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. \[\frac{{{f}_{o}}}{{{f}_{e}}}=\frac{D}{d}\]\[\Rightarrow \frac{300}{{{f}_{e}}}=\frac{15}{0.3}\Rightarrow {{f}_{e}}=6\ cm\]