The rainbow colors are rarely pure or arranged in spectral order, owing to interference of light waves. It is the interference of light waves from different parts of the same drop that produces the bands of alternate maximum and minimum brightness, that lie below the primary bow and beyond the secondary bow. The red or green and red bands of maximum brightness produced thus by interference, are called the supernumerary bows, and they are always found parallel to the primary and secondary bow within the former and above the latter.