After reviewing a simple explanation.
you can trace the path the sunlight makes through a rain drop and derive
a formula for the angle between the incoming sunlight and the refracted
A few basics first:
if you draw a line from the center of the circle through
the surface it intersects the surface at 90 deg
since Snell's law always references the angle between the
light ray and the line which is perpendicular to the surface where the
light ray hits, this extended radius line is the reference for measuring
angles in Snell's law and also for the law of reflection, i.e., the angle
of incidence equals the angle of reflection.
So in the diagram above, the angle A is the incident
angle and the angle B is the transmitted angle. If we assume the
index of refraction for air is 1.0 and the index of refraction for the
n then we have from Snell's law:
sin(A) = n * sin(B)