Question

an object of height 5cm is placed 20cm in front of a concave mirror of focal length 15 cm at what distance from the mirror should a screen be placed to obtain a sharp image

Answer 1

Given Mirror : Concave Mirror.

Mirror Formula:
$\frac{1}{f} = \frac{1}{v} + \frac{1}{u}$


According to New Cartesian Sign Convention, the values of u, v and f are taken as negative.

The given values are
u = - 20 cm
f = - 15 cm

To find : Distance of the Image ( v )

Now Substitution,
$\frac{1}{ - 15} = \frac{1}{v} + \frac{1}{ - 20}$

$\frac{1}{v} = \frac{1}{ - 15} + \frac{1}{20}$

$\frac{1}{v} = \frac{ - 20 + 15}{300}$

$\frac{1}{v} = \frac{ - 5}{300}$

$v = \frac{300}{ - 5}$

v = -60 cm

The distance of image from the concave mirror is - 60 cm.