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, t