Question

The focal length of an objective of a telescope is 3 meter and diameter 15 cm. Assuming for a normal eye, the diameter of the pupil is 3 mm for its complete use, the focal length of eye piece must be

• A. $6cm$
• B. $6.3cm$
• C. $20cm$
• D. $60cm$

Answer 1

The correct option is A $6cm$

Full use of resolving power means whole aperture of objective in use. And for relaxed vision.
$\frac{f_o}{f_e}=\frac{D}{d}\Rightarrow \frac{300}{f_e}=\frac{f_o}{0.3}\Rightarrow f_e=6cm$