JAMB Physics 2024 Paper

Concept:The mirror equation $\frac{1}{f} = \frac{1}{u} + \frac{1}{v}$ relates the focal length $f$, object distance $u$, and image distance $v$ for a convex mirror.Explanation:An object in front of a mirror is taken as negative by the Cartesian sign convention, so $u = -25\,\text{cm}$.The image of a convex mirror is formed behind the mirror, so $v = +5\,\text{cm}$.Substitute these values into the mirror formula:$$\frac{1}{f} = \frac{1}{-25} + \frac{1}{5}$$Simplify the fractions:$$\frac{1}{f} = -\frac{1}{25} + \frac{5}{25} = \frac{4}{25}$$Hence,$$f = \frac{25}{4} = 6.25\,\text{cm}$$Answer:The focal length of the convex mirror is $6.25\,\text{cm}$. The correct option is B.