The photoelectric effect

- [Instructor] The photoelectric effect is another one of these cool things in physics that sounds like it should be sci-fi, but actually describes an everyday phenomenon around us. This and related effects are used for all sorts of things, like solar panels and cameras. And the name itself is the biggest hint into what's going on. Photo, photon, electric, electron. So what is the photoelectric effect? It's the emission of electrons from a metal that has absorbed electromagnetic radiation like light over a certain frequency. A particle of light is called a photon, and when a photon has enough energy, it can actually knock an electron free. What happens is that the photon collides with the metal surface, hitting an electron. When these particles collide, some of the energy of the photon is used to dislodge the electron. That electron is then shot out of the metal, or emitted. The rest of the photon's energy is then transferred to the emitted electron. We know that energy transfers between objects and is conserved in a system, so maybe this photoelectric effect sounds simple, but that's actually why it's such a big deal, so big that it's what Albert Einstein received his Nobel Prize for. Why was this such a big deal? By understanding the photoelectric effect, we also learned a lot about the fundamental properties of light. Today, we know that light can behave as both a particle and a wave, but that wasn't always known. When this effect was discovered, it was clear that light had wave-like properties, but the photo electric effect was a big piece of evidence that light could also behave as a particle. It also showed that the energy of those particles was related to the frequency of the light. So how did scientists figure all this out? Well, let's start by thinking about how to make a metal emit an electron. This sounds an awful lot like work, and we need energy to perform work. It turns out that electrons have what is called a binding energy. Binding energy, which I'm going to refer to as Eb, refers to the amount of energy needed to pull an electron away from the atom that it's orbiting, or that it's bound to, and hence the name binding energy. Electrons are happy to remain where they are, unless something gives them enough energy to overcome this binding energy and bounce them out of place. So the energy hitting an electron has to be greater than its binding energy to cause emission. Scientists had observed the photoelectric effect, but they wanted to know how it worked, because, well, that's what scientists do. So over the years, various scientists designed experiments that tested the effects of different types of light on different materials. And let's look at what they found. They found that if a light was shining on a metal and the metal wasn't emitting electrons, changing the intensity of the light didn't change things, but they did notice that raising the frequency of the light did. So, for example, if you set up an experiment where you're shining a lamp on a piece of metal and it's not ejecting any electrons, shining two of those lamps on the metal won't change anything. But if you replace the bulb with one that emits a higher frequency of light, this might change things. And this was a big hint that light might be behaving as a particle. Why is that? If this was a normal wave, we would expect that by sending more energy at the metal, by increasing the intensity, electrons would gain enough energy to be emitted. But since that didn't happen, it was clear something else was going on. And that eventually led to the idea that light was composed of photons. And these photons need to individually transfer enough energy to an electron that it can bounce out of place. So by changing the frequency of a light, we can change the energy of a photon. Once this energy is higher than that of the binding energy we talked about earlier, it can knock an electron loose. Any extra energy then contributes to the energy of the emitted electron. This is a big deal and enables us to do some pretty cool things. So let's go back to one of the examples at the beginning, solar panels. They have these things called photo cells, which we're pretending I'm drawing right now, and these photo cells use a version of the photoelectric effect to generate electricity. When sunlight hits the cells, they emit electrons. The cells are designed so that these electrons moving around generate a voltage, and voltage can power devices. Now we can see why the photoelectric effect is such a big deal. Not only did it revolutionize our understanding of physics, but it and similar effects are used in tons of technology.