Hi all From http://en.wikipedia.org/wiki/Interference_%28wave_propagation%29: [Broken] "Interference usually refers to the interaction of waves that are correlated or coherent with each other, either because they come from the same source or because they have the same or nearly the same frequency. " Coherent means that they are monochromatic and have a definite phase relationship. Lets say we have two waves of e.g. 16 Hz and 18 Hz. They are not monochromatic and their phase relationship is not constant, even though they start out in phase - but they will still interfere (they will beat against eachother). Here the waves are not coherent, but we still have interference. How is that?