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The refractive index of a converging lens is 1.4. What will be the focal length of this lens if it is placed in a medium of same refractive index ? (Assume the radii of curvature of the faces of lens are R1 and R2 respectively)

1. 1
2. R1R2/(R1 - R2)
3. Infinite
4. Zero

Correct answer: Infinite

Solution

By the lensmaker formula 1/f = (n_lens/n_medium - 1)(1/R1 - 1/R2). When the lens is in a medium of the same refractive index, n_lens/n_medium = 1, so 1/f = 0 and the focal length is infinite.

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