Doing a school paper right now, and would appreciate any answer. Please Register or Log in to view the hidden image! I have read in some places that, for two waves, when there is a difference in the path length that is equal to a wavelength, constructive interference will occur. The amplitude of the wave that results is larger than either of the waves that created itâ€”the light will brighten, sound gets louder, etc... Does the difference in path length always have to be equal to a wavelength for constructive interference to occur? Here the waves have the same wavelength and amplitude, but they are not perfectly parallel to each other: http://images.google.co.uk/imgres?i...&hl=en&rlz=1C1GGGE___GB351&sa=N&start=18&um=1 Constructive interference can occur in this case?

Yes, you're right, constructive interference is occurring in that picture too. The 1 (or any integer) wavelength shift criteria just gives the maximum constructive interference. There is a continuous spectrum between destructive and constructive interference as you vary the phase difference from 1 wavelength (fully constructive) to 1/2 wavelength (fully destructive). Somewhere in the middle you will just get back the same wave, but shifted in phase a little. All this is assuming the waves are identical except their phase btw. You can consider that constructive/destructive interference just describes what is happening at each particular point along the wave. Two waves can constructively interference in some places and destructively interfere in others.