JEE PHYSICS MCQ: RAY OPTICS

The two surfaces of a biconvex lens has same radii of curvatures. This lens is made of glass of refractive index $\mathrm{1}\mathrm{.5}$ and has a focal length $\mathrm{10}\,\,\mathrm{cm}$ in air. The lens is cut into two equal halves along a plane perpendicular to its principle axis to yield two plano-convex lenses. The two pieces are glued such that the convex surfaces touch each other. If this combination lens is immersed in water (refractive index $\mathrm{ = 4}\,\mathrm{/}\,\mathrm{3}$), its focal length (in cm) is