How Thin Must the Oil Film Be to Appear Red?

[matt85]

A thin film of oil (n = 1.49 is spread over a puddle of water (n = 1.33). In a region where the film looks red from directly above (lambda = 629 nm), what is the minimum possible thickness of the film?

______nm

Could someone please help me with this? I have no clue on this one, it was the only of the assignment I couldn't solve...

Thanks!

 

[Doc Al]

thin film interference

What conditions must be met so that the reflection from the top surface (air/oil interface) is exactly in phase with the reflection from the bottom surface (oil/water interface)?