A soap film has thickness 5×10⁻⁵ cm. The reflected light has a maximum at wavelength 5320 Å in the visible region. Find the refractive index of the film.
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Question

A soap film has thickness 5×10⁻⁵ cm. The reflected light has a maximum at wavelength 5320 Å in the visible region. Find the refractive index of the film.

G1 Physics · Accepted Answer

For a reflected maximum from a soap film in air, one reflection has a π phase reversal, so 2μt=(m+1/2)λ. Here t=5×10⁻⁵ cm=500 nm and λ=532 nm. Hence μ=(m+1/2)λ/(2t). Taking m=2 gives μ≈(2.5×532)/1000≈1.33, so option B.