Bear in mind that I am talking about monochromatic light. I looked into this subject and the thing that many say is that when we have thick films, the two reflected waves are not considered to be coherent any more(and something about coherence length). I can't see why this is the case(and I have to say that I don't fully understand coherence in this case). If the above is not the case and we can indeed have interference effects on thick films, why does every book only consider thin film interference? Is there a problem with thick films? I am not talking about the surface not being a plane, but if there is something about the interference itself that can't happen in thick films. Thank you!