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Two thin convex lenses of focal length 30 cm and 10 cm are placed coaxially, 10 cm apart. The power of this combination is :

Options

  1. A5 D
  2. B1 D
  3. C20 D
  4. D10 D

Correct answer

D. 10 D

Step-by-step solution

f ₁=30 ~cm , f ₂=10 ~cm 1 f _ eq = 1 f ₁ + 1 f ₂ - d f ₁ f ₂ , ~d = distance between lens 1 f _ eq = 1 0.3 + 1 0.1 - 0.1 (0.3)(0.1) 1 f _ eq = 1 0.1 Power = 1 f _ eq =10 D

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