Question

An object of height $5\text{cm}$ has been kept at a distance of $20\text{cm}$ in front of a concave mirror with radius of curvature $40\text{cm}$. Which of the following is incorrect as per sign convention?

• A. Focal length of mirror $f=+20\text{cm}$
• B. Object distance is $u=-20\text{cm}$
• C. Height of the object is $h_o=-5\text{cm}$
• D. Radius of curvature of mirror is $R=-40\text{cm}$

Answer 1

The correct option is A Focal length of mirror $f=+20\text{cm}$

As per sign convention,
(1) All distances are measured from the pole.
(2) 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 $u=-20\text{cm}$
Radius of curvature of mirror is $R=-40\text{cm}$
Focal length of mirror $f=\frac{R}{2}=-20\text{cm}$