Which of the following statements correctly describes rays 1 and 2 and their path difference?

Question

The sketch (Intro 1 figure) shows a thin film of soapy water of uniform thickness t and index of refraction n=1.33 suspended in air. Consider rays 1 and 2 emerging from the film: Ray 1 represents the reflected wave at the air-film interface and ray 2 represents the wave that reflects off the lower surface of the film.

http://session.masteringphysics.com/problemAsset/1020409/16/1020409.jpg

Which of the following statements correctly describes rays 1 and 2 and their path difference?

A) Ray 1 undergoes a half-cycle phase shift. Ray 2 does not undergo any phase shift

B) Both rays 1 and 2 undergo a half-cycle phase shift.

C) Ray 2 undergoes a half-cycle phase shift. Ray 1 does not undergo any phase shift.

D) The path difference between rays 1 and 2 is about t.

E) There is no path difference between the two rays.

F) The path difference between rays 1 and 2 is about 2t.

Answer 1

The interference colors from a soap film can be related to the thickness of the film by using the interference condition and noting that there is a 180 degree phase change upon reflection from the film surface, but no phase change for the reflection from the back surface. The color seen depends also upon the angle of view.

A) Ray 1 undergoes a half-cycle phase shift. Ray 2 does not undergo any phase shift