Question

An object of size 7 cm is placed at 27 cm in front of a concave mirror of focal length 18 cm. At what distance from the mirror should a screen be placed, so that a sharp focused image can be obtained ? Find the size and the nature of the image.

Answer 1

Mirror formula :

$\frac{1}{v}+\frac{1}{u}=\frac{1}{f}$...........(1)

So, $u=-27cm$

Concave mirror have negative focal length.

$f=-18cm$

Putting the values in (1),

$\frac{1}{v}+\frac{1}{-27}=\frac{1}{-18}$

We get , $v=-54cm$

Also,

Magnification = $\frac{h_2}{h_1}=\frac{-v}{u}$

$\frac{h_2}{7.0}=-\frac{-54}{-27}$

$h_2=-14.0cm$

Image will be real and inverted and will be of size 14 cm.