First slide

The optical instruments

Question

The magnifying power of a telescope is 9. When it is adjusted for parallel rays, the distance between the objective and the eye-piece is found to be 20 cm. The focal length of the two lenses are

Moderate

A

18 cm, 2 cm
B

11 cm, 9 cm
C

10 cm, 10 cm
D

15 cm, 5 cm

Solution

$In this case | m | = \frac{f_{o}}{f_{e}} = 5 - - - - (i) and length of telescope = f_{0} + f_{e} = 36 - - - (ii) Solving (i) and (ii), we get f_{e} = 6 cm, f_{0} = 30 cm .$

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