Photoelectric Effect

Einstein won the Nobel Prize in Physics in 1921. Lots of people assume that he won it either for his work in relativity or for the immensely influential equation E=mc². However, it was for his groundbreaking discoveries in a physical phenomenon known as the photoelectric effect for which he was awarded the Prize. Herein we will discuss the phenomenon and its subsequent applications and implications.

Explanation:

Simply, the photoelectric effect is the emission of electrons from a metal as a result of incident light. In other words, sometimes, when you shine light on a piece of metal, some of the electrons in the metal come unbound and fly freely though space. I guess we need to back up and talk a little about metals.

One of the properties of metals is the configuration of its electrons. When a whole lot of iron atoms (to use one of many metals in the periodic table) get together, they start to share their electrons. However, unlike other solids, metals share their electrons with the entire solid. The top layer of electrons are free to flow anywhere about the surface of the metal, bound to no specific atom. Incidentally, this property is what makes metals such good conductors of electricity; the "fluid" electrons on the surface carry and transport charge very efficiently in much the same way as it is easier to slide over a wet surface than a dry one.

This property of metals is what makes the photoelectric effect possible. When you shine light on metal, the sea of electrons (as it is often called) receives lots of energy, causing some of the electrons to shoot off. However, not just any kind of light can make it happen. Imagine a swimming pool that is only filled up half way with water. If you were to throw a rock into the pool, you could make some of the water splash out, but only if the rock was traveling fast enough. Even a whole bunch of rocks traveling too slow would only make lots of splashes that didn't remove any of the water. In the same way, light needs to be energetic enough to cause the electrons to escape from the sea. The minimum energy that is required for a photon to remove an electron from a metal is called the work function (symbolized by the Greek letter φ).

Implications:

The implications of this discovery were shattering to the world of physics. There was a huge debate at the time concerning the nature of light--whether it was a particle or a wave. Einstein's discovery helped us to understand the truth. As I mentioned before, only light with a certain minimum energy (equal to φ) could make electrons leave the metal. Einstein discovered that this minimum energy could only be achieved by changing the color of light, not the intensity. That means that red light, no matter how bright, will never induce the photoelectric effect, whereas very very weak ultraviolet light will always do so. We learned some great truths through this. First, the frequency of light (its color) is directly related to its energy. In fact, frequency is the only factor that determines photon energy. Intensity (brightness) of light corresponds not to energy, but to the number of photons hitting the area per unit time.In other words, shining really bright, red light on the metal was like throwing lots and lots of rocks really slowly into the pool. But shining really weak UV light was like throwing just a few rocks really really fast, causing a large splash (but only a few times). To induce a large photoelectric current, one needs only to produce an intense UV source.

Applications:

The applications of the photoelectric effect are many and influential. This is the basic idea that makes solar energy possible (taking light and making electrical energy out of it). Also, from this idea came photomultipliers (which created such devices as night-vision goggles) and CCDs (which are the imaging devices in digital cameras and telescopes), to name just a few.