Rainbow With Supernumerary Bows
July 08, 2004
The photo of the rainbows above was captured on May 30, 2004 in Bloomington, Illinois. If conditions are right, it's possible to see supernumerary bows or arcs comfortably positioned just inside the primary rainbow. Look closely at the color strips adjacent and inside of the blue/violet part of the primary bow. Supernumerary bows result from diffraction. Essentially, interference between two parts of a light wave (sunlight) having traveled through a raindrop along ever so slightly different tracks causes the rays to emerge at nearly the same location but out of phase with each other. In some cases, multiple orders of interference can be detected.
Notice the secondary bow to the right of the primary. Unlike the supernumerary arcs, diffraction isn't required to explain the double bow. The secondary bow occurs when there are two internal reflections of sunlight in raindrops.