Question

From the following three lenses, two lenses are used as an eyepiece and the objective lens, to construct an astronomical telescope. Find the angular magnification for the normal adjustment.

Lenses	Power	Aperture
$L_1$	$3\text{D}$	$8\text{cm}$
$L_2$	$6\text{D}$	$1\text{cm}$
$L_3$	$10\text{D}$	$1\text{cm}$

• A. $3.33$
• B. $4.33$
• C. $5.33$
• D. $6.33$

Answer 1

The correct option is A $3.33$

In astronomical telescope :

$\left(i\right)$ Objective lens has lower power with a longer aperture.

$\left(ii\right)$ Eyepiece has greater power with a smaller aperture.

Therefore, $L_1$ should be used as objective lens while $L_3$ should be used as eyepiece.

So,

$P_o=3\text{D}$ and $P_e=10\text{D}$

Now, magnifying power for the normal adjustment,

$m=\frac{f_o}{f_e}$

$\Rightarrow m=\frac{1/f_e}{1/f_o}=\frac{P_e}{P_o}=\frac{10}{3}=3.33$

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