A person can see clearly only up to a distance of 30 cm. He wants to read a book placed at a distance of 50 cm from his eyes. What is the power of the lens he requires for his spectacles?

The person is myopic. This defect is corrected by using a concave lens. Putting $\mathrm{u}=-50\mathrm{cm}$ and $\mathrm{v}=-30\mathrm{cm}$ in the lens formula $\frac{1}{\mathrm{v}}-\frac{1}{\mathrm{u}}=\frac{1}{\mathrm{f}}$ gives $\mathrm{f}=-75\mathrm{cm}$. Therefore, the power of the lens = -1.33 D.