Supernumerary rainbows are faint bows that appear just inside a primary rainbow. Several supernumerary rainbows can appear at the same time with a small gap between each one.
- The word supernumerary means additional to the usual number. The first supernumerary rainbow forms near the violet edge of the primary bow and is the sharpest. Each subsequent supernumerary bow is a little fainter.
- Supernumerary bows often look like fringes of pastel colours and can change in size, intensity and position from moment to moment.
- Supernumerary rainbows are clearest when raindrops are small and of equal size.
- On rare occasions, supernumerary rainbows can be seen on the outside of a secondary rainbow.
- Supernumerary rainbows are produced by water droplets with a diameter of around 1 mm or less. The smaller the droplets, the broader the supernumerary bands become, and the less saturated are their colours.
- Supernumerary bows result from the wave-like nature of light and are caused by interference between the waves that contribute towards the main bow. In some places, the waves amplify each other, and in others, they cancel each other out.
- The theory is that rays of a similar wavelength have slightly different distances to travel through misshapen droplets affected by turbulence, and this causes them to get slightly out of phase with one another. When rays are in phase, they reinforce one another, but when they get out of phase they produce an interference pattern that appears inside the primary bow.