Question

A simple microscope has a magnifying power of $3.0$ when the image is formed at the near point of a normal eye. What is its focal length?

• A. $15.5\text{cm}$
• B. $12.5\text{cm}$
• C. $18.5\text{cm}$
• D. $17.5\text{cm}$

Answer 1

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

Given:

Magnifying power, $m=3$

We know, when the image is formed at near point of a normal eye, the magnifying power is,

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

$\Rightarrow 3=1+\frac{25}{f}$

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

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