The Feynman Lectures on Physics, Vol I, 31–1 The index of refraction: http://www.feynmanlectures.caltech.edu/I_31.html http://www.feynmanlectures.caltech.edu/I

The Feynman Lectures on Physics, Vol I, 31–1 The index of refraction: http://www.feynmanlectures.caltech.edu/I_31.html http://www.feynmanlectures.caltech.edu/I_31.html "We have said before that light goes slower in water than in air, and slower, slightly, in air than in vacuum. This effect is described by the index of refraction $n$. Now we would like to understand how such a slower velocity could come about. In particular, we should try to see what the relation is to some physical assumptions, or statements, we made earlier, which were the following: That the total electric field in any physical circumstance can always be represented by the sum of the fields from all the charges in the universe. That the field from a single charge is given by its acceleration evaluated with a retardation at the speed $c$, always (for the radiation field). But, for a piece of glass, you might think: “Oh, no, you should modify all this. You should say it is retarded at the speed $c/n$.” That, however, is not right, and we have to understand why it is not."