Question

An object is placed at a distance of $30\text{cm}$ from a concave mirror and its real image is formed at a distance of $30\text{cm}$ from the mirror. The focal length of the mirror is

• A. $15\text{cm}$
• B. $45\text{cm}$
• C. $30\text{cm}$
• D. $20\text{cm}$

Answer 1

The correct option is A $15\text{cm}$

Here, object distance = image distance = $30\text{cm}$
$\Rightarrow$ It means object is placed at centre of curvature of concave mirror.
$\therefore R=30\text{cm}$
$2f=30\text{cm}$
or, $f=\frac{30}{2}=15\text{cm}$
Focal length is $15\text{cm}$