We had quite a long discussion about the variation in the speed of light in different media in another thread.

The following is an extract from the notes I distilled from that discussion. I would be interested to know if others feel that I "got the right end of the stick".

Manifestly, light slows down when passing through any medium denser than a vacuum, but how does it do that? One explanation goes something like this: Consider a single photon passing through a block of glass; for simplicity, have the glass surrounded by a vacuum. It approaches the glass at “c”. Once it enters the glass, it is absorbed by an atom, then re-emitted. This process is repeated as it continues to pass through the glass. In fact it is not a single photon that travels through the glass; it is a succession of new photons, created at each new emission. Any travelling done by the photon within the glass is at “c”; the apparent slowing results from a succession of minute instants during which the photon does not exist. We have, therefore, to regard it as travelling at “c” whatever medium it is travelling through.

Would that explanations were that simple, but of course they are not. It turns out that if this were what actually happened, then the absorption spectrum would be discrete because atoms have only discrete energy states. Yet, in glass for example, we see almost the whole visible spectrum being transmitted with no discrete disruption in the measured speed. In fact, the refractive index (which reflects the speed of light through that medium) varies continuously, rather than abruptly.

The reason for this is that a solid is composed of a network of ions and electrons fixed in a "lattice". Because of this, they have what is known as "collective vibrational modes", sometimes called phonons. These are quanta of lattice vibrations, and it is these vibrational modes that can absorb a photon. So when a photon enters a solid, and it can interact with these phonons, it can be absorbed by the solid and then converted to heat. The solid then becomes opaque to this particular photon (i.e. at that frequency). Unlike the atomic orbitals which are discrete, the phonon spectrum can be broad and continuous over a large frequency range.

If a photon has an energy beyond the phonon spectrum, the solid cannot sustain this vibration, because the phonon mode is not available. So the lattice does not absorb this photon and it is re-emitted but with a very slight delay. This appears to be the origin of the apparent slowdown of the light speed in that particular medium. The emitted photon may encounter other lattice ions as it makes its way through the material and the resulting reactions accumulate to cause the delay.

There never was nothing.