 Another way of saying it is that we'd expect energy to be conserved, so the total energy the photon is going to be split up into the kinetic energy of the electron afterwards and the energy we had to give it to get out of the material in the first place. So Leonard's initial exploration that showed that the energy the emitted charges was proportional to the frequency of the light isn't quite what Einstein's theory predicts. Einstein's theory predicts this extra kink in the graph, whereas just a linear relationship would look like that. But perhaps the experiments hadn't been done sensitively enough in those earlier tests by Leonard. Einstein's theory however does explain why the intensity light gives us a current, because the intensity light just tells us how many photons we have. It doesn't tell us how much energy each of those photons have. And therefore as we increase the intensity light we're just getting more of these photons hitting our surface and so we're going to get more electrons coming off. And so the intensity affects the current, how many electrons per second are coming off, and it's the frequency that affects the energy. And in 1914 Robert Millican did a series of very careful experiments where he showed that Einstein's prediction was exactly correct, and he could calculate the work function of various metals. And that was basically the birth of quantum theory. Previously Planck had discovered that if he assumed the energy came in packets that he could explain radiation, but it was the point where Einstein made a very serious attempt to describe the photoelectric effect saying that that was exactly how the radiation works. That these waves, electromagnetic waves were genuinely coming in these packets that seemed a lot like particles called photons. That's when quantum mechanics really started.