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JEE Main201815 Apr 2018Evening ShiftPhysicsRay OpticsActual

A convergent doublet of separated lenses, corrected for spherical aberration, has resultant focal length of 10 ~cm . The separation between the two lenses is 2 ~cm . The focal lengths of the component lenses

Options

  1. A18 ~cm , 20 ~cm
  2. B10 ~cm , 12 ~cm
  3. C12 ~cm , 14 ~cm
  4. D16 ~cm , 18 ~cm

Correct answer

A. 18 ~cm , 20 ~cm

Step-by-step solution

For minimum spherical aberration separation, d=f₁-f₂=2 ~cm Resultant focal length =F=10 ~cm Using 1 F = 1 f₁ + 1 f₂ - d f₁ f₂ and solving, we get f ₁, f ₂ 18 ~cm and 20 ~cm respectively.

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