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MHT CET202124 Sep 2021Morning ShiftPhysicsRay OpticsActual

A plano-convex lens of refractive index ( ₁ ' fits exactly into a plano-concave lens of refractive index ₂ . Their plane surfaces are parallel to each other. ' R ' is the radius of curvature of the curved surface of the lenses. The focal length of the combination is

Options

  1. AR ₁- ₂
  2. BR 2 ( ₁+ ₂ )
  3. C2 R ₁- ₂
  4. DR 2 ( ₁- ₂ )

Correct answer

A. R ₁- ₂

Step-by-step solution

aligned & 1 f ₁ = ( ₁-1 ) ( 1 R ₁ - 1 R ₂ )= ( ₁-1 ) R & 1 f ₂ = ( ₂-1 ) ( 1 R ₁ - 1 R ₂ )= ( ₂-1 ) - R & 1 f = 1 f ₁ + 1 f ₂ = ₁-1 R - ₂-1 R = 1 R [ ₁-1- ₂+1 ]= ₁- ₂ R & f = R ₁- ₂ aligned

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