Polarization Reflection question

[BobbyBobbyBob]

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.

 

[berkeman]

What thicknesses of ethyl alcohol will result in the yellow light reflected off the top surface and the yellow light reflecting off the alcohol-glass surface being in phase as they head back up to the observer?