Question

An astronomical telescope has a magnifying power $10$. The focal length of the eye piece is $20$ cm. The focal length of the objective is -

• A. $2$ cm
• B. $200$ cm
• C. $\left(\frac{1}{2}\right)\text{cm}$
• D. $\left(\frac{1}{200}\right)\text{cm}$

Answer 1

The correct option is B $200$ cm

magnification formula $\frac{f_0}{f_e}=M$
$f_0$ = focal length of objective
$f_e$ = focal length of eye - piece
$\frac{f_0}{f_e}=10$
focal length of objective is $f_0=10f_e=10\times 20=200\text{cm}$