The anomalous diffraction approximation is valid for large and optically soft spheres. The extinction efficiency in this approximation is given by Q = 2 - \frac{4}{p} \sin{p} + \frac{4}{p^2} (1-\cos{p}), where Q is the efficiency factor of scattering, which is defined as the ratio of the scattering cross section and geometrical cross section πa2;