1. The problem statement, all variables and given/known data an oil stick 122 nm thick (n=1.40), lying on water, is illuminated by white light incident perpendicular to it's surface. What is the wavelength of the color that appears? 2. Relevant equations I'm not sure, but i tried n= Lambda(air)/Lambda coating in order to find the lambda in air but it doesn't match. 3. The attempt at a solution I used the above equation but it didn't work. I'm also having a hard time grasping the concept of index of refraction and how it's related to the thin films. Any strong analogy that you can give me as a tool to understand the concept will be more than helpful to me.