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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.
Solution
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
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