Thin-Film Interference: Coating a Lens -- Answers found, explanation needed! 1. The problem statement, all variables and given/known data Boxed answers are the correct ones. 2. Relevant equations 2L=(m+1/2)(λ/n) for constructive interference (maximum reflection, bright film) 2L=(m)(λ/n) for destructive interference (minimum reflection, dark film) Where: L = thickness of film m = fringe count (for m=0,1,2,3,4...) λ = wavelength of light n = refractive index of coating 3. The attempt at a solution I got the correct answer (a) for Fall2008 #20 by using the equation for constructive interference, plugging in 0 for m, 1000 nm for λ, and 1.8 for n. Now for Sp2011 #11, I took it as essentially the same question except this one is asking for minimum reflection, so I should use the equation for destructive interference. Doing that gets me (e), which is incorrect. The correct answer is (b), which is half of (e), implying that I should use the equation for constructive interference. Am I missing a fundamental difference between the two problems besides one asking for minimum and another asking for maximum reflection? Thanks for the help.