Question

The magnifying power of a telescope is $11$, when it is in normal adjustment. The distance between the objective and the eyepiece is found to be $24\text{cm}$. The focal lengths of objective and eyepiece lenses are, respectively,

• A. $18\text{cm},6\text{cm}$
• B. $22\text{cm},2\text{cm}$
• C. $16\text{cm},8\text{cm}$
• D. $22\text{cm},4\text{cm}$

Answer 1

The correct option is B $22\text{cm},2\text{cm}$

Given:
Magnifying power, $m=11$
Length of telescope, $L=24\text{cm}$
we know, $m=\frac{f_o}{f_e}$
$\Rightarrow 11=\frac{f_o}{f_e}$

$\therefore f_o=11f_e$

Now, for normal adjustment, $L=f_o+f_e$

$\Rightarrow 24=11f_e+f_e$

$\therefore f_o=22\text{cm}$ and $f_e=2\text{cm}$.

Hence, $\left(B\right)$ is the correct answer.

Why this question : Concept: When telescope is adjusted for parallel rays, it means that images is seen in normal adjustment.