Rainbows are caused by light bouncing once off the insides of raindrops;
the colours occur because different colours (wavelengths) of light are
refracted by different amounts.
Many rainbows have a double on the outside, the opposite way round from
the main bow.
This is caused by light bouncing around the inside of water droplets
twice. A third bow,
caused by light bouncing three times, is easily seen in the lab but
almost never in the sky.
Occasionally a rainbow has many extra bands of colour close together
on the inside. These
supernumerary bows show the interference pattern created by light waves
their visibility depends on how uniformly sized the drops are.
A coloured ring around the sun, caused by refraction from cirrostratus.
Like the 22º Halo only bigger, weaker and rarer.
Bright, colourful patches of sky at the same elevation as the sun, just
outside the 22º halo;
the result of refraction by ice crystals in cirrus clouds.
One of the more uncommon effects, this is a rainbow-arc partway around
the zenith of the
sky (its highest point). Also produced by ice crystals.
A coloured ring pretty close to the sun or moon, caused by diffraction
from water droplets in an altostratus or altocumulus layer. Droplets
of different sizes give different-sized coronas, so the effect is smeared
out if the sizes of the droplets vary too much. Although beautiful and
common, these are usually much too bright to look at, which is why most
people have never even noticed them.
Do not try to look for bright colours in the clouds
immediately surrounding the sun without proper sunglasses! Even with
strong shades, take great care not to look for too long at a time, and
block the sun out of your view.
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