Question

An object is held in front of a concave mirror of focal length `15 cm`. The image formed is `3` times the size of the object. Calculate two possible distances of the object from the mirror.

Answer 1

Correct Answer - `- 10 cm ; -20 cm`
Here, `f = -15 cm, m = +- 3, u = ?`
As `m = -(v)/(u) = + 3, v = -3 u`
From `(1)/(v)+(1)/(u)=(1)/(f) , (1)/(-3 u) +(1)/(u)=(1)/(-15)`
or `(2)/(3 u) = (1)/(-15), u = -10 cm`
Similarly, when `m = -(v)/(u) = -3` , we get
`u = -20 cm`.