The following is a solid: The atoms with the electrons in specific energy levels and a lattice where the atoms are held, with vibrational and rotational degrees of freedom. The energy levels for these degrees of freedom can be very small.
A photon impinging on the lattice will interact mainly with the electric field of the lattice. If it has an energy that can raise an electron to a higher level, it is absorbed, and then reemitted in a random direction. If this happens with high probability the lattice is not transparent, but opaque.
For optical frequencies the lattice of a piece of glass is transparent,there are no energy levels that can absorb the optical photons. This means that the photons scatter with the field of the lattice elastically, not loosing momentum and very little energy to the field of the lattice, because the exchanges are very small. (energy loss would be seen in a change in frequency, but it is too small to be observable).
A light beam emerges from a confluence of photons. Photons are quantum mechanical entities, and the superposition of their wavefunctions creates the classical electromagnetic wave.
The slowing down of the light beam through a transparent material comes from the random change in direction by the elastic scatters of the individual photons, which are traveling with speed c. This gives a larger path length to the individual photon, thus a different effective velocity to the light beam which emerges from these photons.
