# Homework Help: When light reflects from a thin film back toward the source, what is the path length?

1. Apr 19, 2012

### McAfee

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

When light reflects from a thin film back toward the source, what is the path length difference?

3. The attempt at a solution

A. It is equal to twice the thickness of the film.
B. It is equal to the thickness of the film.
C. It is equal to half the thickness of the film.

I have rules out C. I think the answer is A but I'm not sure.

2. Apr 20, 2012

### McAfee

Re: When light reflects from a thin film back toward the source, what is the path len

I'm leaning towards A now. Anyone have any ideas?

3. Apr 20, 2012

### Steely Dan

Re: When light reflects from a thin film back toward the source, what is the path len

Tell us why you are leaning towards A and we can help you evaluate that thought process.

4. Apr 20, 2012

### ajit.phys

Re: When light reflects from a thin film back toward the source, what is the path len

I guess the path length difference is between the rays reflected from the two surfaces of the film. When light reflects from a "thin" film back toward the source, the angle of incidence is approximately 90 degree. So, the optical path length difference is twice the film thickness times the refractive index of the film. One also has to take into account the fact that a phase change of 180 degree occurs upon reflection from an optically denser medium.