A soap bubble (index of refraction ) is surrounded by air and is viewed under…
PHYSICS · Multi-part · AP · Physics 2: Algebra-based · Waves Sound and Physical Optics · Waves Sound and Physical Optics
Problem
A soap bubble (index of refraction nb=1.33) is surrounded by air and is viewed under sunlight at normal incidence.
A student notices that certain spots on the bubble appear dark while others appear brightly colored.
Answer the following, referring specifically to reflections at the air–film and film–air interfaces and to the role of film thickness.
(a) Identify which, if any, of the two reflected rays from the front and back surfaces of the film undergo a half-wavelength phase reversal. Justify your answer in terms of the relative indices of refraction at each interface.
(b) Describe qualitatively how the interference between the two reflected rays depends on the local thickness of the soap film. In your explanation, relate variations in thickness to the appearance of bright and dark regions.
(c) For a region that appears perfectly dark when illuminated with monochromatic light of wavelength 560nm in air, explain why that darkness occurs even though some light is reflected from each interface. Your answer should make clear reference to both phase considerations and optical path differences.