1. The problem statement, all variables and given/known data You are coating a glass lens of index of refraction 1.6 with a film of material of index of refraction 1.7. You start with the thinnest film possible that creates a strong reflection for 610 nm light. You gradually increase the film thickness until you again get strong reflection. What is the thickness of the film now? 2. Relevant equations Wavelength in film = wavelength in a vacuum / index of refraction of film 610 nm / 1.6 = my answer? Not sure if this is correct. 3. The attempt at a solution Just need to be pointed in the right direction! Really am not sure where to start for this problem. Any help would be really nice!