Light Reflection: Path Length Difference?

[McAfee]

Homework Statement

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

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.

 

[McAfee]

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

 

[Steely Dan]

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

 

[ajit.phys]

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.