Question

An astronomical telescope has an eye piece of focal length $5cm$. If magnification produced is 14 in normal adjustment, then calculate the length of the telescope.

• A. $75cm$
• B. $9cm$
• C. $50cm$
• D. $55cm$

Answer 1

Answer: (A)

$75cm$

Given, focal length of the eyepiece of an astronomical telescope is $f_e=5cm$,

And, in normal adjustment, magnification is $m=14$.

Now, from the formula, $m=\frac{f_o}{f_e}$, we get,

$14=\frac{f_o}{5}$

$\therefore f_o=14\times 5cm=70cm$

Now, length of the telescope is $L=f_o+f_e=(5+70)cm=75cm$

Thus, the correct option is (A).