Supernumerary rainbows

Supernumerary rainbows

by Konstantinos Kourtidis, Demokritus University of Thrace, Xanthi, Greece

A supernumerary rainbow is rather rare. It consists of several (in this image three) faint rainbows on the inner side of the primary rainbow. Supernumerary rainbows are slightly detached and have pastel colour bands that do not fit the usual pattern.

The faint rainbows are caused by interference between rays of light following slightly different paths with slightly varying lengths within the raindrops. Some rays are in phase, reinforcing each other through constructive interference, creating a bright band; others are out of phase by up to half a wavelength, cancelling each other out through destructive interference, and creating a gap.

This image displays only the RED band of the original RGB image, making the supernumeraries clearly visible. The original RGB image can be viewed at