Question

The focal length of the objective of a telescope is $60$ cm. To obtain a magnification of $20$ for the relaxed eye, the length of telescope should be

• A. $80cm$
• B. $40cm$
• C. $63cm$
• D. $57cm$

Answer 1

Answer: (C)

$63cm$

$f_o=60cm,m=20$
For relaxed eye, i.e. for normal adjustment
$m=\frac{f_o}{f_e}$
$\Rightarrow f_e=\frac{f_o}{m}=\frac{60}{20}=3$
Length of the telescope tube $=L=f_o+f_e=60+3=63cm$.