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 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 an even number of half-wavelengths. a whole number of half-wavelengths. a whole number of wavelengths. zero. an odd number of half-wavelengths.