Although I have not been thinking very much about actual physics for while now I suddenly got the urge to read a little bit about it and I decided to start by checking out the birth of quantum mechanics again. In the book quantum theory by Bohm in chapter 1 the ultra-violet catastrophe and its resolution are explained. It is also mentioned there that Planck originial idea was that the material oscillators in the wall of the cavity only emit energy at certain discrete levels. In the book as well as in my recollection of verious quantum mechanics courses I recall one usually also assumes that the light itsself can only receive energy in quanta. In the book Bohm reasons as follows. He says that, since the only light we can detect couples with matter and the material that the walls of the cavity are made of does not matter, it is equivalent to assume that the light only absorbs energy in quanta or to assume the material only emits energy in quanta. He then continues to demonstrate the quantization of matter with an example about the dependence of the specific heat of a crystal on temperature. I thought this reasoning is a little bit strange since it seems Occams razor would suggest only deciding that matter must emit quanta. It actually seems like Bohm's reasoning tells me no experiment could show that light always absorbs energy in specified quanta. In which case it confuses me that I seem to have always been taught this. So I guess my question is if someone understands my confusion and can help me, or more precisely. Is there some other experiment that shows that the allowed energies of specifically light (of some frequency) only take a discrete set of values?