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In Greek mythology, Iris, the Greek
goddess of rainbows was a divine messenger. Whenever she
embarked on a message delivery, she would leave a rainbow in
her wake. A charming legend no doubt, but we are wiser now
having science on our side to explain such breathtakingly
beautiful natural phenomena as rainbows. The rainbow, a treat
for our eyes is a natural spectrum and is produced by
meteorological phenomena. Try holding up a glass prism so that
sunlight passes through it. And hey presto, you see the
familiar spectrum of colors.
How are rainbows formed?
Rainbows, that curved band of varied colors that has
inspired many a song and poem, are caused by light passing
through raindrops. Refraction and internal reflection of the
light rays that pass through raindrops give us what we know
and feast our eyes on as the rainbow. Each color is bent at a
slightly different angle resulting in the curved bow shape
that we are so familiar with.
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A rainbow can also be described as sunlight
spread out into its spectrum of colors directed to our eyes by
water droplets. The rainbow is actually composed of nearly circular
arcs of color, all the arcs having a common centre. And the centre
of the rainbow is in a direction opposite to that of the sun. Next
time you see a rainbow, notice the fact that the sun is always
behind the person who is looking at the rainbow. This is because
sunlight from behind the observer is striking raindrops that are
falling from cumulonimbus clouds. The raindrops perform the function
of tiny prisms suspended in air. They bend and reflect different
colors of light back to your eyes when you are looking at a
rainbow.
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How many colors?
The traditional answer to that question extends to just seven
colors – red, orange, yellow, green, blue, indigo and violet.
Actually, the rainbow is composed of a whole continuum of colors
ranging from the starting band of red to the last band of violet and
some of these the eye cannot see.
What causes these colors?
Sunlight has a range of colors that the human eye can detect.
The combination of all these colors assumes the color white when
we look at sunlight. This fact was ‘brought to light’ by Sir Isaac
Newton in 1666. When we see colors, light is passing from one
medium to another, as for example from air into water. When we see a
rainbow, sunlight passes from air through raindrops, giving us a
visual treat. A ray of light is bent or is refracted when it passes
through regions that have different densities, water and air in the
case of rainbows. Different colors have different angles of
deviation.
At times a secondary rainbow may be
observed. This secondary rainbow is less intense than the primary
rainbow and one can see the standard color sequence being reversed
here. This bow is a result of sunlight that has undergone two
reflections within raindrops. Similarly there may even be three or
more internal reflections of light resulting in more layers in the
rainbow, but they cannot be observed easily, as each successive
reflection makes the resultant rainbow weaker. At times,
interference after one internal reflection results in the formation
of faintly colored rings that can be seen just within the primary
bow.
Lunar rainbow (Rainbow in the dark)
Seen a rainbow only during the day? Well, there exists a lunar
rainbow too. The light at night is supplied by a full moon and light
is refracted by raindrops. However as moonlight is much fainter than
sunlight, the resultant lunar rainbow is not as bright as a rainbow
that can be seen during the day.
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