1. The problem statement, all variables and given/known data A sheet of glass having an index of refraction of 1.20 is to be coated with a film of material having an index of refraction of 1.42 such that green light with a wavelength of 525 nm (in air) is preferentially transmitted via constructive interference. What is the minimum thickness of the film that will achieve the result? 2. Relevant equations See my solution. 3. The attempt at a solution I did this problem and end up with a thickness half that of the answer. Here is what I did. I first noted that on the first reflection, there is a phase shift of ∏. This is because it there is reflection on a surface of higher index. For the second reflection, there is no phase shift as the glass index is lesser than the film. So the total phase shift due to reflection is just ∏. Then the total phase shift should be related with: Δ∅ = ∏ + 4∏nL/λ, where n = index of refraction of film, L = thickness of film, λ = wavelength of light in vacuum. In order for there to be constructive interference, this phase shift must be equal to 2∏m where m is an integer. So: ∏ + 4∏nL/λ = 2∏m 1/2 + 2nL/λ = m 2nL/λ = m - 1/2 L = λ(m - 1/2) / (2n), m >= 1 In order to minimize L, I set m = 1. So: L = λ(1/2) / (2n) = 1/4 λ/n = 1/4 525 nm / 1.42 = 92.23 nm The answer state is 184.46 nm. Am I incorrect or is it? Thanks!