WAEC Physics 2024 Paper

Concept:Polishing the inside of a hollow sphere creates a reflecting surface on the inner curved side, which behaves as a concave mirror.Explanation:Diameter of the sphere is $20\ \text{cm}$.So, radius of curvature, $R = \dfrac{20}{2} = 10\ \text{cm}$.Focal length, $f = \dfrac{R}{2} = \dfrac{10}{2} = 5\ \text{cm}$.Since the inside surface is polished, the reflecting surface is the concave inner surface.Therefore, it forms a concave mirror with focal length $5\ \text{cm}$.Answer:A. Concave mirror of focal length 5 cm.