One beam of coherent light travels path P1 in arriving at point Q and another coherent beam travels path P2 in arriving at the same point. If these two beams are to interfere destructively, the path difference P1 - P2 must be equal to

Huygen's principle: Every point on a wave front can be considered as a source of tiny wavelets that spread out in the forward direction at the speed of the wave itself. The new wave front is the envelope of all the wavelets - that is, tangent to all of them.