Optics Question 109
Question: 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
[MP PET 1989]
Options:
A) 6 cm
B) 6.3 cm
C) 20 cm
D) 60 cm
Show Answer
Answer:
Correct Answer: A
Solution:
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 $
