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Telescopes

In a terrestr...

Question

In a terrestrial telescope, the telescope, the focal length of objective is 90 cm of inverting lens is 5 cm and of eye lens is 6 cm. If the final image is at 30 cm, then the magnification will be

A

21

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B

12

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C

15

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D

18

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Solution

The correct option is B 18
As we know,
$m = \frac{f_{o}}{f_{o}} (1 + \frac{f_{e}}{D}) \Rightarrow m = \frac{90}{6} (1 + \frac{6}{30}) \Rightarrow m = 18$

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