Question

A real image 2/3 of size of an object is formed by a convex lens when the object is at a distance of 12 cm from it
Find the focal length

Answer 1

size of image = -2/3 × size of object.

-2/3 = v/u

u = (3/2)v = -1.5v ..........(1)

image distance , v = 12cm

object distance , u = -1.5v = -1.5 × 12 = -18cm

use formula, 1/v - 1/u = 1/f

1/12 - 1/-18 = 1/f

1/12 + 1/18 = 1/f

1/f = 5/36

hence, f = 7.2 cm

Answer 2

Answer:

Explanation:

Given :-

m = - 2/3

v = + 12 cm

Solution :-

m = v/u

m = - 2/3

m = 12 u.

- 2u = 36

u = - 18 cm

1/f = 1/v - 1/u

⇒ 1/f = 1/12 - 1/- 18

⇒ 1/f = 3 + 2/36

⇒ f = 36/5

⇒ f = 7.2 cm.

Hence, The focal length is 7.2 cm.

Extra Information :-

The image formed is real (convex lens forms diminished real image.)