1. The problem statement, all variables and given/known data "A glass lens (nglass = 1.52) has an antireflection coating of MgF2 (n = 1.38). (a) For 517-nm light, what minimum thickness of the coating will cause reflection rays R2 and R4 to interfere destructively assuming normal incidence? (b) Interference will also occur between forward moving rays R1 and R3. What minimum thickness will cause these rays to interfere constructively? We are given a figure (attached as temp.jpg). 2. Relevant equations Δphase = π*(2k + 1) for destructive, Δphase = 2kπ for constructive, Δphase = (2*thickness*n / λ) * 2π + π for part (a) because the rays go from the air to the back wall of the coating an reflect back, part (b) has got me stumped 3. The attempt at a solution Ok, so attempting to solve part (a), I've solved the equations for Δphase (setting them equal to each other), I got 2k = ((4 * 1.38) / (517 e -9)) * thickness. Now I don't know where to go from here. As far as part (b), I don't understand what the rays are actually doing according to the figure.