I searched for a topic already about this and i couldn't really see one so here we go. A teacher of mine showed this problem to me and could not see a solution just as i cannot either. A single laser beam is sent on a path as in the diagram enclosed as an attachment. If you look in the diagram at both detectors, the beams will be totally out of phase. The path difference is half a wavelngth and the total difference in relflections between the two is an even integer so the beams are 180 degrees out of phase. Therefore no light should be observed at each detector. How is this possible as this would surely result in complete destructive interference and violate the conservation of energy? Thanks in advance.