I'm starting out with QM (I have a maths degree) and have come up with the following problem to help me understand QM more. Say we have a mirror lying flat on the ground. We shine a laser from the East on the mirror at say X degrees, to be reflected to the West at X degrees. At the quantum level all we have is photons being absorbed by the reflective coating atoms of the mirror (eg silver), then being re-emitted shortly later. I'm trying to understand is why the photons are reflected at the same angle they are absorbed. Once the photon is absorbed I cant see how the silver atom "decides" to emit it at the same angle. Is it to do with QED or conservation of momentum? Is there a good book to get me started with this?