I am learning quantum mechanics from Feynman Lectures on Physics. Where he says that amplitude of alternatives that can't be distinguished interfere with each other. I am trying to understand some simple phenomenons using this principle. My query is, when photons of different energy come from two different source and are detected using a 'single' detector, do their amplitudes interfere. It seems quite straight forward from the aforementioned principle , that the answer is no. Because when the detector receives a photon , its energy can be determined and we'll know from which source it came. So these two alternatives are distinguishable and should not interfere with each other. But quick google search revealed to me that photons of different energy do interfere with each other for reasons I am incapable of understanding at this moment. Plz explain the matter in basic terms for I am a novice (I can easily follow Feynman, though .. .. pity he haven't explained it there :( ). Thx in advance .