Question

The focal length of the objective of a terrestrial 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

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

Answer 1

Answer: (A)

$164cm$

Magnification for parallel rays
$m=\frac{f_o}{f_e}$
$\Rightarrow 20=\frac{80}{f_e}$
or $f_e=4cm$
If the focal length of erecting lens is $20cm$ then the length of the telescope
$L_{\infty }=f_o+4f+f_e$
[where $f$ is the focal length of erecting lens]
$=80+4\times 20+4=164cm$