Question

A simple microscope has a magnifying power of 3.0 when the image in formed at the near point (25 cm) of a normal eye. (a) What is its focal length? (b) What will be its magnifying power if the image is formed at infinity?

Answer 1

The simple microscope has,

m = 3

when image is formed at D = 25 cm

(a) $m=1+\left(\frac{D}{f}\right)$

$\Rightarrow 3=1+\left(\frac{25}{f}\right)$

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

$\Rightarrow f=\frac{25}{2}=12.5cm$

(b) When the image is formed at infinity in the normal adjustment.

Magnifying power $=\frac{D}{f}=\frac{25}{12.5}=2.0$