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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 :

1. f 1 + f 2
2. f 1 – f 2
3. 1 2 – R μ μ
4. 2 1 2 1 f f f f 2 +

Correct answer: 1 2 – R μ μ

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