A thin film is viewed both in reflected and transmitted light at normal incidence. The figure shows

Question:

A thin film is viewed both in reflected and transmitted light at normal incidence. The figure shows the strongest two rays for each. Show that if rays 1 and 2 interfere constructively, then rays 3 and 4 must interfere destructively, and if rays 1 and 2 interfere destructively, then rays 3 and 4 interfere constructively. Assume that n 2 is the largest of the three indices of refraction.