Now, all this means nothing if I tell it to you this way. It may begin to mean something if I give you some examples. One of these examples is the Raman Effect. Raman, a Hindu, discovered that when molecules scatter light, the frequency of light is essentially unchanged. But, in addition to that unchanged frequency, there are weaker frequencies in the scattering. In comes a frequency of light, the frequency is connected with the energy; the higher the frequency, the higher the energy. Most frequently, the light will come out with the same frequency. But sometimes, the light will come out with less energy and less frequency, enough frequency and enough energy just to excite one quantum of the vibration of the molecule. In quantum mechanics, that's very simple. What happens in classical theory? And the simplicity is really changed into something complex. When you do not talk about a simple diatomic molecule like nitrogen having two nuclei that vibrate against each other. Instead, take benzene: six carbons and six hydrogens. Take carbon tetrachloride: one carbon atom and two- and four chlorines around it. There are many vibrations that this molecule can execute. And some of them will appear in the Raman effect, some vibrations can retain a part of the energy that have come in, and some not. This is governed by what are called, obviously, selection rules. How do we explain these selection rules? In quantum mechanics it's straight forward but a little involved.