WAEC Physics 2024 Paper

Concept:For a convex lens to form a real image on a screen, the object and screen must be separated by at least four times the focal length.Explanation:Let $u$ be the object distance and $v$ be the image distance from the lens.For a real image, both $u$ and $v$ are positive.Using the lens formula: $\frac{1}{f} = \frac{1}{u} + \frac{1}{v}$.The distance between the object and the screen is $D = u + v$.From the formula, $\frac{1}{f} = \frac{u+v}{uv} = \frac{D}{uv}$, so $uv = fD$.Since $u+v \ge 2\sqrt{uv}$, we get $D \ge 2\sqrt{fD}$.Squaring both sides gives $D \ge 4f$.The minimum separation is achieved when $u = v = 2f$, giving $D = 4f$.Hence, to form a real image, the object-screen distance must be four times the focal length.Answer:D. four times the focal length of the lens.