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