The two surfaces of a biconvex lens have the same radii of curvatures. This lens is made of glass of refractive index $1.5$ and has a focal length of $10$ 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. Then they are put in contact with their convex surfaces touching each other. If this combination lens is immersed in water (refractive index $= 4 / 3$ ), its focal length (in cm) is :
$5$
$10$
$20$
$40$

Question

The two surfaces of a biconvex lens have the same radii of curvatures. This lens is made of glass of refractive index $1.5$ and has a focal length of $10$ 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. Then they are put in contact with their convex surfaces touching each other. If this combination lens is immersed in water (refractive index $= 4 / 3$ ), its focal length (in cm) is :
$5$
$10$
$20$
$40$

Toppr Admin · Accepted Answer