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Two identical thin planoconvex glass len...

Two identical thin planoconvex glass lenses (refractive index `1.5)` each having radius of curvature of `20cm` are placed with their convex surfaces in contact at the centre. The intervening space is filled with oil of refractive index `1.7`. The focal length of the combination is

A

`-25 cm `

B

` -50 cm `

C

` 50 cm`

D

`-20 cm `

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The correct Answer is:
B
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