Confused on Constructive/Destructive Interference!!! 1. The problem statement, all variables and given/known data A film with n=1.6 is deposited on glass. What is the thinnest film that will produce constructive interference in the reflection of light with a wavelength of 575 nm? 2. Relevant equations 3. The attempt at a solution I have 2 equations that I can choose from... Constructive Interference for 0 to 2 refl. phase changes Destructive Interference for 1 refl. phase changes 2T = m * t/n t = wavelength Destructive Interference for 0 to 2 refl. phase changes Constructive Interference for 1 refl. phase changes 2T = (m + 1/2) * t/n t = wavelength How do I know which one to use? I chose the 2nd one because I assumed there was only 1 phase change. How do figure out what m =???