First slide

The optical instruments

Question

The focal length of the objective of a terrestrial telescope is 80 cm and it is adjusted for parallel rays, then its magnifying power is 20. If the focal length of erecting lens is 20 cm, then full length of telescope will be

Moderate

A

84 cm
B

100 cm
C

124 cm
D

164 cm

Solution

$m = \frac{f_{o}}{f_{e}} \Rightarrow \frac{80}{f_{e}} = 20 \Rightarrow f_{e} = 4 cm Hence length of terrestrial telescope = f_{0} + f_{θ} + 4 f = 80 + 4 + 4 \times 20 = 164 cm$

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