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A rainbow is an optical or meteorological phenomenon that causes a (nearly) continuous spectrum of light to appear in the sky when the sun shines onto falling rain. It is a multicoloured arc with red on the outside and violet on the inside; the full sequence is red, orange, yellow, green, blue, indigo and violet. See also the Colour article for information on the rainbow spectrum of colour.
The rainbow effect can be observed whenever there are water drops in the air and sunlight shining from behind the observer at a low altitude or angle. The most spectacular rainbow displays when half of the sky is still dark with draining clouds and the observer is at a spot with clear sky overhead. Another common place to see the rainbow effect is near waterfalls. Rainbow fringes can sometimes be seen at the edges of backlit clouds and as vertical bands in distant rain or virga. The effect can also be artificially created by dispersing water droplets into the air during a sunny day.
In a very few cases, a moonbow, or night-time rainbow, can be seen on strongly-moonlit nights. As human visual perception for colour in low light is poor, moonbows are perceived to be white.
Physics of rainbows
A double rainbow features reversed colours in the outer (secondary) bow.
The rainbow's appearance is caused by dispersion of sunlight as it is refracted by (approximately spherical) raindrops.
The light is first refracted as it enters the surface of the raindrop, undergoes total internal reflection from the back of the drop, and is again refracted as it leaves the drop. The overall effect is that the incoming light is reflected back at an angle of about 40-42°, regardless of the size of the drop. Since the water of the raindrops is dispersive, the amount that the sunlight is bent depends upon the wavelength (colour) of the light's constituent parts. Blue light is refracted at a greater angle than red light, but because of the reflection from the back of the raindrop, the red light appears higher in the sky, and forms the outer colour of the rainbow.
The rainbow does not actually exist as a location in the sky, but is an optical illusion whose apparent position depends on the observer's location. All raindrops refract and reflect the sunlight in the same way, but only the light from some raindrops reaches the observer's eye. These raindrops are perceived to constitute the rainbow by that observer. Its position is always in the opposite direction of the sun with respect to the observer, and the interior is actually a magnified image of the sun, which can be seen to be slightly brighter than the exterior. The bow is centered on the shadow of the observer's head, appearing at an angle of approximately 40-42° to the line between the observer's head and its shadow (this means that if the sun is higher than 42° the rainbow is below the horizon and cannot be seen unless the observer is at the top of a mountain or a similar vantage point). Similarly it is difficult to photograph the complete arc of a rainbow, which would require an angle of view of 84°. For a 35mm camera, a lens with a focal length of 19mm or less would be required, whilst most photographers are only likely to have a 28mm wide-angle lens.
Sometimes, a second, dimmer rainbow is seen outside the primary bow, caused by a double reflection of the sunlight inside the raindrops, and appears at an angle of 50-53°. Because of the extra reflection, the colours of the bow are inverted compared to the primary bow, with blue on the outside and red on the inside. From an aeroplane one has the opportunity to see the whole circle of the rainbow, with the plane's shadow in the centre.
Even more rarely is a triple rainbow seen and a few observers have reported seeing quadruple rainbows in which a dim outermost arc had a rippling and pulsating appearance.
The first accurate theoretical explanation of a rainbow came from Descartes in 1637. Knowing that the size of raindrops didn't appear to affect the observed rainbow, he experimented with passing rays of light through a large glass sphere filled with water. By measuring the angles that the rays emerged, he concluded that the primary bow was caused by a single internal reflection inside the raindrop and that a secondary bow could be caused by two internal reflections. He was able to back this up with a derivation of the law of refraction (subsequently, but independently of Snell) and correctly calculated the angles for both bows. However, he was unable to explain the colours.
Isaac Newton was the first to demonstrate that white light was composed of the light of all the colours of the rainbow, which one glass prism could split into the full spectrum of colours, and another could recombine into a beam of white light. He also showed that red light gets refracted less than blue light which led to an essentially complete explanation of a rainbow's optical effect.
 White light refracts as it enters drops of water, and the refraction spreads the light according to its wavelength. The same spreading repeats as the light reflects off the back of the raindrop and as it exits in an angle 42° off from the angle at which the light entered. |
 In the formation of a primary rainbow, light enters from one direction (typically a straight line from the sun), reflects off the back of the raindrop and fans out the front separated into colors. |
 With particularly bright sunlight and the observer in the correct position, light reflects twice inside the raindrop before exiting to the viewer, producing a secondary rainbow. |
Rainbow in legends and the Bible
The rainbow has a place in legend due to its beauty and the difficulty in explaining the phenomenon before Galileo's treatise on the properties of light. In Greek mythology, it is a path made by a messenger (Iris) between Earth and Heaven. The Irish leprechaun's secret hiding place for his crock of gold is usually said to be at the end of the rainbow. In Chinese mythology, the rainbow was a slit in the sky sealed by Goddess Nüwa using stones of seven different colours. In Hindu mythology, the rainbow is called Indradhanush- meaning the bow of Indra, the God of lightning and thunder. In Norse Mythology, a rainbow called the Bifröst Bridge connects the realms of Asgård and Midgård, homes of the gods and humans, respectively.
It is mentioned in Genesis 9:13 [1] (http://sources.wikipedia.org/wiki/Bible%2C_English%2C_King_James%2C_Genesis#Chapter_9) , as a sign of God's covenant with mankind. After Noah survives the flooding of the earth in the story of Noah's Ark God sent the rainbow to promise that he would never again send such a flood to destroy the world.
Rainbow in literature
The rainbow has also been used in more contemporary settings, such as the song Over the Rainbow in the musical film of The Wizard of Oz, and in selling Lucky Charms by alluding heavily to leprechaun mythology.
One of the poems of William Wordsworth goes...
- My heart leaps up when I behold
- A rainbow in the sky:
- So was it when my life began;
- So is it now I am a man;
- So be it when I shall grow old,
- Or let me die!...
However, the Newtonian deconstruction of the rainbow is said to have provoked John Keats to lament....
- Do not all charms fly
- At the mere touch of cold philosophy?
- There was an awful rainbow once in heaven:
- We know her woof, her texture; she is given
- In the dull catalogue of common things.
- Philosophy will clip an Angel’s wings,
- Conquer all mysteries by rule and line,
- Empty the haunted air, and gnomed mine -
- Unweave a rainbow
In contrast to this is Richard Dawkins; talking about his book Unweaving the Rainbow: Science, Delusion and the Appetite for Wonder .....
"My title is from Keats, who believed that Newton had destroyed all the poetry of the rainbow by reducing it to the prismatic colours. Keats could hardly have been more wrong, and my aim is to guide all who are tempted by a similar view, towards the opposite conclusion. Science is, or ought to be, the inspiration for great poetry."
See also
References
- Robert Greenler, Rainbows, Halos, and Glories, (1980) ISBN 0-521-38865-1
- David K. Lynch & William Livingston, "Color and Light in Nature", 2nd edition (2001) ISBN 0-521-77504-3
- M.G.J. Minnaert, "Light and Color in the Outdoors", 1995 ISBN 0-387-97935-2
- M. Minnaert, "The Nature of Light and Color in the Open Air", 1973 ISBN 0-486-20196-1
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