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The focal length of a plano-convex lens ...

The focal length of a plano-convex lens is f and its refractive index is 1.5. It is kept over a plane glass plane with its curved surface touching the glass plate. The gap between the lens and the glass plate is filled by a liquid. As a result, the effective focal length of the combination becomes 2f. Then the refractive index of the liquid is

A

`1.5`

B

2

C

`1.25`

D

`1.33`

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