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KCET2007PhysicsRay Optics

Two thin equiconvex lenses each of focal length 0.2 ~m are placed coaxially with their optic centres 0.5 ~m apart. Then the focal length of the combination is

Options

  1. A-0.4 ~m
  2. B0.4 ~m
  3. C-0.1 ~m
  4. D0.1 ~m

Correct answer

A. -0.4 ~m

Step-by-step solution

Equivalent focal length (F) of two lenses separated by distance d is given by aligned 1 F &= 1 f₁ + 1 f₂ - d f₁ f₂ &= 1 0.2 + 1 0.2 - 0.5 (0.2)(0.2) &=5+5-0.5 5 5 &=10-12.5=-2.5 F &=- 1 2.5 =-0.4 ~m aligned

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