Question

In a simple microscope of focal length $6.25\text{cm}$, if the final image is located at infinity then its magnifying power is(integer only)

Answer 1

The magnification of a simple microscope is given by,

$m=\frac{D}{u}$

Here, D is the least distance of the distinct vision.

It is given that the final image is located at infinity. This indicates that the object is placed at the focal point of the lens. So, $u=f$

$\Rightarrow m=\frac{D}{f}$

Here, $f=6.25\text{cm}\text{and}D=25\text{cm}$

$\Rightarrow m=\frac{D}{f}=\frac{25}{6.25}$

$\therefore m=4$

Why this question? Keep in mind that when the image is formed at the infinity, the magnification is given by $m=\frac{D}{f}$.