A thin convex lens has radii of curvature 10 cm and 15 cm. Its focal length is 12 cm. The refractive index of the lens material is:
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Question

A thin convex lens has radii of curvature 10 cm and 15 cm. Its focal length is 12 cm. The refractive index of the lens material is:

G1 Physics · Accepted Answer

For a thin bi-convex lens in air, using magnitudes of radii, 1/f = (μ-1)(1/R₁ + 1/R₂). Given, f = 12 cm, R₁ = 10 cm, R₂ = 15 cm. Thus, 1/12 = (μ-1)(1/10 + 1/15) = (μ-1)(5/30) = (μ-1)/6. Therefore, μ-1 = 1/2 ⇒ μ = 3/2 = 1.5. Correct option: B.