1. The problem statement, all variables and given/known data Part of a longer question, what is the phase change, if any for a system of three substances of different n where there is surface reflection between n1 and n_f which is meant to constructively interfere with another surface reflection between n_f and n3.where n1 < n_f > n3 with n3 > n1? 2. Relevant equations None, the text book I am using only says that when n1 = n3 there will be a pi phase change and there will be none if n1>n_f>n3 or n1<n_f<n3. 3. The attempt at a solution None, other than an intelligent guess it will be some sort of function with 0 < f(n_f) < pi.