question_answer

The focal length of an objective of a telescope is metre 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

Correct Answer: A

Solution :

Full use of resolving power means whole aperture of objective in use. And for relaxed vision. \[\frac{{{f}_{o}}}{{{f}_{e}}}=\frac{D}{d}\,\,\,\Rightarrow \,\,\frac{300}{{{f}_{e}}}=\frac{15}{0.3}\,\,\,\Rightarrow \,\,{{f}_{e}}=6\,cm\]