First slide

Ray optics

Question

The focal length of the objective of a terrestrial telescope is 80 cm and it is adjusted for normal vision to produce a magnification of 20. If the focal length of erecting lens is 20cm, then full length of telescope tube will be ___

Easy

A

84cm
B

100cm
C

124 cm
D

164 cm

Solution

For normal vision,
$\frac{f_{o}}{f_{e}} = m \Rightarrow \frac{80}{f_{e}} = 20 \Rightarrow f_{e} = 4 cm ∴ L = f_{o} + 4 f + f_{e} = (80 + 4 \times 20 + 4) cm = 164 cm$

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