First slide

The optical instruments

Question

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

Moderate

A

21
B

12
C

18
D

15

Solution

$m = \frac{f_{0}}{f_{e}} (1 + \frac{f_{e}}{D}) \Rightarrow m = \frac{90}{6} (1 + \frac{6}{30}) \Rightarrow m = 18$

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