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What Causes a Rainbow?
An optical effect produced by sunlight shining on raindrops, a rainbow is most commonly seen during a sunshower, when the sun shines brightly despite the falling rain. The rainbow appears as a multi-colored, semi-circular band in the sky, with both ends generally apparently disappearing into the ground. It contains all the colors of the spectrum, from red through orange, yellow, green and blue to violet. These bands of color merge into each other.
The cause of a rainbow can best be understood by considering the interaction between a ray of sunlight and a single raindrop.
When the ray meets the front surface it enters the water but is refracted, the direction of its path being altered. The refracted ray is internally reflected from the far surface of the water and re-emerges through the front, suffering a second refraction. A side effect of the double refraction is the dispersion of light, the raindrop acting as a prism to separate the white sunlight into the colors of the spectrum.
Violet light is observed from raindrops closest to the ground and red light from raindrops farthest from the ground; the intermediate colors of light are seen between these two extreme positions.
What are the colors of the rainbow?
Most people know the colors of the rainbow: red, orange, yellow, green, blue, indigo, violet. But few have been fortunate enough to see some of the rarer manifestations of this natural phenomenon.
Some rainbows are all purple, all red, or even white. Some, instead of arching into the sky, lie flat or stand vertically in glowing pillars. And sometimes from an airplane, one can see a complete circle of multicolored light.
Rainbows may be visible whenever droplets of water catch the rays of the sun. The splashes from a ship plowing through the waves, the mist remaining after a shower on a sunny day, dew on a spider's fragile web, or the spray rising over a waterfall- each can put on a vibrant light show. To see the spectacular effects, one needs only to be in the right place at the right time.
Purple rainbows are seen only before or at sunrise. A rare occurrence, they can be formed by high clouds that scatter the blue and violet light, which raindrops reflect back to the observer. At sunset, when the sun is low in the sky, a rainbow may be a dramatic red arc, because the shorter wavelengths (blue,green, and yellow) have been dispersed during their relatively long trip through the atmosphere.
White rainbows can appear either in daylight or in moonlight, but for entirely different reasons. During the day, rays of sunlight may be reflected from very small droplets of moisture- so small that the emerging bands of color are close enough to overlap, creating white light. But a white rainbow seen by moonlight is not white at all.
It only seems so because the eye cannot detect color in light as weak as what the moon is reflecting. However, a photograph of a lunar rainbow, taken at the correct exposure, will be in full color.
A curious feature of double rainbows is that the colors of the two halves are mirror images. This is because the light in the outer rainbow is reflected not once but twice inside the raindrops, and emerges at such an angle that the order of the colors is reversed.
Rainbows that appear to be horizontal are sometimes seen above dew-covered fields or water-often with a second, "conventional" rainbow behind them.
Standing like pillars at the end of a curved rainbow, straight rainbows are usually seen over a large expanse of water. Scientists suggest that reflections from the water create a number of rainbows, one above the other, but that only the ends of the rainbows are visible.
Why is a rainbow different colors unlike the uniform "white" light of the sun?
Sunlight is a mixture of light of different wavelengths. Although we can see each as a separate color, combined they appear to be white. As a ray of sunlight enters a raindrop, its colored components are bent, or refracted, at different angles and then reflected off the raindrop's inner surface. Then, as it leaves the raindrop, the light is bent once again. Red, the longest wavelength, is reflected to the highest or outermost arc; violet, the shortest wavelength, to the lowest or innermost arc; yellow and green, to arcs between the two.