White light is incident normally on a thin film which has n = 1.5 and a thickness of 5000 Å. For what wavelengths in the visible spectrum (4000 - 7000 Å) will the intensity of the reflected light be a maximum?
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Question

White light is incident normally on a thin film which has n = 1.5 and a thickness of 5000 Å. For what wavelengths in the visible spectrum (4000 - 7000 Å) will the intensity of the reflected light be a maximum?

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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: 4285 Å, 6000 Å. Hence the correct answer is C.

White light is incident normally on a thin film which has n = 1.5 and a thickness of 5000 Å. For what wavelengths in the visible spectrum (4000 − 7000 Å) will the intensity of the reflected light be a maximum?