# Oil on puddle of water

1. Nov 4, 2004

### GreenDinos

A thin film of oil (n = 1.46 is spread over a puddle of water (n = 1.33). In a region where the film looks red from directly above ( = 626 nm), what is the minimum possible thickness of the film?

I thought this was simple but it says my answer is wrong. Ok the light goes from a smaller to larger n value when it goes from air to the oil, so it shifts 180 deg. The light that goes through the oil reflects off the water and doesn't shift so now the waves have to be lined up so they're not destructive. The minimum thinkness that would cause the wave reflecting off the water to match up constructively with the first reflection off the oil should cause the wave to shift by 1/2 wavelength- so the minimum thickness should be wavelength/4 ...but it's not. I even found an equation for this in the book
2t = m wavelength
and since we have to shift it by .5 it would be
2t = (m+.5) wavelength

but i'm doing something wrong.
thanks

2. Nov 4, 2004

### Bystander

Wavelength in air is related to wavelength in oil in what way?