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Rays leaving raindrops after three reflections produce a tertiary
rainbow. Unlike the primary and secondary bows which are opposite
the sun and centered on the antisolar point, the tertiary appears
sunwards and centered on the sun. With a nominal radius of 42.5º
it is similar in size to, but very much broader than, the primary
bow.
Why
can we not see it?* Its
total brightness is 24% of the primary bow and we might think
that it ought to be easily visible. There are two snags,
its light is swamped by that of rays that pass through raindrops
without any internal reflection. These generate an intense
glare around the sun, the
zero order glow that masks the tertiary.
Another factor is that the tertiary is much broader than the primary
and secondary, thus its luminosity per unit solid angle of sky
is correspondingly less.
But perhaps not enough people have looked hard enough.** Next
time there is a rainbow, shield both your
eyes very carefully from the sun and look for a 3rd order
bow - of course, rain must be falling in that direction.
The 4th order bow (four internal reflections) is close to the 3rd
order but with reversed colours. Its brightness is only 15% of that
of the primary.
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