1. The problem statement, all variables and given/known data A thin film of oil (n=1.5) is floating on top of water. When looking directly at this film, a large portion appears red (700nm). What is the thickness of the red part of the layer? 2. Relevant equations 3. The attempt at a solution Since n for oil > n for air, there will be a phase shift of lambda/2 There will be no phase shift for the next reflection, but it travels 2x the thickness of the film (2d) Since it appears red, i said it was constructive interference. So we get mlambda = 2d + lambda/2 If they asked for minimum i would set m=0 and use the 700nm to solve for d, but they don't specify. What should i do?