Supernumerary rainbow over Scholars Green

Look at the underside of this rainbow. There are some other colours in two smaller bows. These are called supernumerary rainbows. They are formed by interference not by refraction. As a light wave bends to go into a small water droplet, some wavefronts go a longer distance through the drop that others because of the rounded edges of the drop. In other words, if the drop curves away from that part of the wave, the wavefront travels further. This puts wavefronts out of phase with each other resulting in constructive and destructive interference. This is a good A Level topic but it is the only way to explain these extra bits of the rainbow. See http://www.atoptics.co.uk/rainbows/supform.htm and http://www.atoptics.co.uk/rainbows/supdrsz.htm for more information. Apparently smaller drops are better and the supernumerary bows are often better nearer the top of the rainbow because smaller raindrops are found there.