Physics Question

[obiwankenobi]

Hey guys,

I have a question about the speed of light in a medium, more specifically glass. I understand why it's slower than a vacuum according to the classical model : the waves become out of phase and the sum result is a slower wave whose direction is determined by optic laws. However, the quantum model confuses me. Photons are discreet energy packets. The reason why they can traverse glass is because the molecules can't absorb photons at the energy of the visible light spectrum. But if that's the case, why do photons travel more slowly through glass? I read that this phenomenon occurs because molecules absorb then reemit photons, which slow light down. But that explanation contradicts the idea that photons can go through glass because they are not absorbed.