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

Options

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

Answer

D

Explanation

(d) For terrestrial telescope magnification,
`M=f_(o)/(f_(e))=80/(f_(e))=20`
`rArr f_(e)=4 cm`
Hence, length of terrestrial telescope
=`f_(o)+f_(e)+4f`
`=80+4+4xx20`
=164 cm