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Rainbows are optical and meteorological phenomena that cause a spectrum of light to appear in the sky when the Sun shines onto droplets of moisture in the Earth's atmosphere. They take the form of a multicoloured arc, with red on the outer part of the arch and violet on the inner section of the arch. More rarely, a double rainbow is seen, which includes a second, fainter arc with colours in the opposite order, that is, with violet on the outside and red on the inside.
Even though a rainbow spans a continuous spectrum of colours, traditionally the full sequence of colours is most commonly cited and remembered as red, orange, yellow, green, blue, indigo and violet. ("Roy G. Biv" and "Richard Of York Gave Battle In Vain" are popular mnemonics.)
Though rainbows are bow-shaped in most cases, there are also phenomena of rainbow-coloured strips in the sky: in the shape of stripes, circles, or even flames. (See Circumhorizontal arc)

Scientific explanation
Occasionally, a second, dimmer, and thicker secondary rainbow is seen outside the primary bow. Secondary rainbows are caused by a double reflection of sunlight inside the raindrops, and appear at an angle of 50°–53°. As a result of the second reflection, the colours of a secondary rainbow are inverted compared to the primary bow, with blue on the outside and red on the inside. The dark area of unlit sky lying between the primary and secondary bows is called Alexander's band, after Alexander of Aphrodisias who first described it.
A third, or triple, rainbow can be seen on rare occasions, and a few observers have reported seeing quadruple rainbows in which a dim outermost arc had a rippling and pulsating appearance. These rainbows would appear on the same side of the sky as the Sun, making them hard to spot.
High-order rainbows have been described long time ago by Felix Billet (1808-1882) who has depicted angular positions up to the 19th-order rainbow. A pattern he called "rose". In the laboratory, it is much easier to observe high order rainbows by using extremely bright and well collimated light, namely, laser beams. The sixth-order rainbow was first observed by K. Sassan in 1979 using a HeNe laser beam and a pendant water drop. Up to the 200th-order rainbow have been reported by Ng et al. in 1998 using a similar method but an argon ion laser beam.
Infrequently another rainbow phenomenon is observed, consisting of several faint rainbows on the inner side of the primary rainbow, and very rarely also outside the secondary rainbow. They are slightly detached and have pastel colour bands that do not fit the usual pattern. They are known as supernumerary rainbows, and it is not possible to explain their existence using classical geometric optics. The alternating faint rainbows are caused by interference between rays of light following slightly different paths with slightly varying lengths within the raindrops. Some rays are in phase, reinforcing each other through constructive interference, creating a bright band; others are out of phase by up to half a wavelength, cancelling each other out through destructive interference, and creating a gap. Given the different angles of refraction for rays of different colours, the patterns of interference are slightly different for rays of different colours, so each bright band is differentiated in colour, creating a miniature rainbow. Supernumerary rainbows are clearest when raindrops are small and of similar size. The very existence of supernumerary rainbows was historically a first indication of the wave nature of light, and the first explanation was provided by Thomas Young in 1804.
Other rainbow variants are produced when sunlight reflects off a body of water. Where sunlight reflects off water before reaching the raindrops, it produces a reflection rainbow. These rainbows share the same endpoints as a normal rainbow but encompass a far greater arc when all of it is visible. Both primary and secondary reflection rainbows can be observed.
A reflected rainbow, by contrast, is produced when light that has first been reflected inside raindrops then reflects off a body of water before reaching the observer. A reflected rainbow is not a mirror image of the primary bow, but is displaced from it to a degree dependent on the Sun's altitude. Both types can be seen in the image to the right.
Another rainbow-like variant is produced when sunlight is reflected off clouds. The fire rainbow or circumhorizontal arc can sometimes be seen in cirrus clouds with ice crystals (normally at least 6 km above sea level) and with the sun at least 58° above the horizon.
You can create your own rainbow by facing 180 degrees from the sun and spray mist from a garden hose in front of you in a circular motion, outlining a 360 degree "rainbow".

Variations
The Persian astronomer, Qutb al-Din al-Shirazi (1236–1311), is thought to have first given a fairly accurate explanation for the rainbow phenomenon. This was elaborated on by his student, Kamal al-Din al-Farisi (1260–1320), who gave a more mathematically satisfactory explanation of the rainbow.

Scientific history

Culture

Main article: Rainbows in mythology Art
Virginia Woolf in To the Lighthouse highlights the transience of life and Man's mortality through Mrs Ramsey's thought,
"it was all as ephemeral as a rainbow"
A poem of William Wordsworth from 1802, "My Heart Leaps Up When I Behold The Rainbow", begins:
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 in his poem "Lamia" in 1820:
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."
The Rainbow is the title of a 1915 novel by British author D.H. Lawrence;
Dawn and sunset were the feet of the rainbow that spanned the day, and she saw the hope, the promise.

Literature
The rainbow has also been used in more contemporary settings, such as the song "Over the Rainbow" in the musical film The Wizard of Oz, the hit song "The Rainbow Connection" from The Muppet Movie, and in selling Lucky Charms by alluding heavily to leprechaun mythology.
The Greenpeace ship, Rainbow Warrior, was named after a Cree Native American prophecy that stated "When the world is sick and dying, the people will rise up like Warriors of the Rainbow.…"
Rainbow Coalition is a local social action group that was started in Chicago, Illinois by Jesse Jackson from which he launched his national political and social reform platform.
Rainbow Gatherings are gatherings of hippies who come together on public lands with a stated mission to espouse the ideas of peace, love, freedom and community. In the 1960s, artists like Peter Blake made use of Rainbow colours in iconic prints such as Babe Rainbow,
Historically, a rainbow flag was used in the German Peasants' War in the 16th century as a sign of a new era, of hope and of social change. Rainbow flags have also been used as a symbol of the Cooperative movement; as a symbol of peace, especially in Italy; to represent the Tawantin Suyu, or Inca territory, in Peru and Ecuador; by some Druze communities in the Middle east; and by the Jewish Autonomous Oblast.
The rainbow has become the universal symbol of Lesbian, Gay, Bisexual, and Transgender (LGBT) persons, who are also known as 'rainbow people', a reference to their diversity. The rainbow appears on the Gay pride flag, designed by Gilbert Baker for the 1978 San Francisco's Gay Freedom Celebration. The colours of the rainbow are displayed as horizontal stripes, with red at the top and purple at the bottom. The rainbow flag was inspired by the song Somewhere Over the Rainbow.

Popular culture
All the Roy G. Biv mnemonics follow the tradition of including the colour indigo between blue and violet. Newton originally (1672) named only five primary colours: red, yellow, green, blue and violet. Only later did he introduce orange and indigo, giving seven colours by analogy to the number of notes in a musical scale. There is also some evidence that Newton's use of the terms blue and indigo map to the modern hues turquoise and blue respectively.
Since rainbows are composed of a nearly continuous spectrum, different people, most notably across different cultures, identify different numbers of colours in rainbows.

See also