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Question

The focal lengths of the lens of an astronomical telescope are 50 cm and 5 cm. The length of the telescope when the image is formed at the least distance of distance vision is ( in cm )

A
2506
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B
2756
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C
3256
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D
3506
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Solution

The correct option is C 3256
focal length of objective fo=50 cm
focal length of eye piece fe=5 cm
Length of telescope is L=f0+|ue|
for least distance of distinct vision D=|ve|=25 cm

Lens formula at eye piece
1fe=1ve1ue
15=1251ue
1ue=[125+15]=625
ue=256 cm
L=f0+ue=50+256=3256cm

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