Question

The focal length of a concave lens is 20 cm. The focal length of a convex lens is 25 cm. These two are placed in contact with each other. What is the power of the combination? Is it diverging, converging or undeviating in nature?

Answer 1

Given f₁ = - 20 cm (concave lens), h = 25 cm (convex lens)

The focal length of the combination is,

`1/"F" = 1/"f"_1 + 1/"f"_2`

`= 1/(- 20) + 1/25`

`= - 1/100`

or F = - 100 cm = - 1.0 m

The lens system is a diverging lens.

Power of the lens system is

P = `1/("F" ("in meter")) = - 1/1.0` = - 1.0 D