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AP Physics 2
14.9 Thin-Film Interference
IntermediateMCQConceptual21.6k
A schematic showing a vertical rectangular wire frame holding a soap film. Light rays labeled Incident Light travel horizontally from the left toward the film. To the right of the frame, a magnified side-profile cross section shows the soap film shaped as a wedge, extremely thin at the top where thickness t is much less than lambda_0, and wider at the bottom. A horizontal dark region is indicated at the very top of the film. No other labels, lines, text, or axes appear.
Soap film held vertically in a frame illuminated by light.
A vertical soap film with index of refraction \(n = 1.33\) suspended in air (\(n = 1.00\)) drains under gravity, forming a wedge shape whose thickness increases from top to bottom. Monochromatic light of wavelength \(\lambda_0\) in air shines normally onto the film. As draining continues, a band at the very top of the film thins until its thickness \(t\) is much smaller than \(\lambda_0\) (\(t \ll \lambda_0\)). Which of the following correctly describes the appearance of this top region of the film and provides the correct physical justification?

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