Question

An astronomical telescope has an angular magnification of magnitude 5 for an object. The separation between the objective and the eye-piece is 36 cm. The final image is formed at infinity. The focal length $f_0$ of the objective and $f_e$, of the eye-piece are :

• A. 45 cm and - 9 cm
• B. 50 cm and 10 cm
• C. 72 cm and 5 cm
• D. 30 cm and 6 cm

Answer 1

The correct option is D 30 cm and 6 cm

Magnification, $m=-5=\frac{-f_0}{f_e}\Rightarrow f_0=5f_e$
$\Rightarrow L=f_0+f_e=36$
$6f_e=36\therefore f_e=6cm$
$f_0=5f_e=30cm$