You are given two converging lenses of focal lengths 1.25 cm and 5 cm to design a compound microscope. If it is desired to have a magnification of -30, find out the separation between the objective and the eyepiece.

Given, $f_e=5cm;f_o=1.25cm$ and M=-30.

Let L be the tube length (distance between the objective and the eyepiece)

$\therefore M=\frac{-L}{f_o}(1+\frac{d}{f_e})$

[d is a constant and equal to the normal distance of clear vision of the human eye]

Hence, $-30=-\frac{L}{1.25}(1+\frac{25}{5})$

$\therefore L=\frac{30\times 1.25}{6}$

$=6.25cm$

Hence, the the separation between the objective and the eyepiece should be 6.25 cm to get the desired magnification.