Subtle question. I think we can answer it in terms of classical electromagnetism, without invoking quantum photons.
What happens to the light (electromagnetic wave) as it enters the glass? By coating the glass with an anti-reflective layer one can in principle eliminate the effects of reflection at each surface, helping keep things simple. In the glass the wave gets mixed up with oscillations of the charged particles, mainly electrons. The wave traveling through the glass is a combined wave of electromagnetic field and oscillating electrons. It does travel more slowly than the bare electromagnetic wave. So I think it is legitimate to say that the light 'decelerated'. In the process however, its momentum actually increased
. That's easiest to see in the photon picture, since the slower wave has a shorter wavelength, and hence more momentum per photon. It is also true, however in the classical picture. You can think of it as adding some momentum from the particles which are now contributing to the wave. Net momentum is conserved because the glass itself moves slightly, opposite to the light.
When the light leaves the glass, it goes back to the initial speed and momentum.
(published on 09/14/2009)