Rainbows: Unfiltered and Filtered
August 31, 2009
This series of rainbow images shows the primary bow viewed with no filter (at top) as well as through interference filters at 532 nm (green light) and at 633 nm (red light). The narrow bandwidths (10 nm) of the filters make each rainbow almost monochromatic. The green/red bows had to be exposed 12/20 times more than the ordinary non-filtered bow. The photographs were all made within one minute in order to minimize changes in the bow.
To the left, we've scanned the intensity (grey value) distribution across identical sections of the bows. The dark and bright variations inside these bows are two supernumerary bows, which are hard to detect in the ordinary bow. Actually, we would expect to see four to five supernumeraries in such monochromatic bows. This, however, demands that the raindrops in the shower are about the same size and this was not the case.
These images also demonstrate why absolutely pure colors don't exist in rainbows. The ordinary rainbow results from the overlap of roughly thirty broad monochromatic bows. The position (angular size) of each bow is determined by refraction while the bow's brightness is determined by the color distribution of the Sun weighted against the color response of our eyes. In this context, it is amazing rainbows can be colorful at all! Photo taken on September 20, 2008 from Rugeldalen, Norway.