the rainbow's colours are described as red,
orange, yellow, green,blue, indigo and violet. In fact,
our eyes can discern many more indivual hues.
Rainbow colours are not "pure". The classical
is made up of overlapping bows of individually pure spectral colours.
Only a few examples are shown at left.
The final colours that we see are mixtures. Any one colour of the
bow has longer wavelength (towards red) colours mixed in. Look for instance at
the blue; there are significant amounts of green present and lesser
amounts of yellow and red.
Compare this classical bow with one made from small raindrops.
An alternative look. At left are plotted rainbow intensities versus angular distance from the rainbow
center. Red is farthest from the center and violet nearest.
The peaks are at angles of minimum deviation.
The angular size of the sun broadens each peak but there
is a more severe smearing effect. Rays deviated
more than the minimum deviation angle send light inside the
bow and so the light of any particular wavelength falls off slowly
towards the bow's center. Think of rainbows more as overlapping
bright edged disks rather than narrow coloured rings. The
peak intensity for any wavelength has mixed in with it contributions
from all the longer wavelengths. Yellow is contaminated with reds,
green with yellows and reds and so on. Well inside the main bow
all these colours mix to form white. Rainbow colours are not pure.
Those of the circumzenithal
arc ice crystal halo are purer.
The classical rainbow is one made from sufficiently large drops that
diffraction effects are not apparent (when made by the sun - but see this bow). The colour cross sections at
top were computed using geometric optics to trace rays through a spherical
drop and then summing the contributions at each deviation angle. Results
were convoluted over the angular diameter of the sun. The composite
bow's colours were then weighted by the spectral intensity distribution
of sunlight at the earth's surface.