Physics · Interference: Young's double-slit experiment and expression for fringe width, coherent sources and sustained interference of light
A soap bubble has a thickness of and a refractive Index of 1.35 Given that the w
A soap bubble has a thickness of \( 100 n m \) and a refractive Index of 1.35 Given that the wavelength of red light is about \( 700 n m \) and that of blue light is \( 400 n m, \) what colour does the bubble appear to be at the point on its surface closest to an observer when it is illuminated by white light?
- A. White
- B. Black
- C. Red
- D. Green
Step-by-step solution
For a soap bubble (air on both sides), reflection at the top surface (air to film) undergoes a phase change of π, while reflection at the bottom surface (film to air) has no phase change. The condition for constructive interference in reflected light is 2nt = (m + 1/2)λ, with n=1.35, t=100 nm. For m=0, λ = 4nt = 540 nm, which lies in the green region of the spectrum. Blue (400 nm) is closer to destructive, and red (700 nm) is not strongly reflected. Thus, the bubble appears green at that point.
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