Reflection is not considered to be a measurement. Reflection, like using lenses is considered to be a time-reversible process. It is not a measurement and it does not erase the coherence (interference pattern) Here is what I dont understand What happens to the momentum of the photon? The light pressure? If I put a mirror (flying in a zero gravity and vacuum) right behind 2 slits, mirror will start rotating very slowly in a different directions depending on the path of the photon Where is a flaw?