Question

The size of image of an object by a convex lens of focal length 20 cm

is observed to be reduced 1/3 rd of its size.Find the distance of the

object from the optical centre of the lens.

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Answer 1

The distance of the object from the optical centre of the lens will be 40 cm

Answer 2

Answer:

The distance of the object is 80 cm.

Explanation:

Given that,

Focal length f = 20 cm

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

We know that,

The magnification is

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

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

The distance of the object is

Using lens formula

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

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

$u = -80\ cm$

Hence, The distance of the object is 80 cm.