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A thin convex lens of refractive index 1.5 has 20 c m focal length in air. If the lens is completely immersed in a liquid of refractive index 1.6 , its focal length will be

Options

  1. A- 160 c m
  2. B- 100 c m
  3. C+ 160 c m
  4. D+ 100 c m

Correct answer

A. - 160 c m

Step-by-step solution

From lens formula 1 f = μ g a - 1 1 R 1 - 1 R 2 = 1.5 - 1 1 R 1 - 1 R 2 .....(i) Also, μ g l = μ g μ l = 1.5 1.6 ∴ 1 f ′ = μ g - 1 l 1 R 1 - 1 R 2 1 f ′ = 15 16 - 1 1 R 1 - 1 R 2 ...(ii) Dividing Equation (i) by Equation (ii), we get f f ′ = 15 16 - 1 1.5 - 1 = - 1 8 f ' = − 8 f = − 160 c m

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