Question

An astronomical telescope, consists of two thin lenses set 36 cm apart and has a magnifying power 8. Calculate the focal length of the lenses.

Answer 1

In normal adjustment the image forms at infinity

So $f_o+f_e=36$ , o is for objective and and e for eye piece

Also magnification is 8

$8=\frac{f_o}{fe}$

$f_o=8f_e$

on solving we get

$f_o=32cm$

$f_e=4cm$