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Q.

Focal length of the objective of a terrestrial telescope is 80 cm and it is adjusted for parallel rays, then its magnfying power is 20. If the focal length of erecting lens is 20 cm, then full length of telescope will be (in cm)

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a

124

b

164

c

84

d

100

answer is D.

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Detailed Solution

Magnification for parallel rays=fofe=20

fo =80 so fe=4 total length of the telescope fo+4f+fe

f  is the focal length of the erecting lens

length of the telescope =80+4(20)+4=164 cm

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