Question

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

Answer 1

Given :

Least distance of clear vision, $$D = 25$$ cm

Magnifying power, $$m = 3$$

Let the focal length be f.

When the image is formed at $$D = 25$$ cm, the magnifying power of the simple microscope is given by

$$m=1+\dfrac Df$$

$$3=1+\dfrac{25}f$$

$$\dfrac{25}f=2$$

$$f=25/2=12.5$$cm

When the image is formed at infinity in normal adjustment, the magnifying power is given by

$$m=\dfrac Df$$

$$m=\dfrac {25}{12.5}$$

$$m=2.0$$

So, the magnifying power is 2 if the image is formed at infinity.