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

• A. 84 cm
• B. 100 cm
• C. 124 cm
• D. 164 cm

Answer 1

Answer: (D)

164 cm

Answer 2

$m = \frac{f_{o}}{f_{e}} = \frac{100}{2} = 50$cm

Hence length of terrestrial telescope

$= f_{o} + f_{e} + 4f = 80 + 4 + 4 \times 20 = 164\mspace{6mu} cm$