Question

An object $AB$ of height $5\text{cm}$ has been kept at a distance of $30\text{cm}$ in front of a concave mirror with radius of curvature $50\text{cm}$. Which of the following is incorrect as per sign convention?

• A. Focal length of mirror is $f=+25\text{cm}$
• B. Object distance is $u=-30\text{cm}$
• C. Focal length of mirror is $f=-25\text{cm}$
• D. Radius of curvature of mirror is $R=-50\text{cm}$

Answer 1

The correct option is A Focal length of mirror is $f=+25\text{cm}$

As per sign convention,
$\left(1\right)$ All distances are measured from the pole.
$\left(2\right)$ Distance in the direction of the incident ray is taken as positive and in the opposite is taken as negative.

Based on these rules,
Object distance is $u=-30\text{cm}$
Radius of curvature of mirror is $R=-50\text{cm}$
Focal length of mirror is $f=\frac{R}{2}=-25\text{cm}$