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, known as a dark fringe, occurs due to destructive interference of the reflected light waves. At the top of the film, the reflected light waves from both the top and bottom surfaces of the film are out of phase with each other. This means that the light waves are canceling each other out, creating a dark fringe. The phase change of 180 degrees occurs when the reflected wave undergoes a change in direction due to reflection at the top surface of the film, resulting in a path difference of half a wavelength between the two reflected waves. This choice is the best because it provides a direct and accurate explanation for the occurrence of the dark fringe.

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

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