Light Paths in Water Droplet

Of the many paths taken by parallel light rays through a spherical water droplet, several bunch together near a minimum deviation angle, and these rays together enhance the intensity at that particular angle to produce the primary rainbow. The ray which emerges lowest is often called the Descartes ray, since Decartes is credited with being the first to work out these ray paths through a spherical droplet of water. The Descartes ray is analogous to the ray at the angle of minimum deviation through a prism. All the other rays emerge at higher angles from the droplet, so they will reach the eye from droplets lower than the primary rainbow. The other rays contribute to the lighter sky beneath the rainbow and to interference effects seen in the supernumerary arcs.