Question

A thin convex lens of focal length 10 cm is .placed in contact of a thin concave lens of focal length 15 cm. Find:

1. The focal length
2. Power, and
3. The nature of the combined lens system.

Answer 1

(a) The focal length of the combination is given by

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

Here, f₁ = 10 cm (convex) and f₂ = - 15 cm (concave)

`therefore 1/"f" = 1/10 - 1/15 = (3 - 2)/30 = 1/30`

or f = 30 cm = 0.3 m

(b) The power of the combination is

P = `1/("f" ("in meter")) = 1/0.3`D = + 3.33 D

(c) The combination is a converging lens of focal length 30 cm.