I've been reading a bit around Quantum Electrodynamics (QED) which says that when you shine monochromatic light onto a thin film, the fact of whether it produces a dark or bright band is purely a probability function. It seems to say you use Feynman arrows, one for the first surface and one for the second. You set the arrows spinning in a circle and then stop them as soon as the light is detected from each surface. The vector difference in the two arrows represents the radius of a circle and the bigger the area of the circle the higher the probability of getting a bright band. My textbook says that thin film patterns are caused by wave interference of the light and so it depends on the phase difference of the light waves. I was just wondering if these two concepts agree with each other. It seems to imply that, in terms of thin films, QED has nothing to do with wave interference when explaining thin films? Does this mean wave interference is a dated classical theory?