1. The problem statement, all variables and given/known data Two parallel glass plates of index of refraction n are separated by an air film of thickness d. Light of wavelength λ in air, normally incident on the plates, is intensified on reflection when, for some integer m a) 2d=mλ b) 2d=mλ/n c) 2d=mnλ d) 2d=(m+1/2)λ e) 2d=mλ/2 2. Relevant equations If only one ray is π shifted: 2nd=mλ (min), 2nd=(m+1/2)λ (max) If both rays π shifted: 2nd=mλ (max), 2nd=(m+1/2)λ (min) 3. The attempt at a solution I've attached a diagram of two different for how I picture light should be transmitted in this problem. I'm not sure which one would be a correct approach. Going with the first diagram, This would result in a net π shift, so it's conditions would be 2nd=mλ (min) and 2nd=(m+1/2)λ (max). I chose answer (B) 2d=mλ/n, since the question asks for "intensity increase" so I am assuming it is asking about the condition for a bright fringe; and also because it's one of the only options that takes the index of refraction (n) into account. I would just like to verify that my reasoning is correct.