# The two surfaces of a biconvex lens has same radii of curvatures. This lens is made of glass of refractive index 1.5 and has a focal length 10 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 = 4/3), its focal length (in cm) is

## Question ID - 152248 | Toppr Answer The two surfaces of a biconvex lens has same radii of curvatures. This lens is made of glass of refractive index 1.5 and has a focal length 10 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 = 4/3), its focal length (in cm) is

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If a lens of focal length  is divided into two equal parts as shown in figure (i) and each has a focal length  then

, each part will have focal length 2

Now if these parts are put in contact as in figure (2), then resultant focal length of the combination will be

For this combination,

Now, if this combination is immersed in liquid, then

∴ F’