Supernumerary Bows

January 05, 2008


Provided and copyright by: Verena Tießen
Summary authors & editors: Jim Foster, Verena Tießen

The colorful stripes that brighten this cloudy sky were captured near Berlin, Germany on September 18, 2007. Inside the primary rainbow are a series of fainter bows that result from diffraction, rather than refraction, of sunlight in small water droplets. Wave theory and not geometrical optics is required to explain how supernumerary bows are formed. In essence, when two parts of a light wave pass through a water droplet along slightly different trajectories, they re-emerge at nearly the same but not exactly the same position. This produces interference, which we see as colored rings just inside the primary bow. Rainbows are most likely to be accompanied by supernumeraries when falling raindrops are nearly constant in size. Note that a dim portion of the secondary rainbow is seen at upper right.

Photo taken one hour before sunset (6:30 p.m. local time) with a Canon EOS 350D camera and a Heliopan polarization filter.