Question

A plano-convex lens is made of material having refractive index 1.5. The radius of curvature of curved surface is 40 cm. The focal length of the lens is ____________ cm.

Options

• - 80

• 120

• 80

• - 120

Answer 1

A plano-convex lens is made of material having refractive index 1.5. The radius of curvature of curved surface is 40 cm. The focal length of the lens is 80 cm.

Explanation:

The focal length of a plano-convex lens is,

`"f" = "R"/"n" - 1`

`therefore "f" = 40/1.5 - 1`

` = 40/0.5`

` = 80  "cm"`