1. The problem statement, all variables and given/known data Given a cellphone layered with an anti reflection film with width L. A ray denoted with lambda hits the screen and some of the ray passes through while some of it reflects off from both inside the film and where the ray hits the film. The index of reflection from top to bottom is denoted by n1, n2, and n3 and n1 < n2 < n3. At what thickness L would the two rays interfere. If you are confused here is a picture of what I'm talking about. Just the diagram, the labels are different. http://upload.wikimedia.org/wikipedia/commons/f/f7/Optical-coating-1.png It's the bottom picture, just flip it then rotate it -90 degrees. 2. Relevant equations Snell's Law n1sinx = n2siny 3. The attempt at a solution I just tried applying Snell's Law and using concepts of terminal refraction. I'm just confused with the whole question in general... thanks.