Question

An erect image 2.0 cm high is formed 12 cm from a lens, the object being 0.5 cm high. Find the focal length of the lens.

Answer 1

Given:

Image distance, v = −12 cm (Image is erect.)

Height of the object, h = 0.5 cm

Height of the image, h' = 2.0 cm

Applying the magnification formula, we get:

m = `v/u = (h')/h`

`-12/u = 2.0/0.5`

u = −3 cm

Applying the lens formula, we get:

`1/v- 1/u = 1/f`

`1/(-12)-1/(-3) = 1/f`

`1/f = 3/12`

f = 4.0 cm