Question

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

Answer 1

It is given that,

Focal length

f=−20cm

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=−

u

v

=−3

v=3u

Using mirror’s formula

f

1

=

v

1

+

u

1

−20

1

=

3u

1

+

u

1

u=

3

−80

cm

Case 2. For virtual image

m=

u

−v

=+3

v=−3u

Again using mirror’s formula

−20

1

=

−3u

1

+

u

1

u=

3

−40

cm

The possible values of u are

3

−80

cmand

3

−40

cm.