A thin film of ethyl alcohol (n=1.36) is spread on a flat glass plate and illuminated with white light. When illuminated and viewed from directly above, it shows a coloured pattern in reflection. If the only visible light reflected by a certain region of the film is yellow (lambda=560nm), how thick is the film there? Justify (assume visible spectrum is 400-7800nm) I have no idea where/how to start and what it involves. I have a feeling it invovles Huygen's Principle and Polarization by Reflection.