Halliday, Resnick & Walker — Interference: Problem 53

53 In Fig. 35-30, a light wave along ray \(r_1\) reflects once from a mirror and a light wave along ray \(r_2\) reflects twice from that same mirror and once from a tiny mirror at distance \(L\) from the bigger mirror. (Neglect the slight tilt of the rays.) The waves have wavelength \(350 \text{ nm}\) and are initially in phase. (a) What is the smallest value of \(L\) that puts the final light waves exactly out of phase? (b) With the tiny mirror initially at that value of \(L\), how far must it be moved away from the bigger mirror to again put the final waves out of phase?