1. The problem statement, all variables and given/known data Three experiments involving a thin film (in air) sandwiched between two layers of air are shown with nair < nfilm. If t denotes the film thickness and λ denotes the wavelength of the light in the film, which experiments will produce constrictive interference as seen by the observer? The incident light is nearly normal to the surface. 2. Relevant equations 2nt = (m+0.5)λ 3. The attempt at a solution I think the solution is I and III will produce constructive interference. I'm not totally sure why, but I think in experiment 1 and experiment 3 light undergoes one phase reversal and t satisfys (2nt = m+0.5)λ. Experiment 2 has 2 phase reversals?