Question

Paraxial rays parallel to each other but not parallel to the principal axis fall on a concave mirror as shown in the figure. Find the perpendicular distance of the image from the principal axis.

• A. $f\cot \alpha$
• B. $f\alpha$
• C. $f$
• D. None of the above

Answer 1

The correct option is B $f\alpha$

Since rays are parallel to each other but not parallel to principal axis, they meet on the focal plane upon reflection.


Let $I$ be the image formed on the focal plane.

From $△POI$:

$\tan \alpha =\frac{h}{f}....\left(1\right)$
Since rays are paraxial, $\alpha$ is very small $\therefore \tan \alpha \approx \alpha$

So, we can rewrite $\left(1\right)$ as

$\alpha \approx \frac{h}{f}\Rightarrow h\approx \alpha f$

Hence, option (b) is the correct answer.

Let's try :- If two paraxial rays are parallel to each other as well as parallel to principal axis, where does the rays meet on the principal axis?