Before the scientific secrets of rainbows were discovered, these colorful bands of light were wrapped in mystery and folklore. Every culture had its own theory for the rainbow’s purpose, and many times it had religious significance. Rainbows have been called such things as the tongue of the sun, road of the dead, bride of the rain, hem f the sun-god’s coat, road of the thunder god, bridge between heaven and earth, window to heaven, and bow of God. Biblical accounts establish the rainbow as a covenant, or promise, between God and every living creature that the earth never again will be destroyed by flood.
Superstitious beliefs also surround the rainbow. One tribe in South America believed a rainbow oer the sea was a good sign, but when it appeared over land, it was the sign of an evil spirit looking for a victim. In Eastern Europe it was believed that an angel put gold at the ends of the rainbow, but only a nude man could find it. An old Romanian folktale claimed that the end of a rainbow stood in a river, anyone creeping in on hands and knees for a drink of the rainbow-touched water would be changed instantly to the opposite sex. A similar tale claimed that anyone passing beneath a rainbow’s arch would be changed into the opposite sex.
Rainbow colors also have had their significance in folklore. Some people believed that when red was the most brilliant, or noticeable, it meant war; green meant abundance; and yellow meant death. Such superstitious beliefs seem foolish to us today, but these earlier cultures, with their limited scientific knowledge, had no way of knowing the colorful bands of light were produced by sunlight being refracted (bent) and reflected (turned back) by raindrops.
In order to understand how this happens, we must learn a little about light. Sunlight is a mixture of color rays, and these rays travel in wavy lines. The distance between the tops of the waves, which varies in different rays from 14- to 28-millionths of an inch, is called the wave length. Each different wave length produces a different color. Red and yellow wave lengths are longer than those which produce blue and violet light, and the combination of all colors produces white light.
You can separate the colors contained in sunlight by using a simple prism, which is a triangular bar of glass. As light waves enter the prism, they are refracted. Since each color has a slightly different wave length, each is bent at a slightly different angle. This separates them, and they emerge from the prism in bands of colors—red, orange, yellow, green, blue, indigo, and violet. These seven bands of color are called the solar spectrum.
When sunlight passes through air filled with water droplets, each of the raindrops acts as a tiny prism to bend and separate the light into its many colors. But instead of allowing the light to pass through, as a prism does, the inner surface of the raindrop reflects the color. Upon leaving the raindrop the color is bent again. The result of this refraction and reflection is a rainbow.
All of the colors of the solar spectrum are present in a rainbow, but since they blend or overlap somewhat, you rarely see more than four or five colors clearly. The color on the outer edge of the arch is red, and violet lies on the inner edge. The width of the color bands depends upon the size of the raindrops forming the rainbow.
Certain conditions must exist before a rainbow can be seen. The sun must be behind you and low enough on the horizon for its rays to be reflected at the proper angle to reach your eyes. The rain must be somewhere in front of you. Since sunshine and rain showers appear together most frequently in the summer, more rainbows are seen during this season.
At this point, you may be wondering how a rainbow remains the same while the raindrops are falling. Each drop contributes to the color for only a second, but since each falling drop is quickly replaced by another, the reflected rays give the appearance of never changing. We do not see all of the rays reflected by the many raindrops present in a shower. Those reflected at forty- to forty-two-degree angles form the primary rainbow. Violet rays arrive at our eyes at a forty-degree angle, red at forty-two degrees and the remaining five colors at degrees between these two.
A secondary rainbow, located a short distance outside the primary one, sometimes can be seen. Its rays reach the eye at fifty- to fifty-four-degree angles. This secondary rainbow, which some people incorrectly think is a reflection of the primary one, has a full spectrum of colors; however, the colors always are fainter and the order reversed. Red lies on the inner edge of the secondary rainbow and violet on the outer edge. The light rays forming the secondary rainbow strike the raindrops from a higher angle and are reflected twice before leaving the raindrop. This double reflection accounts for the fainter appearance and the reversed order of color.
You may think you have seen the complete rainbow as it arched from one point on the horizon to another, but you really haven’t. Rainbows can for a full circle. If you were standing on a high mountain and the sun appeared low enough on the horizon to create a rainbow, you might see a round round one. Passengers in airplanes occasionally do when conditions are right.
With the help of a harden hose, you can make your own rainbow. During the early morning or late afternoon, put the sun at your back and spray a fine mist of water into the air in front of you. A circular rainbow should be reflected by the water droplets. Increase the size of the water droplets and notice whether the width of the bands of color changes. Mist from a plunging waterfall will produce a similar rainbow on a sunny day.
Proving the scientific theory that sunlight is a combination of colors can be a fascinating experience. Perhaps you already have had a chance to experiment with a simple prism in your science class and have separated sunlight into its various colors, but have you ever produced the solar spectrum with a water prism? A water prism is easy to make. All you need is an oblong, blass dish at least two inches deep, a small mirror, a piece of foil large enough to wrap around the mirror, a rock, and a large, white card.
Fill the dish with water and set it in the sunlight. Cut a window one-inch tall by one-half-inch wide in the middle of the piece of foil. Center the window on the mirror and wrap the foil around the edges to hold the window in place. Put the foil-covered mirror in the water with the window facing the sun. The top, back edge of the mirror should be leaning against the side of the dish. Adjust the angle of the mirror until a color spectrum is reflected on a nearby wall. Use the rock to keep the mirror from slipping once you have it at the proper angle. Insert the white card between the dish and the wall image and the spectrum should appear on the card. The water refracts the light, separating it into its colors, and the mirror reflects these colors onto the card.
To bring the colors back together, place a magnifying glass in the path of the reflected light, holding it several inches from the card and facing the mirror. A rectangle of white light the shape of the foil window should now be projected on the white card instead of a color spectrum. By adjusting the angle of the magnifying glass, you should get a sharp outline of the window. The curved lens of the magnifying glass has brought the color rays back together, producing white light, but you have proved with your water prism that sunlight is a combination of colors.
Up to now, all of the rainbows mentioned have been caused by reflected sunlight, but a study of rainbows would not be complete without mentioning that moonlight occasionally is responsible for rainbows. The feebleness of moonlight results in very faint colors, making a lunar rainbow very difficult to see; however, the lunar rainbow differs from those made by the sun only in the intensity of its color. All rainbows, whether produced by sunlight or moonlight, are a result of light being refracted and reflected in moisture.
1983 Crystals. Young Naturalist. The Louise Lindsey Merrick Texas Environment Series, No. 6, pp. 110-113. Texas A&M University Press, College Station.