Question

The magnifying power of a telescope is $11$. When it is adjusted for parallel rays, the distance between the objective lens and eyepiece is found to be $24\text{cm}$. The focal length of lenses are

• A. $f_e=2\text{cm}$
• B. $f_e=4\text{cm}$
• C. $f_o=18\text{cm}$
• D. $f_o=22\text{cm}$

Answer 1

Answer: (A), (D)

$f_o=22\text{cm}$

Given:
Magnifying power, $m=11$

Length of telescope, $L=24\text{cm}$

we know that,
$m=\frac{f_o}{f_e}$

$\Rightarrow 11=\frac{f_o}{f_e}$

$\therefore f_o=11f_e...\left(1\right)$

Now, $L=f_o+f_e$

$\Rightarrow 24=11f_e+f_e$

$\Rightarrow f_e=2\text{cm}$

From equation $\left(1\right)$,
$f_o=22\text{cm}$

Hence, options $\left(A\right)$ and $\left(D\right)$ are correct.