Question

A real image ,4/5 size of the object is formed at a distance of 18 cm from a lens .calculate the focal length of lens?

Answer 1

Since the image is real and diminished, the lens must be convex and the object must be placed beyond 2F.

Given:

v=+18 cm

m=h'/h=4/5

we have, m=h'/h = v/u

v/u = 4/5     ⇒ 4u= -5u

4u = -5 × 18

u = -22.5 cm

also focal length is give by

1/v - 1/u = 1/f

1/18 - 1/-22.5 = 1/f

1/18 + 1/22.5 = 1/f

9/90 = 1 /f

∴ f = 10 cm