Q. The focal length of an objective of a telescope is 3 $metre$ 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) $6\ cm$
B) $6.3\ cm$
C) 20 $cm$
D) 60 $cm$
Explanation
(a) Full use of resolving power means whole aperture of objective in use. And for relaxed vision
$\frac{f_{o}}{f_{e}} = \frac{D}{d} \Rightarrow \frac{300}{f_{e}} = \frac{15}{0.3} \Rightarrow f_{e} = 6\ cm$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.