[pif]

They don't. A photon absorbed by an atom/molecule can be re-emitted in any direction. Imagine a thick piece of glass being traversed by a light beam: most of the photons will traverse it undisturbed (and thus maintain their original direction), but a few will be scattered all around (and that's the light you can see if you watch "inside" the glass from one of its sides). On the other hand, depending on the light frequency and the crystalline structure of the material, photons can be scattered by the whole lattice itself, rather than by the single atoms/molecules. In this case, the direction follows the "rule of the mirror", which is dictated by quantum mechanics.

[TheLoneWolfling]

Problem with that:

When you shine light through glass the maximum propagation speed, the speed at which you start getting photons out the other side, is about 2/3 the speed of light. This implies that the overwhelming majority of photons are adsorbed and re-emitted. So this explanation cannot be correct, as I can look through a pane of glass, but you're saying any photon adsorbed is re-emitted in any direction, and the overwhelming majority of photons must be adsorbed and re-emitted.

[pif]

Sorry, my fault! We were talking about "absortion"/"stop over" as a way to describe in layman's term the interaction of photons with the glass atomic structure. In this sense, as others pointed out, we are talking about scattering and the final direction is indeed related to the original one. Anyway, light scattering is a quantum process and there is no way of observing the "moment" between "absorption" and "re-emission".

In my comment, I talked about actual absorption, which means that there is a finite time interval when the photon does not exist and the atom/molecule who absorbed it can be observed in a different state than usual (electron in a higher orbital for an atom, different vibrational modes for a molecule). Later, the atom/molecule will go back to its normal state emitting a photon with the same energy as the first one, or several lower energy photons. This actual re-emission will not have a favourite direction. Depending on the typical time scale of the re-emission, you may call this process fluorescence or phosphorescence (http://en.wikipedia.org/wiki/Phosphorescence).

[imaginenore]

> most of the photons will traverse it undisturbed

But then why is the speed of light in glass smaller than in vacuum?