Why does glass allow light through, and how does it speed up again?

Dave - The easiest way to explain the reason why glass lets light through it is that it just can't absorb the light. It's a smooth surface, so light can get in to it, and it just doesn't happen to have any electrons which are able to absorb light of visible frequencies in there, so it can't absorb, so light carries on going as if nothing had happened to it. The only effect that it does have is that the electrons in the glass do slow the light down a bit. It doesn't take any energy away from it. All it does is slow it down because essentially it's moving through a denser medium, a medium with more inertia to it. So once it leaves the glass again, it moves into the air or a vacuum which is again a less dense medium, at which point, the light has still got all the energy it had before and it can carry on at its original speed.