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One plano-convex and one plano-concave lens of same radius of curvature 'R' but of different materials are joined side by side as shown in the figure. If the refractive index of the material of 1 is μ 1 and that of 2 is μ 2 , then the focal length of the combination is : μ 2 μ 1 1 2

1. ) – ( – 2 R 2 1 μ μ
2. ) – ( 2 R 2 1 μ μ
3. 2 1 – R μ μ
4. 2 1 – R 2 μ μ

Correct answer: 2 1 – R μ μ

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