|MadSci Network: Physics|
That's a very good question! The speed of light itself doesn't change as it goes through different media; instead, the decrease in the "travel time" is a reflection of the interactions of light with matter. As I was preparing my answer, I did a little search at my two favorite sites and found answers there to your question that were much more coherent than mine was shaping up to be. So, I hope you don't mind if I defer to their answers.
The first interpretation is from MadSci Admin extraordinaire John Link and was originally presented here; the abridged version follows:
Another of your statements is "Light will slow down when entering an optically dense medium." Actually this is not quite true. While it is true that we measure a lower AVERAGE speed of the light through a medium, the propagation of light through the medium, between atoms, is actually at the normal vacuum speed of light, c. What happens is that the light moves at speed c between atoms, but photons are "absorbed" by the medium's atoms. By "absorption" I mean that the energy of the photon causes an electron of the atom to be kicked to a higher energy level, and the photon ceases to exist. Then, after a very small time delay, the electron goes back to its original (usually ground state) energy and "emits" a photon of the same energy (and thus same frequency and thus same wavelength) as the original "absorbed" photon. (In fluorescent materials the energy of the photon is downshifted, but I am talking here of "elastic", or non-energy shifting, absorptions.) It is this very small time delay which makes us measure the average "speed of light" through the medium as slower than the vacuum speed of light. But, again, between atoms the light does travel at the speed c.The second interpretation is from Prof. Louis Bloomfield's incredible site, How Things Work, where I ran a search for keyword: transparent. Here is an abridged (albeit very detailed) response from there:
When a light wave enters matter, the light wave's electric field causes charged particles in the matter to accelerate back and forth. That's because an electric field exerts forces on charged particles. The light wave gives up some of its energy to these charged particles and is partially absorbed in the process. However, the charged particles don't retain the light's energy very long. They are accelerating and accelerating charged particles emit electromagnetic waves. In fact, they reemit the very same light wave that they absorbed moments earlier. Overall, the light wave is partially absorbed and then reemitted by each electrically charged particle it encounters, so that the light continues on its way as though nothing had happened. However, something has happened--the light wave has been delayed ever so slightly. This absorption and reemission process holds the light wave back so that it travels at less than its full speed. If the charged particles in the matter are few and far between, this slowing effect is almost insignificant. But in dense materials such as glass or diamond, the light wave can be slowed substantially. Actually, higher frequency violet light is slowed more than lower frequency red light because violet light is more effectively absorbed and reemitted by the atoms in most transparent materials. That's because when a high frequency light wave encounters the electrons in an atom, the jiggling motion is so rapid and the electrons' motions are so small that the electrons never reach the boundaries of the atom. As a result, those electrons are able to jiggle back and forth as though they were free electrons and they do a good job of slowing the light wave down. But when a low frequency light wave encounters the electrons in an atom, the jiggling motion is slower and the electrons' motions are so large that they quickly reach the boundaries of the atom. As a result, those electrons aren't able to jiggle back and forth as far as they should and they don't slow the light wave down as well. When a light wave passes through matter, the charged particles in that matter do respond--the light wave contains an electric field that pushes on electrically charged particles. But how a particular charged particle responds to the light wave depends on the frequency of the light wave and on the quantum states available to the charged particle. While the charged particle will begin to vibrate back and forth at the light wave's frequency and will begin to take energy from the light wave, the charged particle can only retain this energy permanently if doing so will promote it to another permanent quantum state. Since light energy comes in discrete quanta known as photons and the energy of a photon depends on the light's frequency, it's quite possible that the charged particle will be unable to absorb the light permanently. In that case, the charged particle will soon reemit the light. In effect, the charged particle "plays" with the photon of light, trying to see if it can absorb that photon. As it plays, the charged particle begins to shift into a new quantum state--a "virtual" state. This virtual state may or may not be permanently allowed. If it is, it's called a real state and the charged particle may remain in it indefinitely. In that case, the charged particle can truly absorb the photon and may never reemit it at all. But if the virtual state turns out not to be a permanently allowed quantum state, the charged particle can't remain in it long and must quickly return to its original state. In doing so, this charged particle reemits the photon it was playing with. The closer the photon is to one that it can absorb permanently, meaning the closer the virtual quantum state is to one of the real quantum states, the longer the charged particle can play with the photon before recognizing that it must give the photon up. A colored material is one in which the charged particles can permanently absorb certain photons of visible light. Because this material only absorbs certain photons of light, it separates the components of white light and gives that material a colored appearance. A transparent material is one in which the charged particles can't permanently absorb any photons of visible light. While these charged particles all try to absorb the visible light photons, they find that there are no permanent quantum states available to them when they do. Instead, they play with the photons briefly and then let them continue on their way. This playing process slows the light down. In general blue light slows down more than red light in a transparent material because blue light photons contain more energy than red light photons. The charged particles in the transparent material do have real permanent states available to them, but to reach those states, the charged particles would have to absorb high energy photons of ultraviolet light. While blue photons don't have as much energy as ultraviolet photons, they have more energy than red photons do. As a result, the charged particles in a transparent material can play with a blue photon longer than they can play with a red photon--the virtual state produced by a blue photon is closer to the real states than is the virtual state produced by a red photon. Because of this effect, the speed at which blue light passes through a transparent material is significantly less than the speed at which red light passes through that material. Finally, about quantum states: you can think of the real states of one of these charged particles the way you think about the possible pitches of a guitar string. While you can jiggle the guitar string back and forth at any frequency you like with your fingers, it will only vibrate naturally at certain specific frequencies. You can hear these frequencies by plucking the string. If you whistle at the string and choose one of these specific frequencies for your pitch, you can set the string vibrating. In effect, the string is absorbing the sound wave from your whistle. But if you whistle at some other frequency, the string will only play briefly with your sound wave and then send it on its way. The string playing with your sound waves is just like a charged particle in a transparent material playing with a light wave. The physics of these two situations is remarkably similar.
I hope that helps! I know the last response was probably a little more detail than you were looking for but I think it's better to give too much information than too little. But if you still have any questions, please feel free to drop me an email.
Try the links in the MadSci Library for more information on Physics.