Light of wavelength 6000 Å is incident on a thin glass plate of refractive index 1.5. The angle of refraction inside the plate is 60°. Find the smallest thickness for which the plate appears dark in reflected light.
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Question

Light of wavelength 6000 Å is incident on a thin glass plate of refractive index 1.5. The angle of refraction inside the plate is 60°. Find the smallest thickness for which the plate appears dark in reflected light.

Options

G1 Physics · Accepted Answer

For reflected darkness, 2μt cosr=mλ. For the least non-zero thickness m=1. With μ=1.5, r=60° and λ=6000 Å, t=λ/[2μcos60°]=6000/[2×1.5×0.5]=4000 Å=4×10⁻⁷ m. Option B.