Question

An astronomical telescope has a length of 44 cm and ten-fold magnification. The focal length of the objective lens is

• A. $4$ cm
• B. $40$ cm
• C. $44$ cm
• D. $440$ cm

Answer 1

The correct option is B $40$ cm

Length of the telescope $L=f_o+f_e$

$\therefore$ $f_o+f_e=44$ cm

We get $f_e=44-f_o$

Magnification $|M|=\frac{f_o}{f_e}$

$\therefore$ $10=\frac{f_o}{44-f_o}$

$\therefore$ $f_o=\frac{440}{11}=40$ cm