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

Answer

A

Explanation

Magnification for parallel rays,
`m=(f_(v))/(f_(e))Rightarrow 20=(80)/(f_(e)) or f_(e)=40cm`
If the focal length of erecting lens is 20cm, then the length of the telescope
`L_(v)=f_(v)+4f+f_(v)`(where f is the focal length of erecting lens)
`=80+4xx20+4`
=80+80+4=164cm