An object is placed 12 cm in front of a convex mirror of focal length 18 cm, the value of the image distance is

7.2 cm

The correct answer is option (3) : 7.2 cm

For, mirror, $\frac{1}{v}+\frac{1}{u}=\frac{1}{f}$

We have been given a convex mirror, $⇒f=18\, cm$

$\frac{1}{v}+\frac{1}{u}=\frac{1}{f}$ and $f=18\, cm , u = -12\, cm$

$\frac{1}{v}-\frac{1}{12}=\frac{1}{18}⇒\frac{1}{v}=\frac{1}{18}+\frac{1}{12}$

$⇒\frac{1}{v}=\frac{30}{12\times 18}⇒v=7.2 \,cm$