Two identical thin planoconvex glass len...

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

`-25 cm`

`-50 cm`

50 cm

`20 cm `

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