1. The problem statement, all variables and given/known data Light falls at normal incidence onto a transparent film on a substrate as shown i. what is the optical path length difference in the case of normal incidence? ii. if nf>no and nf>ns, what is the condition for constructive interference in the case of normal incidence? explain your answer. 2. Relevant equations OPL=nd 3. The attempt at a solution ok im studying for an exam and this qn has been on the last 2 exams, and i have absolutely no idea how to do it, it cant be that, just an understanding issue. ok i know that the OPL=nd, i got no idea what the questions is asking me, or how to do it, plz can anyone give me some info?