WAEC Physics 2022 Paper

Concept:The power of a lens gives its focal length, but the radius of curvature of a lens also depends on the lens shape and refractive index.Explanation:For a lens, power is given by:$$P = \frac{1}{f}$$where $P$ is in dioptres and $f$ is in metres.Given $P = +2.5\,\text{D}$:$$f = \frac{1}{2.5} = 0.4\,\text{m} = 40\,\text{cm}$$The relation $f = \frac{r}{2}$ is valid for spherical mirrors, not for a lens.A lens has two spherical surfaces, and each has its own radius of curvature.To find the radius, the lens maker’s formula is needed:$$P = (n - 1)\left(\frac{1}{R_1} - \frac{1}{R_2}\right)$$Since the refractive index $n$ and the lens geometry are not given, the radius cannot be uniquely determined.Answer:The radius of curvature cannot be determined from the given information; none of the options A–D is correct.