Now, the whole Raman effect can be explained and, indeed, has been explained, in classical physics. That has actually been done in the very early days by two excellent Frenchmen, Cabannes and Rocard, and I want to give you their explanation. Because, it turns out, and that was a work in which I participated, the ideas of Cabannes and Rocard are very careful to establish, understand, to see explicitly, why some vibrations are excited and others are not. What Cabannes and Rocard say is this: what is scattering? Light comes in, it's an electromagnetic vibration, primarily changing, oscillating electric fields. The atom or molecule that scatters that light is polarizable, that is, under the influence of light the electrons move to some extent and give a dipole moment to the atom or molecule that changes with the same frequency as the incoming light. This dipole moment then radiates and that is the normal scattering, not the Raman effect. What is the Raman Effect? Take a molecule, a nitrogen molecule. Two nitrogen atoms with electrons all around them. Now, assume that the nitrogen molecule vibrates. The nitrogen molecule is polarizable. The electrons move along with the incoming vibration of high frequency. But this polarizability is a little different if the nu- if the two nitrogen nuclei are close together and again different if they are a little farther apart. And if the molecule vibrates, then the induced dipole moment will vary with the frequency, the high frequency of the light that is coming in, but the amplitude of this frequency will change as the molecule vibrates relatively slowly. I can show you. Oscillation, low amplitude, high amplitude, low amplitude, high amplitude. In this kind of motion can in classical physics be shown very easily; that to an observer it would look like a sum of the high frequency and the low frequency re-describes the envelope. And that is the Raman effect. Completely classical. To my mind, a beautiful example discovered by Cabannes and Rocard of an application of the correspondence principle. You can explain a phenomenon by quantum states or you can explain it by purely classical processes.