When the Sun's light is refracted on passing through tiny rain droplets falling in the air a rainbow is formed. The reason that the arc of a rainbow cannot be followed below the horizon is because the droplets in the air below the horizon cannot be seen.
In all rainbows the bands of color form in a perfect half circle shape when seen from the earth's surface. There are two different types of rainbows. These types being a primary rainbow and a double rainbow. The primary rainbow consists of the colors, in order from the top edge to the bottom, red, orange, yellow, green, blue, and violet. However, the brightness of the color bands, and its width, may vary from minute to minute, and also from rainbow to rainbow.
Sometimes a double rainbow can be seen. This occurs when a second rainbow curves above the primary rainbow. The order of colors is backwards in the second rainbow. Also, a darker band of sky divides the two rainbows.
About 2,000 years ago, Aristotle, a famous Greek scholar, said that a rainbow
is not an object that you can touch in space. Instead, a rainbow is a series of places in the sky
where people can see light spread out, or scattered. The places where the light scattering can be seen
depends on where you are standing in relation to the Sun. A rainbow seems to tag along with you as you move around.
When a beam of sunlight passes through the curved surface of a raindrop prism. It is then seperated
and spread out in the bands of color. The light bounces off the rear surface once it enters the droplet of water and exits
through the forward surface. This causes the light beam to get refracted a second time fanning out the colors
even more. Each individual water droplet makes its own primary rainbow. However, since there are millions upon millions
of water droplets spread over the sky, a large single rainbow can be seen.
Usually when rainbows can be seen is after a rain shower or in
a fine spray of a waterfall or fountain.
The top edge of the primary rainbow forms an angle of 42 degrees with the
horizon while the top edge of the secondary rainbow forms an angle of 50 degrees. This
is because the light forming the two types of rainbows exits from the water droplets
at a different angle.
The colors in a primary rainbow result from white light
being seperated into its full array of colors. The water droplets
in the sky act very much like a prism when it comes to seperating
the light into its seperate colors of light.
The water droplets unscramble the light of the sun into the individual colors
of red, orange, yellow, green, blue, and violet light.
Each color of light can be thought of as a train of waves. Violet
light has the shortest waves, or wavelength. Red, being at the opposite
edge, has the longest wavelength. Moving from the violet edge
through the colors to red at the opposite edge the wavelengths get longer. The
color bands of a rainbow always have the same order since the shorter wavelengths
of light are refracted more than the longer wavelengths.
Some of the light entering a water droplet is bounced off the inner
wall twice before exiting the droplet. When the light beam is reflected
twice it produces a second rainbow. The colors in a secondary rainbow are
fainter than the colors in a primary rainbow due to the fact that since
some of the light escapes on the second trip around
the inside of the water droplet.
These rainbows can be more expected when the sunlight is strong and the
water droplets are small.
Rainbows, Mirages, and Sundogs - The Sky as a Source of Wonder
By: Roy A. Gallant
About Rainbows
http://www.unidata.ucar.edu/staff/blynds/rnbw.htm
Links to other sites on the Web
Jeremy Raver's Homepage about physics and Purdue University
Nikki Stahley's Homepage about Hot Air Balloons (she's my physics buddy!)
Mark Grella's Homepage about Black Holes (he to is my physics buddy!!!)
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