Question

The focal length of objective of telescope is 60 cm. To obtain magnification of 20 focal length of eye piece should be?

Answer 1

If the telescope is in normal mode, angular magnification is given by $M = \frac{f_o}{f_e}$ where $f_o$ is the focal length of objective lens and $f_e$ is the focal length of eye piece lens.
Given that M = 20
Focal length of objective lens, $f_o =$60 cm.
From this, focal length of eye piece, $f_e = \frac{f_o}{M} = \frac{60 cm}{20} = 3 cm.$
So to obtain magnification of 20 focal length of eye piece should be 3 cm.