OT:Tempted to get this book

[Aiken]

Slowed light is virtual phenomenon. Once photon from light ray collides with atoms of medium it transfers its energy to atom and disappears, but atom goes to hugher energy state. After some time atom re-emits photon with similar wavelenght in the same (if medium has no lens ability) or different direction and then atom goes back to initial energy state. This effect will occur many times until photon leaves meduim. Since cycle of "absorbtion-reemission" is not instant, it gives effect of "slowed light".

edit: note, that it's a very primitive explanation which assumes that photon bahaves itself as a particle. But in real world it also behave itself as a wave. And things become slightly more complicated. But the principle remains the same.