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Two lens of focal lengths -20 ~cm and +10 ~cm are put in combination, find the power of the combination.

Options

  1. A-1 D
  2. B-2 D
  3. C+5 D
  4. D+2 D

Correct answer

C. +5 D

Step-by-step solution

Here f₁=-20 ~cm , f₂=+10 ~cm Focal length of combination is, aligned & 1 F = 1 f₁ + 1 f₂ = 1 -20 + 1 10 & 1 F = 1 20 F=20 ~cm aligned Power of the combination, P= 100 F = 100 20 =+5 D

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