Question

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

Answer 1

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Answer 2

Answer:

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

Explanation:

Given that,

Focal length f = 20 cm

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

The magnification is the ratio of the distance of the image and the distance of the object.

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

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

Using lens formula

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

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

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

$u = -80\ cm$

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