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How does it work?
Have you ever been in one of those restrooms where the faucets come on automatically when you wave your hands underneath them? Or walked through an electric door that opened just as you approached? Maybe your home is fitted with invisible “magic-eye” beams that “trip up” intruders by sounding an alarm? Or perhaps you’ve got a calculator that makes power with a little built-in solar panel? All these things are examples of photoelectric cells(sometimes called photocells)—electronic devices that generate electricity when light falls on them. What are they and how do they work? Let’s take a closer look!
What is photoelectricity?
“Photo” means light, so photoelectricity simply means electricity produced by a light beam. That idea doesn’t seem at all unusual in the 21st century, when most people have heard of solar panels (lumps of material, such as silicon, that generate an electric current when sunlight shines on them). But imagine how amazing the photoelectric effect must have seemed a little over a century ago, in 1887, when it was first discovered by German physicist Heinrich Hertz (1857–1894), one of the pioneers of radio. It remained something of a mystery for almost 20 years until Albert Einstein weighed in with an almost complete explanation of the phenomenon in 1905.
Yet, when photoelectricity was first explained, in 1905, it marked the beginning of a scientific revolution. The person who did the explaining, Albert Einstein (1879–1955), showed that a light beam, shining on something like a piece of metal, could be thought of as a train of energetic particles called photons. The photons passed their energy in fixed quantities to atoms inside the metal, knocking some of their electrons out of them, so producing an electric current.
As Einstein showed mathematically, the energy of the incoming photons was precisely related to the frequency or wavelength of the light shining and equal to the energy of the electrons they ejected. Einstein’s explanation of the photoelectric effect was powerful evidence that energy could exist only in fixed amounts called quanta. (In other words, you can get energy in family-sized packs but you can’t split the packs up any smaller!) This became the central element of quantum theory: a complex, mathematical explanation of the mysterious world of atoms and the particles lurking inside them. And it was for this work on photoelectricity that Einstein won the Nobel Prize in Physics 1921.
Small solar panels on such things as calculators and digital watches are sometimes referred to as photovoltaic cells. They’re a bit like diodes, made from two layers of semiconductor material placed on top of one another. The top layer is electron rich, the bottom layer, electron poor. When you shine light on the top layer, electrons leap up from the bottom layer to the top, making a voltage that can drive current through an external circuit—so providing what we think of as solar power.