The focal length of the objective of an astronomical telescope is 1 m and it is in normal adjustment. Initially the telescope is focused to a heavenly body. If the same telescope is to be focused to an object at a distance of 21 m from the objective, then identify the correct choice

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Eye piece should be displaced by 5 cm towards the objective

Eye piece should be displaced by 5 cm away from the objective

Eye piece should be displaced by 2 cm towards the objective

Eye piece should be displaced by 2 cm away from the objective

answer is D.

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Detailed Solution

$\frac{1}{f_{o}} = \frac{1}{u_{o}} + \frac{1}{v_{0}}$

$\frac{1}{1} = \frac{1}{21} + \frac{1}{v_{0}} ∴ v_{0} = \frac{21}{20} m$

Distance the eyepiece should be moved away $= v_{0} - f_{0} \frac{21}{20} - 1 = \frac{1}{20} m = 5 c m$

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