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The power of a lens (biconvex) is 1.25 m –1 in particular medium. Refractive index of the lens is 1.5 and radii of curvature are 20 cm and 40 cm respectively. The refractive index of surrounding medium

1. 1.0
2. 7 9
3. 2 3
4. 3 4

Correct answer: 7 9

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