1. The problem statement, all variables and given/known data http://i.imgur.com/Mp77mDR.png 2. Relevant equations Path length interference equation. 3. The attempt at a solution Why is the answer to (a) a minima? 1. When the light wave hits the top surface of the top glass, a wave will be reflected with phase change of pi. 2. The non-reflected goes to the bottom surface of the top glass to be reflected again with no phase change. 3. The non-reflected part still goes to the top of the down glass and is reflected with phase change pi. 4. We don't care about other reflections as amplitude diminishes too much. Now, We have three waves who are reflected towards our eye. 2 and 3 automatically cancel each other. There is only wave 1 left. So, this is not perfectly destructive. How can they call this a minima?