How Are Rainbows
Formed?
A rainbow is sunlight spread
out into it's spectrum of colors and diverted to the eye of the
observer by the water droplets. The "bow" part of the
word describes the fact that the rainbow is a group of nearly
circular arcs of color all having a common center. The sun is
always behind the person looking behind the rainbow, which makes
the center of the circular arc of the rainbow in the direction
opposite to that of the sun. The traditional description of a
rainbow is that it is made up of seven colors- red, orange,yellow,green,blue,indigo,and
violet. Actually, the rainbow is a whole continuum of colors from
red to violet and even beyond the colors that the eye can see!
The colors of the rainbow arise from two basic facts:
* Sunlight is made up of
the whole range of colors that the eye can detect. The range of
sunlight colors,(when combined) looks white to the eye. This property
of sunlight was first demonstrated by Sir Issac Newton in 1666.
*Light of different colors
are refracted by the amounts when it passes from one medium (air,
for example) into another (water or glass are two examples).
Descartes and Willebrord
Snell had determined how a ray of light is bent, or refracted,
as it traverses regions of different densities, such as air and
water. When the light paths through a raindrop are for red and
blue light, one finds that the angle of deviation is different
for the two colors because the blue light refracts more than the
red.
Another type of rainbow
is a Lunar Rainbow. A full moon is bright enough to have it's
light refracted by raindrops just as is in the case for the sun.
Moonlight is much fainter, of course, so the lunar rainbow is
not as nearly as bright as the one produced by sunlight. Lunar
rainbows have infrequently been observed since the time of Aristotle
or before.
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