A rectangular soap film has length 0.020 m and height 0.030 m. White light is incident at 30° to the normal, and the reflected light shows green wavelength λ₀=500 nm. If μ=1.33 and density=1000 kg m⁻³, find the mass of the thinnest film.
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Question

A rectangular soap film has length 0.020 m and height 0.030 m. White light is incident at 30° to the normal, and the reflected light shows green wavelength λ₀=500 nm. If μ=1.33 and density=1000 kg m⁻³, find the mass of the thinnest film.

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G1 Physics · Accepted Answer

For a thin film, use optical path 2μt cosr and include phase reversal at reflection. With one phase reversal, reflected maxima satisfy 2μt cosr=(m+1/2)λ and reflected minima satisfy 2μt cosr=mλ. Choose the least positive m when minimum thickness is asked. After substitution/checking the statements, the matching result is C: 0.06 mg. Hence the correct answer is C.