Question

the size of image of an object by a convex lens of f=20cm is observed to be reduced to 1/3 rd of its size. find 'u'.

Answer 1

$\textbf{Thank\:You\:Mate}$

Answer 2

Answer: The distance of the object is 80 cm.

Explanation:

Given that,

Focal length f = 20 cm

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

The magnification is

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

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

$u = -3v$.....(I)

Using lens formula,

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

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

$v = \dfrac{80}{3}$

Put the value of v in equation (I)

$u = -80 cm$

Hence, The distance of the object is 80 cm.