Question

an object is placed in front of a concave mirror of a focal length 20 cm . the image formed is 3 times the size of the object . calculate two possible distance of the object from the mirror.​

Answer 1

Answer:

Explanation:

It is given that,

Focal length

 $f = -20 cm$

$h_{i} = 3h_{o}$

​The image is three times the size of the object. So there are two cases.

Case 1. For real image

$m=-\frac{v}{u} =-3$

$v=3u$

Using mirror’s formula

$\frac{1}{f} =\frac{1}{v} +\frac{1}{u}$

$\frac{1}{-20} =\frac{1}{3u} +\frac{1}{u}$

$u = \frac{-80}{3} cm$

Case 2. For virtual image

$m=\frac{-v}{u} = 3$

$v=-3u$

Again using mirror’s formula

$\frac{1}{-20} =\frac{1}{-3u} +\frac{1}{u}$

$u=\frac{-40}{3} cm$

The possible values of u are $\frac{-80}{3} cm$  and  $\frac{-40}{3} cm$