I've worked out the scattering of a photon off an atom. Now I'm trying to figure out how I can use the same formulas to figure out scattering off a material. Suppose you have a light photon incident normal to a piece of glass. Classically it should go straight through or get reflected straight back up. Quantum mechanically it can be scattered in all sorts of directions or it can go straight through. So how would I go about calculating where the photon will come out? I wouldn’t have to sum over all paths because not all paths would take the same time. It seems complicated because not only can the photon also be absorbed and not scattered on the way (and when this happens how does the electron’s energy get into vibration of the atom?), but you have to keep track of whether the photon is LHC or RHC after each scatter – oh and by the way did I say that photon can just go straight through at the speed of light in vacuum, making it difficult to decide which paths to sum because variable speeds and variable paths make variable times? Lastly, what is QED? I thought QM contained electrodynamics.