Yellow light is viewed by reflection from a thin vertical soap film. Let $\lambda$be the wavelength of the light within the film. Why is there a large dark space at the top of the film?
A) no light is transmitted through this part of the film
B) the film thickness there is $\lambda$/4
C) the light reflected from exactly one of the two surfaces undergoes a 180$\degree$ phase change
D) the film is too thick in this region for thin film formulas to apply
E) the reflected light is in the infrared

Question

Yellow light is viewed by reflection from a thin vertical soap film. Let $\lambda$be the wavelength of the light within the film. Why is there a large dark space at the top of the film? 
A) no light is transmitted through this part of the film 
B) the film thickness there is $\lambda$/4 
C) the light reflected from exactly one of the two surfaces undergoes a 180$\degree$ phase change 
D) the film is too thick in this region for thin film formulas to apply 
E) the reflected light is in the infrared

Quizplus · Accepted Answer

The large dark space at the top of the film is due to destructive interference of the light reflected from the top and bottom surfaces of the film. This occurs because the reflected light undergoes a 180 degree phase change when it reflects from the bottom surface, while the top surface does not change the phase of the reflected light. This leads to the interference pattern that creates the dark space. Therefore, choice C is the correct answer.

Yellow Light Is Viewed by Reflection from a Thin Vertical λ\lambdaλ

Yellow Light Is Viewed by Reflection from a Thin Vertical $\lambda$