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The science of the rainbow

Posted in Science on Friday, 29 July 2011

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This edited article about the rainbow originally appeared in Look and Learn issue number 998 published on 25 April 1981.

Rainbow, picture, image, illustration

A rainbow over the Victoria Falls by A Gigli

If you ran fast and far enough, could you come to the end of a rainbow? The answer is no. Do you know why?

A rainbow is formed when the sun shines on falling rain. The raindrops refract or bend the light, and then reflect it back, making a rainbow. When light is refracted, it is split up into colours. When it is reflected, it is bounced back from a surface in the direction from which it came, in the same way that a mirror bounces back your image.

The English scientist Sir Isaac Newton was the first man to discover that sunlight is really a mixture of coloured light. In air, the colours all travel at nearly 300,000 kilometres (186,00 miles) per second.

In substances such as glass or water, however, the colours travel at different speeds. The substance shows up coloured light by different amounts, and refracts it. Red light is slowed up least when it enters a substance, and violet light most.

A raindrop acts like a glass prism. The light is refracted into its separate colours as it comes into the prism. The colours leave the prism at different points, and make a band of colours. The order of the colours – red, orange, yellow, green, blue, indigo and violet – is the same as in a rainbow.

Violet light is bent the most by glass because it has short wavelengths. Light travels in waves, and the distance between the crest of two successive waves is called its wavelength. The shorter the wavelength of light, the more it is bent by glass or water. Red light has long wavelengths and so it is bent only slightly.

Raindrops refract the light in just the way that a prism does, but in the raindrop the water does the refracting. Once the light is broken into its separate colours, the back of the raindrop reflects the light back into our eyes.

If each raindrop refracts light, why are there bands of colour in a definite pattern in a rainbow instead of a mixture?

If we follow the paths of two colours inside a raindrop, we can see why. In the diagram, red light is refracted on entering the drop, is reflected from the back of the drop, and is bent once more when it leaves. The light enters our eye, and we see a red colour.

But the violet light is bent more, and so it leaves the drop lower down. The light shoots away into space, and does not enter the eye. A drop lower down, however, would be at just the right point to reflect the violet light back into our eye.

This principle holds good for all the colours of the rainbow. The red light comes from the highest raindrops, the orange, yellow, green, blue, indigo and violet in a series getting lower and lower.

An ordinary rainbow is usually made up of a number of rainbows one on top of another. Because raindrops are round, they reflect light in all directions. This means that people can see a rainbow at the same time from different places.

But no two people see exactly the same rainbow, as the light entering the eyes is different for each of them. This means that the rainbow has no one end. Its end is always changing as you change your position.

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