1. The problem statement, all variables and given/known data What is the thinnest film of MgF2 (n=1.21) on glass that produces a strong reflection for the light with a wavelength of 531 nm? 2. Relevant equations Open-Closed Standing wave Fn= (nv/4L) v = Wave Length * F 3. The attempt at a solution Basically, I tried to plug in the data. I solved for Frequency by using velocity for light (300,000,000 m/s) = (5.31*10^-7m) * F Then used the equation Fn= (nv/4L) using n = 1.21 Alternatively the eq can be simplified into L= ((n*wave length)/4) I get 1.606*10^-7m, but I get the problem wrong. I think I overlooked something, but I'm not sure, can any one help out. Most appreciated.