To prevent reflection from a camera lens, a thin film of magnesium is coated. In order that the light of wavelength 5000 Å not be reflected the minimum thickness of the film should be (μ for magnesium = 1.25 and μ for glass = 1.5)
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Question

To prevent reflection from a camera lens, a thin film of magnesium is coated. In order that the light of wavelength 5000 Å not be reflected the minimum thickness of the film should be (μ for magnesium = 1.25 and μ for glass = 1.5)

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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 B: 10⁻⁷ m. Hence the correct answer is B.