JAMB Physics 2021 Paper

Concept:A spherical mirror is concave when its reflecting surface is the inner curved surface of the sphere.Explanation:The inside of the hollow sphere is polished, so the reflecting surface is the inner concave surface.Therefore, the polished portion forms a concave mirror.For a spherical mirror, the focal length is related to the diameter by:$$f = \frac{d}{4}$$Given diameter $d = 20\text{ cm}$:$$f = \frac{20}{4} = 5\text{ cm}$$Answer:Concave mirror of focal length $5\text{ cm}$. Option A.