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A rainbow is an optical
and meteorological phenomenon that causes a (nearly) continuous
spectrum of light to appear in the sky when the sun shines onto
falling rain. It is a multicoloured arc with red on the outside
and violet on the inside; the full sequence is red, orange, yellow,
green, blue, indigo and violet. See also the Colour article for
information on the rainbow spectrum of colour. The rainbow effect
can be observed whenever there are water drops in the air and
sunlight shining from behind the observer at a low altitude or
angle. The most spectacular rainbow displays when half of the
sky is still dark with draining clouds and the observer is at
a spot with clear sky overhead. Another common place to see the
rainbow effect is near waterfalls. Rainbow fringes can sometimes
be seen at the edges of backlit clouds and as vertical bands in
distant rain or virga. The effect can also be artificially created
by dispersing water droplets into the air during a sunny day.
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In a very few
cases, a moonbow, or night-time rainbow, can be seen on strongly-moonlit
nights. As human visual perception for colour in low light is
poor, moonbows are perceived to be white. The rainbow's appearance
is caused by dispersion of sunlight as it is refracted by (approximately
spherical) raindrops. The light is first refracted as it enters
the surface of the raindrop, reflects off the back of the drop,
and is again refracted as it leaves the drop. The overall effect
is that the incoming light is reflected back over a wide range
of angles, with the most intense light at an angle of about 40°–42°,
regardless of the size of the drop.
Above Images Are From The NOAA
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