 Now in 1893, Wilhelm Wien applied the laws of thermodynamics to light itself, and he showed that if light is in thermal equilibrium with its surroundings, then its frequency spectrum has to shift proportionally with its temperature. In other words, if you have some objects, say an iron bar, and this iron bar is a very good absorber or emitter of light at all frequencies, it's what's called technically a black body, then if you heat that up, it goes hotter and hotter, and it's going to give out radiation, and that radiation is going to get hotter and hotter, and we know that if we heat iron enough, then it will turn red, and that's because if we take the power coming off from the light as a function of frequency, then it's going to have some peak, then for the low frequencies we're going to have the red end of the spectrum, and above that we're going to have the rest of the rainbow, then at a given temperature we're going to see that we're going to get mostly a peak around the red area of the spectrum, and therefore that thing's going to look red. Now as we make this hotter, Winslow says that if we increase the temperature by a certain amount, then we're going to take this frequency pattern and shift it up a little bit, we can see there that what we're going to get is lots of these colours coming through, and if we have lots of these colours, then we tend to get something that's white, and so that's when something gets hotter, it turns to white hot, and when it gets hotter again, we just shift out the frequency once more, and we can see that we're going to get mainly blue, so we're mainly going to get these blue colours, and very little of the rest, and so it's going to be blue hot, and so that's why when you heat up a piece of steel, say, first it goes red, then it goes white, then it goes blue, because we're just shifting up the peak in our distribution there. And then over the turn of the century two things happened in rapid succession. The first is that this calculation was done in a lot more detail by Rayleigh and Sheens, and they got the functional form for this power spectrum, where this is the frequency, this is Boltzmann's constant from thermodynamics, the T is the temperature, and C is the speed of light, and there are two important things to realise from this. The first is that it works very well for the low frequencies, so the Rayleigh-Sheens law got these early frequencies very accurately correct, and the second thing is that it's obviously wrong because it grows without bound. You can see that this frequency, as the frequency goes up, more and more power comes out without bound, and so there's this infinite amount of power coming out at high frequencies, and this is called the ultraviolet catastrophe. And it was called that simply because the Rayleigh-Sheens law was quite accurate for low frequencies like infrared or visible frequencies, but when spectacularly wrong in the ultraviolet regime and above. So despite the fact that the Rayleigh-Sheens law was properly treating thermodynamics and it was properly treating light as an electromagnetic wave, its steadfast refusal to act reasonably, never mind like experimentally observed fact, was extremely frustrating to another person working on the same problem, who was Max Planck. In 1900 he became so frustrated with the difficulty of making his model fit experimental observation that he started making some crazy assumptions. In particular he made one assumption that led him to get the right form for the black body law.