Chapter 36, Problem 48CP

The condition for constructive interference by reflection from a thin film in air as developed in Section 36.5 assumes nearly normal incidence. What If? Suppose the light is incident on the film at a nonzero angle θ₁ (relative to the normal). The index of refraction of the film is n, and the film is surrounded by vacuum. Find the condition for constructive interference that relates the thickness t of the film, the index of refraction n of the film, the wavelength λ of the light, and the angle of incidence θ₁.