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# The two surfaces of a biconvex lens has same radii of curvatures. This lens is made of glass of refractive index 1.5 has a focal length 10 cm in air. The lens is cut into two equal halves along a plane perpendicular to its principal axis to yield two plano-convex lenses. The two pieces are glued such that the convex lenses. The two pieces are glued such that the convex surface touch each other. If this combination lens is immersed in water(refractive index=4/3), its focal length (in cm) is :

$\begin {array} {1 1} (a)\;5 & \quad (b)\;10 \\ (c)\;20 & \quad (d)\;40 \end {array}$

(d) 40

+1 vote