Scientific American presents Everyday Einstein by Quick & Dirty TipsScientific American and Quick & Dirty Tips are both Macmillan companies.

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.

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