Question

Two light waves of equal wavelength, lambda, are emitted in phase from separate sources and propagate to a common point P. Light wave 1 must travel a longer distance (d1) than light wave 2 (d2) to reach point P, where d1 – d2 is equal to the path difference between the two light waves. If the two waves interfere constructively at point P, what must be true about the path difference between the two light waves?

Answer 1

The path difference must be equal to an integral (1 * lambda, 2 * lambda, -------n * lambda) number of wavelengths for constructive interference to occur.