# Homework Help: Thin film interference problem

1. Apr 27, 2010

### Terraist

Hi, I'm a high school physics student and I'm having a lot of trouble with thin film interference

1. The problem statement, all variables and given/known data

A thin layer of oil (noil=1.25) is floating on water (n=1.33). How thick is the oil in the region that strongly reflects green light (λ=556nm)?

2. Relevant equations

noil/nwater = λwater/λoil

t = mλ/2, where t is thickness of the film, m is order of interference maxima (i'm not sure about this one)

3. The attempt at a solution

1.25/1.33 = 556/λ
λ=592 nm

t=mλ/2
=(1)(592)/2
=296 nm

(ans in book 222 nm)

I'm not entirely sure of what I'm doing here. I'm trying to just find the right formula to plug the variables in and I don't grasp the principles behind it. If someone could shed some light on the subject and provide some tips on how to solve similar problems it would be greatly appreciated.

2. Apr 27, 2010

### flyers

Everything is right except you're not supposed to use n oil and water but rather n air and oil, because the ray is coming from above in air and not from the water right?( I think this is a trick question or something) So your new wavelength would be 556/1.33 (n air is 1) = 444 and you should get the books answer.