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A thin plano-convex lens acts like a concave mirror of focal length 0 . 2 m , when silvered on its plane surface. The refractive index of the material of the lens is 1 . 5 . The radius of curvature of the convex surface of the lens will be

Options

  1. A0 . 1   m
  2. B0 . 2 m
  3. C0 . 4   m
  4. D0 . 8   m

Correct answer

B. 0 . 2 m

Step-by-step solution

F = - 0.2 m , f m = ∞ μ = 1.5 f l = R μ - 1 , 1 f F = 1 f m - 2 f l ⁡ ⇒ F = - f l ⁡ 2 ⇒ F = - R 2 μ - 1 ∴ - 0.2 = - R 2 1.5 - 1 ∴ 0.2 = R 1 ∴ R = 0.2 m

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