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