Question

the size of image of an object by a convex lens of focal length 20 cm is observed to be reduced to 1 /3rd of its size. find the distance of the object from the optical centre of the lens​

Answer 1

Here is ur answer         they have given convex lens so yhe focal length will be +ve

Answer 2

Answer:

The distance of the object from the optical center of the lens is 80 cm.

Explanation:

Given that,

Focal length f = 20 cm

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

Magnification is

$m= -\dfrac{v}{u}$

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

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

Using lens formula

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

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

$\dfrac{-4}{u}=\dfrac{1}{20}$

$u = -80\ cm$

Hence, The distance of the object from the optical center of the lens is 80 cm.