JEE Main & Advanced
Physics
Ray Optics
Question Bank
Critical Thinking
question_answer
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]
A) 6 cm
B) 6.3 cm
C) 20 cm
D) 60 cm
Correct Answer:
A
Solution :
Full use of resolving power means whole aperture of objective in use. And for relaxed vision. \[\frac{{{f}_{o}}}{{{f}_{e}}}=\frac{D}{d}\]\[\Rightarrow \frac{300}{{{f}_{e}}}=\frac{15}{0.3}\Rightarrow {{f}_{e}}=6\ cm\]