A monochromatic point source S and a plane mirror are arranged as shown. At point P, at distance x from the mirror, the direct and reflected rays interfere to form a maximum. Find the minimum possible value of x.

Question

G1 Physics · Accepted Answer

Replace the reflected ray by a ray from the virtual image of the source. Reflection at the mirror contributes a phase reversal π, so for a maximum the geometrical path difference must be an odd half-multiple of λ. Using the small-angle geometry shown gives the minimum x=62.5λ. Option C.