An object is placed at a distance of 30 cm from a concave mirror of focal length 20 cm. Where will image be formed?

We have, u = – 30 cm and f = – 20 cm.

$$\begin{gathered} \text{ Also, }\frac{1}{\mathrm{v}}+\frac{1}{\mathrm{u}}=\frac{1}{\mathrm{f}}\text{ or }\frac{1}{\mathrm{v}}=\frac{1}{\mathrm{f}}-\frac{1}{\mathrm{u}}=\frac{1}{-20 \mathrm{cm}}-\frac{1}{-30 \mathrm{cm}}=-\frac{1}{60 \mathrm{cm}} \\ \Rightarrow \mathrm{v}=-60 \mathrm{cm} \end{gathered}$$

So, the image will be formed 60 cm from the mirror.

Since v has a negative sign, the image is formed to the left of the mirror, i.e., in front of it.