Rainbow

Rainbow is a circular arc of colors that appears in the sky when raindrops are illuminated by sunlight. A rainbow is not a physical object. Rather, it is a pattern of light to which a great number of raindrops contribute. A rainbow may spread across the entire sky, and its ends may seem to rest on the earth. Not all rainbows form complete arcs, however, because a rainbow cannot appear in a part of the sky where there is no rain.

You are at the center of the rainbow you see. A person standing next to you would be at the center of a different rainbow--that is, a rainbow to which a different set of raindrops contributes. Thus, no two people ever see the same rainbow.

How to find a rainbow. A rainbow in the form of a complete arc attracts our attention. Sometimes, however, only patches of a rainbow are visible. Knowing when and where to look will help you find them.

Rainbows are most likely to be seen toward the end of the day, especially where local thunderstorms build up during hot summer days, yield rain in the late afternoon, and break up by evening. To locate a rainbow, turn your back to the sun. Next, locate your antisolar point, which will be in the direction of the shadow of your head. Scan the sky in an arc about 42º above the antisolar point. A rainbow at this location is called a primary rainbow. It will be red on its outer edge and violet on its inner edge--with many other colors in between.

If you scan about 9º above this rainbow, you may see a less bright secondary rainbow with its color order reversed. Between the rainbows will be a relatively dark region called Alexander's dark band.

How rainbows appear. To understand some of the general features of rainbows, it helps to pretend that sunlight can be divided into many parallel rays. The rays are uniformly spaced when they arrive at the surface of a raindrop. It also helps to know about the wave nature of light, and how a prism bends sunlight.

The wave nature of light. Light is a form of energy that behaves in some ways like waves. Light waves have a range of wavelengths. A wavelength is the distance between any point on one wave and the corresponding point on the next wave. Visible light of different wavelengths appears as different colors. Light with the longest wavelengths appears red. Light with the shortest wavelengths appears violet.

Sunlight has a mixture of wavelengths. We see this mixture as white light. People often specify the colors in sunlight, from the longest wavelength to the shortest, as red, orange, yellow, green, blue, and violet. All these colors shade into their neighboring colors, however, and each shading is itself a color. Nature produces many more colors than people have ever named.

How a prism bends sunlight. When light passes through a prism, the light is refracted (bent). Light of a given wavelength bends at only one specific angle. Therefore, when sunlight--with its mixture of wavelengths--passes through a prism, it separates into a rainbowlike band of colors. Light with the longest wavelengths bends the least and appears red. Light with the shortest wavelengths bends the most and appears violet.

How droplets form a rainbow. When uniformly spaced rays of white light enter a raindrop, the raindrop acts as a prism. Thus, each ray of white light is separated into many rays corresponding to all the colors in sunlight. Each ray of colored light bends at a different angle.

Some of the rays of colored light reflect once off the inside surface of the raindrop, then exit the drop. As they exit, they bend again. The exiting rays are highly concentrated at angles of about 42º relative to the paths of entry of the original rays of white light.

Such concentrations of rays exit from many raindrops. These rays reach an observer who is scanning the sky about 42º above the antisolar point. As a result, the observer sees a primary rainbow with its colors in the following order, from outer edge to inner edge: red, orange, yellow, green, blue, violet.

Other rays of colored light reflect twice off the inside surface of raindrops. The rays then exit the drops concentrated at angles of about 51º. An observer therefore sees a secondary rainbow at about 51º above the antisolar point.

Other kinds of bows. People sometimes use the term rainbow to refer to colored arcs formed in sea spray or the spray of waterfalls, garden hoses, or lawn sprinklers. You may see such bows even when the sun is high in the sky.

Contributor: Craig F. Bohren, Ph.D., Distinguished Prof. of Meteorology, The Pennsylvania State Univ.

Rainbow

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