”

To Hertz, the photoelectric effect was little more than a curiosity, but to his student, Philipp Lenard, it was an intriguing puzzle. Lenard knew that all wavelengths of light have energy. So why would only certain wavelengths of light cause these electrons to move? And how much energy did those electrons have when they made the jump?

To investigate, Lenard built a similar apparatus and shone light on one node but negatively charged the other, so it would push electrons away. Thus, electrons required a certain kick of energy to overcome that push and make it to the other side. In other words, he could estimate how much energy the electrons received from different light sources.

Like with Hertz, ultraviolet light energized the electrons enough to make their way across the gap. But red light did not. Even when Lenard turned his red lamp up to its most intense setting, not a single electron made it. Yet when he put his ultraviolet light source on its lowest setting, he was able to generate a small but definitive trickle of electrons.

That was deeply unsettling for Lenard. If electrons absorbed the energy from light as continuous electromagnetic waves, then the low energy waves should have built up, eventually giving an electron enough energy to move. But that didn’t happen.

This was not a problem with Lenard’s experiment but with the wave theory of light, as a young Albert Einstein realized. In his 1905 paper for which he won the Nobel Prize, he proposed that energy in light is not continuously distributed along a wave, but instead “consists of a finite number of energy quanta which are localized at points in space, which move without dividing, and which can only be produced and absorbed as complete units.”

Put another way, to knock an electron into an electrical current requires a single powerful jolt of energy all at once, like shooting a pinball out onto the board of a pinball machine. If you don’t pull the shooter back far enough, the ball won’t have enough energy to make it up the track.

And it doesn’t matter how many times you hit the ball—either the shooter gives the ball enough energy or it doesn’t. Since even intense red light (the metaphorical equivalent of many ineffective tugs on the shooter) could not move electrons, Einstein figured that these inadequate wavelengths were unable to add up cumulatively into something more powerful. Instead they were coming in distinct packets, each lacking the energy needed to knock the electron all the way off the metal atoms. The packets became known as photons, particles of light energy.

So, while the slits between our fingers suggest that light is a wave, the experiments of Lenard and company suggest that it comes in discrete particles. And there we remain today. These pioneers were like the blind wise men arguing about the nature of the elephant. “It’s like a rope,” says the one feeling the tail. “No, it’s like a spear” says the one feeling the tusk. To describe light as either a wave or a particle is insufficient. Both are correct.

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