<P> In physics, interference is a phenomenon in which two waves superpose to form a resultant wave of greater, lower, or the same amplitude . 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 . Interference effects can be observed with all types of waves, for example, light, radio, acoustic, surface water waves or matter waves . </P> <P> The principle of superposition of waves states that when two or more propagating waves of same type are incident on the same point, the resultant amplitude at that point is equal to the vector sum of the amplitudes of the individual waves . If a crest of a wave meets a crest of another wave of the same frequency at the same point, then the amplitude is the sum of the individual amplitudes--this is constructive interference . If a crest of one wave meets a trough of another wave, then the amplitude is equal to the difference in the individual amplitudes--this is known as destructive interference . </P>

Two waves can interfere only if they have