Light of wavelength 5880 Å is incident on a thin glass plate of refractive index 1.5. The refraction angle in the plate is 30°. Find the minimum thickness for which the plate appears dark in reflected light.
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Question

Light of wavelength 5880 Å is incident on a thin glass plate of refractive index 1.5. The refraction angle in the plate is 30°. Find the minimum thickness for which the plate appears dark in reflected light.

Options

G1 Physics · Accepted Answer

For reflected darkness from a glass plate in air, 2μt cosr=mλ. For the least non-zero thickness take m=1. With μ=1.5, r=30° and λ=5880 Å, t=5880/[2×1.5×cos30°] Å≈2263 Å. Option C.