Question

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

Answer 1

First case when the image is virtual, m = 3

m = − `"v"/"u"`

3 = − `"v"/"u"`

v = -3u

`1/"f" = 1/"u" + 1/"v"`

`- 1/20 = 1/"u" - 1/"3u"`

`"3u"/4` = - 20

3u = - 40

u = – `40/3` cm

Second case when the image is real

m = -3

m = − `"v"/"u"`

-3 = − `"v"/"u"`

V = 3u

`1/"f" = 1/"u" + 1/"v"`

`- 1/20 = 1/"u" + 1/"3u"`

`"3u"/4` = - 20

u = - `80/3` cm