 So in 1905, Einstein published a model of the photoelectric effect. He said let's assume that in each atom, whether it be in a metal or a gas, these electrons are stuck down the bottom of a potential well, and so they have to get a certain amount of energy just to get out. And we'll call that energy the work function. Now since this object is an atom, that work function is probably a very small amount of energy. And indeed it's probably something like 10 to the minus 19 joules or something around an electron volt, which is the charge of one electron times one volt. And it turns out that that energy is somewhat comparable to just one of these quanta of Planck's imaginary quanta, where he said that let's imagine that the energy for light of a particular frequency comes in units of this h times the frequency. Now Planck didn't take that hypothesis seriously. He used it in order to get the answer that he wanted, but he thought that was just a fiction. But what Einstein did was he said, perhaps that's real and perhaps the energy in light is actually coming in these quantized packets, these definite blocks of energy. We now call these blocks of energy photons. And the key point is if you have a photon, you have to absorb a whole photon at once, you either absorb a photon or you don't, then an electron trapped in a material has to get enough energy from that photon to get it out of the work function. So it doesn't get enough energy from that photon, then it just stays trapped. And that starts to explain why the frequency of the light is so important, because that frequency of the light, remember, is how much energy we have. And so this energy here is hf. And if we don't have enough, we're not going to get any emission at all. And then the extra energy from the photon, whatever that's going to be, is going to be the kinetic energy electron once it's kicked out. So if we make a plot of the kinetic energy of the emitted electrons versus the frequency of the incident light, what we'd expect to see for low frequencies where we didn't have enough energy to overcome the work function is that there would be no electrons coming out. So they'd have no kinetic energy, no electrons. And then as we just had enough energy to get over the work function, we'd have electrons coming out, but they'd have almost no kinetic energy at all. And then as we gave the photons more and more energy, we'd expect a linear relationship, we'd expect more and more energy coming from the photons going straight into electrons. We'd expect a plot something like that.