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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

`-20` cm

B

`25` cm

C

`-50` cm

D

50 cm

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The correct Answer is:
C
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