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Two plano-convex lenses of focal lengths 20 cm and 30 cm are placed together to form a double convex lens. The final focal length will be

Options

  1. A12   cm
  2. B60   cm
  3. C20   cm
  4. D30   cm

Correct answer

A. 12   cm

Step-by-step solution

Equivalent focal length 1 F = 1 f 1 + 1 f 2 = 1 20 + 1 30 ∴ F = 20 × 30 20 + 30 = 600 50 = 12 cm

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