Question

A simple microscope has a magnifying power of $3.0$ when the image is formed at the near point $\left(25\text{cm}\right)$ of a normal eye. What is its focal length?

Answer 1

Given,
Magnifying power, $m=3$, when the image is formed at $D$.

We know,

$m=1+\frac{D}{f}$

$\Rightarrow 3=1+\frac{25}{f}$ $(\because D=25\text{cm})$

$\Rightarrow \frac{25}{f}=2$

$\therefore f=12.5\text{cm}$

Hence, $\left(C\right)$ is the correct answer.