Question

What focal length should the reading spectacles have for a person for whom the least distance of distinct vision is $50cm$?

Answer 1

Let focal length is $f$

$\begin{array}{c}u=-50\\ v=-25\\ \frac{1}{v}-\frac{1}{u}=\frac{1}{f}\\ \Rightarrow -\frac{1}{25}+\frac{1}{50}=\frac{1}{f}\\ \Rightarrow -\frac{1}{50}=\frac{1}{f}\\ Hence,\\ \Rightarrow f=-50cm\end{array}$

$\therefore concavelens$