Walker, Halliday & Resnick — Interference: Problem 45

45 The rhinestones in costume jewelry are glass with index of refraction 1.50. To make them more reflective, they are often coated with a layer of silicon monoxide of index of refraction 2.00. What is the minimum coating thickness needed to ensure that light of wavelength 500 nm and of perpendicular incidence will be reflected from the two surfaces of the coating with fully constructive interference?