That is it really. Why is light slower when not in a vacuum?
If I understand it right–and that’s a big if–it’s due to internal refraction. Basically, the light can’t travel in a straight path inside a non-vacuum cause it hits particles and bounces off of them.
Electromagnetic propagation depends on the magnetic permeability and electric permittivity of the medium it is propagating through. Actually it depends on the inverse square root of the product of the two values. For mediums other than a vacumm the values are larger. So when X[sub]1[/sub] is larger than X[sub]0[/sub] the resulting value of 1/X[sub]1[/sub] will be smaller then 1/X[sub]0[/sub].
As light passes through something (not a vacuum) it is likely to strike atoms along the way. The light will either bounce off or be absorbed and then re-emitted. This absorb/emit process takes time. Kinda like passing a ball from person to person. It is faster if you can pass the ball all the way to the destination (generally) rather than have people in between catch the ball then turn around and throw the ball.
One way of thinking of it is that the interaction between the photons and the medium cause the photons to acquire an ‘effective mass’ (i.e. they behave as if they have mass), which means they can no longer travel at the speed of light in a vacuum.
Remember that matter, even solids, is mostly empty space, so the reduced speed of light is not so much due to actual collisions between photons and atomic particles as it is due to the interaction exerted by the matter on the photons at a (very small) distance. This interaction manifests itself as a difference in permeability and permittivity, as Grey said, although that doesn’t describe the interaction itself. The Wikipedia article Photon has some information about the interaction, but, as with anything in Wikipedia, I’m not sure how trustworthy it is. (A lot of articles represent someone’s pet theory rather than the commonly-accepted one; it’s just that I happened to be reading that article earlier, and it came to mind.)
Matter is indeed mostly empty space but nevertheless light passing through a medium generally hits plenty of things along the way (depending on a number of factors of course such as density). If that were not so then we’d likely be able to see through a lot more things than we do. A thin sheet of aluminum foil is more than sufficient to stop any light from passing through it…it doesn’t have to be, say, a foot thick to work before enough stuff gets in the way to block light.
Near as I can tell the absorb/emit model is used when thinking about light as a particle. The permittivity/permeability bit is used when thinking of light as a wave.
Again, it can be determined from Maxwell’s equations. Where the group velocity of the EM wave winds up being 1/Sqrt(E[sub]0[/sub]U[sub]0[/sub]).
The math can be found here.