Question

size of image of an object by convex lens of focal length 20cm is observed to be reduced to one upon third of its side find the distance of the object from the optical centre of a lens

Answer 1

Distance of object from optical centre = 40 cm

Answer 2

Answer:

The distance of the object from the optical center of a lens is 40 cm.

Explanation:

Given that,

Focal length f = 20 cm

Magnification $m = \dfrac{1}{3}$

Magnification is

$m= \dfrac{1}{3}$

$\dfrac{v}{u}=\dfrac{1}{3}$

$v= -\dfrac{1}{3}u$

Using lens's formula

$\dfrac{1}{f}=\dfrac{1}{v}-\dfrac{1}{u}$

$\dfrac{1}{20}=-\dfrac{3}{u}-\dfrac{1}{u}$

$\dfrac{1}{u}=-\dfrac{1}{40}$

$u = -40 cm$

Hence, The distance of the object  from the optical center of a lens is 40 cm.