Thin film diffraction - to start

[big_boi]

Thin film diffraction -- need help to start

Homework Statement

I attached a picture of the problem, could anyone tell how I would begin to proceed with this problem? I believe that it is more complicated than simply a Snell's law problem, due to the thin film of oil.

Thanks for your consideration.

Homework Equations

The Attempt at a Solution

 

[flatmaster]

All you need is snell's law. However, you will need to use it twice.