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Einstein's Legacy: The Photoelectric Effect

Despite the popularity of Einstein's theories of relativity and his musings on black holes, Einstein's Nobel Prize in physics was actually awarded for his discovery of the photoelectric effect. This discovery revolutionized our understanding of the world around us. But what is the photoelectric effect?

By
Sabrina Stierwalt, PhD,
Episode #158

When you think of Albert Einstein, what do you think of? General relativity? Black holes? Crazy hair? While he certainly made significant contributions to all of those topics during his lifetime, Albert Einstein was perhaps even more well known in his time for his work to understand the photoelectric effect. In fact, when he was awarded the Nobel Prize in Physics in 1921, the honor was stated to be “for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect."

This discovery is so important—and Nobel Prize worthy—because Einstein suggested for the first time that light is both a wave and a particle. This phenomenon, known as the wave-particle duality of light, is fundamental to all of quantum mechanics and has influenced the development of electron microscopes and solar cells.

What Is the Photoelectric Effect?

When light with energy above a certain threshold hits a metal surface, an electron that was previously bound to the metal is knocked loose. Each particle of light, called a photon, collides with an electron and uses some of its energy to dislodge it from the metal. The rest of the photon’s energy is transferred to the now free-roaming negative charge, called a photoelectron.

So why does this happen? What determines the energies (and speeds) of the emitted electrons? To understand the answers to these questions, we need to dig a little into the history of the discovery of the photoelectric effect.

A Mysterious Result

In the late-1800s, experimental physicists had a huge task in front of them. In 1865, the mathematical physicist James Clerk Maxwell published his theory of electromagnetism in which he claimed electricity and magnetism both moved through space as waves traveling at the speed of light. Experimentalists then set out to find observational evidence of Maxwell’s theories which so nicely explained the properties of light—at least mathematically.

Success came in 1887, when Heinrich Hertz was the first to generate and detect the electromagnetic radiation predicted by Maxwell. Hertz created a spark between two pieces of brass using a high voltage induction coil, and then was able to detect the resultant radiation from that spark when its oscillations created a second spark between a copper wire and a brass sphere (his “receiver”) up to 50 feet away.

This second spark was very faint, however, so to get a better view of it, Hertz tried enclosing his receiver in a dark box. Unexpectedly, he found the enclosure diminished the receiver’s spark, but only when the box was made of certain materials. After what was probably months of investigation, Hertz concluded that the best way to increase the receiver’s spark was to use ultraviolet light (i.e. light at a higher frequency than optical light).

However, at the end of his investigation, Hertz still did not know why such an effect occurred. He stated, “I confine myself at present to communicating the results obtained, without attempting any theory respecting the manner in which the observed phenomena are brought about."

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