Primary, Secondary and Supernumerary Bows
August 10, 2004
This classic double rainbow was captured above Cascade, Colorado on June 23 of this year at 18:30 local time. Notice the well defined supernumerary bows just inside the primary rainbow (lower bow). Whereas, the primary and secondary bows are produced by refraction and reflection, supernumerary bows result from the process of diffraction. 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. On the photo above, multiple orders of interference can be detected. The spacing of the supernumeraries depends largely on the size of the raindrops. Also note the dark band between the primary and secondary bows (Alexander's band). When a secondary rainbow is visible, in addition to the sky being brighter on the inside of the primary bow, it's also brighter to the outside of the secondary bow. Thus, the strip between these 2 bows appears darker.
Photo details: 24 mm lens with a polarizer