Geometrical OpticsHard
Question
Focal length of the objective and eye-piece of an astronomical telescope are 20 cm and 10cm. If final image is at the minimum distance of clear vision. then length of the tube will be:-
Options
A.27.1 cm
B.25 cm
C.30 cm
D.None
Solution
for eye-piece 
Ue = - 7.1 cm
so length of the tube
L = |f0| + |ue| = 20 + 7.1 = 27.1cm
Ue = - 7.1 cm
so length of the tube
L = |f0| + |ue| = 20 + 7.1 = 27.1cm
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