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A thin lens made of glass (refractive index = 1.5 ) of focal length f = 16 c m is immersed in a liquid of refractive index 1.42 . If its focal length in liquid is f l , then the ratio f l / f is closest to the integer:

Options

  1. A1
  2. B9
  3. C5
  4. D17

Correct answer

B. 9

Step-by-step solution

1 f a = μ g μ a - 1 1 R 1 - 1 R 2 1 f m = μ g μ m - 1 1 R 1 - 1 R 2 ⇒ f a f m = μ g μ m - 1 μ g μ a - 1 = 1.50 1.42 - 1 1.50 1 - 1 = 0.08 1.92 0.5 f a f m = 1.42 0.05 0.08 = 8.875 ≈ 9

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