Optics Question 423
Question: A telescope has an objective of focal length 50 cm and an eye piece of focal length 5 cm. The least distance of distinct vision is 25 cm. The telescope is focused for distinct vision on a scale 200 cm away. The separation between the objective and the eye-piece is
[Kerala PET 2002]
Options:
A) 75 $ cm $
B) 60 $ cm $
C) 71 $ cm $
D) 74 $ cm $
Show Answer
Answer:
Correct Answer: C
Solution:
$ f _{o}=50\ cm, $
$ f _{e}=5\ cm, $
$ D=25\ cm $ and $ u _{o}=200\ cm. $
Separation between the objective and the eye lens is $ L=\frac{u _{o}f _{o}}{(u _{o}-f _{o})}+\frac{f _{e}D}{(f _{e}+D)} $
$ =\frac{200\times 50}{(200-50)}+\frac{5\times 25}{(5+25)}=71\ cm $
