Halliday, Resnick & Walker — Interference: Problem 36

36 A thin film of liquid is held in a horizontal circular ring, with air on both sides of the film. A beam of light at wavelength \(\lambda = 620\) nm is directed perpendicularly onto the film, and the intensity \(I\) of its reflection is monitored. Figure 35-27 gives intensity \(I\) as a function of time \(t\); the horizontal scale is set by \(t_s = 20.0\) s. The intensity changes because of evaporation from the two sides of the film. Assume that the film is flat and has parallel sides, a radius of 1.2 cm, and an index of refraction of 1.33. Also assume that the film’s volume decreases at a constant rate. Find that rate.