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?

• A. $8\text{cm}$
• B. $2.5\text{cm}$
• C. $12.5\text{cm}$
• D. $8.3\text{cm}$

Answer 1

The correct option is C $12.5\text{cm}$

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.