The power of a lens is defined as the reciprocal of its focal length in metres: P = \(\frac{100}{f}\) → 2.5 = \(\frac{100}{f}\) where P is the power in diopters and f is the focal length in metres. 2.5 * f = 100 f = \(\frac{100}{2.5}\) f = 40 Where f = \(\frac{r}{2}\) radius of the curvature[r] = f * 2 → 40 * 2 r = 80cm