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Q.

The focal length of the objective of a terrestrial telescope is 80 cm and it is adjusted for normal vision to produce a magnification of 20. If the focal length of erecting lens is 20cm, then full length of telescope tube will be ___

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a

164 cm

b

100cm  

c

84cm

d

124 cm

answer is D.

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Detailed Solution

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For normal vision,

fofe = m  ⇒ 80fe = 20    ⇒ fe = 4cm ∴ L = fo+4f+fe = 80+4×20+4cm = 164cm

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