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.
Snell's Law n1sinx = n2siny
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...