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BITSAT2015PhysicsRay OpticsActual

A planoconvex lens fits exactly into a planoconcave lens. Their plane surface are parallel to each other. If the lenses are made of different materials of refractive indices ₁ & ₂ and R is the radius of curvature of the curved surface of the lenses, then focal length of combination is

Options

  1. AR ₁- ₂
  2. B2 R ₁- ₂
  3. CR 2 ( ₁- ₂ )
  4. DR 2- ( ₁+ ₂ )

Correct answer

A. R ₁- ₂

Step-by-step solution

If F be the equivalent focal length, then aligned 1 F = 1 f₁ + 1 f₂ 1 F = ( ₁-1 ) ( 1 + 1 R ) + ( ₂-1 ) ( 1 -R - 1 )= ₁- ₂ R F= R ₁- ₂ aligned

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