First slide

The optical instruments

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

Easy

A

6 cm
B

6.3 cm
C

20 cm
D

60 cm

Solution

Full use of resolving power means whole aperture of objective in use. And for relaxed vision.
$\frac{f_{o}}{f_{e}} = \frac{D}{d} \Rightarrow \frac{300}{f_{e}} = \frac{15}{0.3} \Rightarrow f_{e} = 6 c m$

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