Question

Focal length of objective lens and eyepiece of an astronomical telescope are $100\text{cm}$ and $2\text{cm}$ respectively. Find the angular magnification produced in the case of normal adjustment:

• A. $100$
• B. $50$
• C. $200$
• D. $25$

Answer 1

The correct option is B $50$

Given,
$f_0=100\text{cm}$ and $f_e=2\text{cm}$

In case of normal adjustment, the angular magnification is given by,

$m=\frac{-f_0}{f_e}=\frac{-100}{2}=-50$

$\therefore |m|=50$

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

Key Concept: 1. In case of normal adjustment, final image obtained is at infinity. 2. Negative sign in the angular magnification represent that the final image is inverted.