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A plano - convex lens (focal length f 2 , refractive index μ 2 , radius of curvature R) fits exactly into a plano - concave lens (focal length f 1 , refractive index μ 1 , radius of curvature R). Their plane surfaces are parallel to each other. Then, the focal length of the combination will be:

Options

  1. AR μ 2 - μ 1
  2. Bf 1 - f 2
  3. Cf 1 + f 2
  4. D2 f 1 f 2 f 1 + f 2

Correct answer

A. R μ 2 - μ 1

Step-by-step solution

1 f 1 = μ 1 - 1 1 - R - 1 ω 1 f 2 = μ 2 - 1 1 R - 1 ω When joined together 1 f e q = 1 f 1 + 1 f 2 = μ 2 - 1 - R + μ 2 - 1 R = μ 2 - μ 1 R ⇒ f e q = R μ 2 - μ 1

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