Question

The magnifying power of an astronomical telescope can be increased if we :

• A. increase the focal length of the objective
• B. increase the focal length of the eye-piece
• C. decrease the focal length of the objective
• D. decrease the focal length of the objective and at the same time increase the focal length of the eye piece

Answer 1

The correct option is A increase the focal length of the objective

The magnifying power of astronomical telescope:
$m=\frac{-f_0}{f_e}(1+\frac{f_e}{D})$
where $f_0$ is the focal length of the object
where, $f_e$ is the focal length of eyepiece
$D$ is least distance of clear vision
So, clearly increasing the focal length of objective increases the magnifying power of the telescope. Increasing the focal length of the eye-piece decrease the magnifying power of telescope.