First slide

The optical instruments

Question

The magnifying power of an astronomical telescope is 8 and the distance between the two lenses is 54 cm. The focal length of eye lens and objective lens will be respectively

Moderate

A

6 cm and 48 cm
B

48 cm and 6 cm
C

8 cm and 64 cm
D

64 cm and 8 cm

Solution

$f_{o} + f_{e} = 54 and \frac{f_{0}}{f_{e}} = m = 8 \Rightarrow f_{o} = 8 f_{e} \Rightarrow 8 f_{e} + f_{e} = 54 \Rightarrow f_{e} = \frac{54}{9} = 6 \Rightarrow f_{0} = 8 f_{e} = 8 \times 6 = 48$

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