Hello, this question might not be particularly clear, but I'd like a quantum mechanical description of the classical idea of external reflection by a medium. Now, the way I see it, a photon (or numerous photons if necessary) approach the boundary of a medium. Classically, the reflected wave has a phase retardation of 180 degrees. I figure this same effect must be observed quantum mechanically, leading to interference of photons in a way corresponding to the classical picture. My problem is seeing where this effect comes from and why it's not observed for internal reflection. To quote http://www.mellesgriot.com/products/optics/oc_2_1.htm Does anyone have any insight on why this is? Or a way for me to grasp it. My maths is sufficiently complicated and my knowledge of quantum mechanics good enough that any answer could get quite technical if desired, but it would be nice to have an answer which argues with basic physical principles and easy to understand language at the same time.