First slide

Ray optics

Question

The magnifying power of an astronomical telescope for normal vision is 7 and its tube length is 1.6 m. Then focal lengths of eye piece and objective are

Moderate

A

20 cm and 160 cm
B

20 cm and 140 cm
C

15 cm and 105 cm
D

10 cm and 70 cm

Solution

for normal vision $M = \frac{f_{0}}{f e} and L = f_{0} + f_{e}$
$∴ L = f_{e} (1 + M) \Rightarrow f_{e} = \frac{L}{1 + M} = \frac{160}{(1 + 7)} cm = 20 cm$
and $f_{0} = 7 \times 20 cm = 140 cm$

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