The Mie algorithm assumes scattering from a single sphere, but a rainbow involves scattering from millions of raindrops.  In practice, raindrops are not of uniform size.  MiePlot simulates the effect of size dispersion by performing N calculations for sizes described by Normal or Log-Normal statistical distributions.  Fig. 5 shows that even very small variations in the size of the drops (e.g. a standard deviation of 0.1%) can have a considerable effect.

Scattering angle (degrees)	Scattering angle (degrees)
Fig. 5   Intensity v. scattering angle for red light (0.65 µm wavelength) by a water drop of nominal radius 100 µm for perpendicular polarisation