Question

A convex mirror has a focal length $20\text{cm}$. A convergent beam tending to converge to a point $20\text{cm}$ behind the mirror on principal axis falls on it. The image is formed at

• A. Infinity
• B. $40\text{cm}$
• C. $20\text{cm}$
• D. $10\text{cm}$

Answer 1

The correct option is A Infinity

Using mirror fornula
$\frac{1}{v}+\frac{1}{u}=\frac{1}{f}$
Here we have a virtual object so sign of $u$ is positive.

Here $f=+20$,$u=+20$
$\frac{1}{v}+\frac{1}{20}=\frac{1}{20}\Rightarrow \frac{1}{v}=0$
$v=\infty$
Image is formed at very large distance from the mirror.