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Focal length of the objective of a terrestrial telescope is 80 cm and it is adjusted for parallel rays, then its
magnifying 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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100

124

164

answer is D.

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Detailed Solution

$\begin{array}{l} m = \frac{f_{0}}{f_{e}} \Rightarrow 20 = \frac{80}{f_{e}}; f_{e} = 4 cm \\ L = f_{0} + f_{e} + 4 f = 20 + 4 + 4 (20) = 164 cm \end{array}$

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