Question

Focal length of the objective of a terrestrial telescope is $80\text{cm}$ and it is adjusted for parallel rays, then its magnifying power is $20$. If the focal length of erecting lens is $20\text{cm},$ then full length of telescope will be ( in $\text{cm}$)

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

Answer 1

The correct option is A $164$

Given that
$m=20;f_o=80\text{cm};f=20\text{cm}\text{(erecting lens)}$
Magnification
$m=\frac{f_o}{f_e}\Rightarrow 20=\frac{80}{f_e}\Rightarrow f_e=\frac{80}{20}=4\text{cm}\text{(eye piece)}$
Length of terrestial telescope
$L=fo+4f+fe$
$L=80+(4\times 20)+4$
$L=80+80+4$
$L=164\text{cm}$