Rainbow

A rainbow is an optical and meteorological phenomenon that causes a spectrum of light to appear in the sky when the Sun shines on to droplets of moisture in the Earth's atmosphere. It takes the form of a multicoloured arc, with red on the outer part of the arc and violet on the inner section. A rainbow spans a continuous spectrum of colours; the distinct bands are an artifact of human colour vision. The most commonly cited and remembered sequence, in English, is Newton's sevenfold red, orange, yellow, green, blue, indigo and violet (popularly memorized by mnemonics like Roy G. Biv). Rainbows can be caused by other forms of water than rain, including mist, spray, and dew. 

Rainbows can be observed whenever there are water drops in the air and sunlight shining from behind at a low altitude angle. The most spectacular rainbow displays happen when half the sky is still dark with raining clouds and the observer is at a spot with clear sky in the direction of the sun. The result is a luminous rainbow that contrasts with the darkened background.

The rainbow effect is also commonly seen near waterfalls or fountains. In addition, the effect can be artificially created by dispersing water droplets into the air during a sunny day. Rarely, a moonbow, lunar rainbow or nighttime rainbow, can be seen on strongly moonlit nights. As human visual perception for colour is poor in low light, moonbows are often perceived to be white.^[1] It is difficult to photograph the complete semicircle of a rainbow in one frame, as this would require an angle of view of 84°. For a 35 mm camera, a lens with a focal length of 19 mm or less wide-angle lens would be required. Now that powerful software for stitching several images into a panorama is available, images of the entire arc and even secondary arcs can be created fairly easily from a series of overlapping frames. From an aeroplane, one has the opportunity to see the whole circle of the rainbow, with the plane's shadow in the centre. This phenomenon can be confused with the glory, but a glory is usually much smaller, covering only 5–20°.

At good visibility conditions (for example, a dark cloud behind the rainbow), the second arc can be seen, with inverse order of colours. At the background of the blue sky, the second arc is barely visible. 

The seven colours of the rainbow

A rainbow spans a continuous spectrum of colours—there are no "bands." The apparent discreteness is an artefact of the photopigments in the human eye and of the neural processing of our photoreceptor outputs in the brain. Because the peak response of human colour receptors varies from person to person, different individuals will see slightly different colours, and persons with colour blindness will see a smaller set of colours. However the seven colours listed below are thought to be representative of how humans everywhere,^[35] with normal colour vision, see the rainbow.

Newton originally (1672) named only five primary colours: red, yellow, green, blue and violet. Later he included orange and indigo, giving seven colours by analogy to the number of notes in a musical scale.^[36]

Red:     . Orange:     . Yellow:     . Green:     . Blue:     . Indigo:     . Violet:    .

Mythology

The end of a rainbow

Main article: Rainbows in mythology

The rainbow has a place in legend owing to its beauty and the historical difficulty in explaining the phenomenon.

In Greek mythology, the rainbow was considered to be a path made by a messenger (Iris) between Earth and Heaven. In Chinese mythology, the rainbow was a slit in the sky sealed by goddess Nüwa using stones of five different colours.

In Hindu mythology, the rainbow is called Indradhanush, meaning "the bow (Sanskrit and Hindi: dhanush is bow) of Indra, the god of lightning, thunder and rain". Another Indian mythology says the rainbow is the bow of Rama, the incarnation of Vishnu. It is called Ramdhonu in Bengali, dhonu (dhanush) meaning bow. Likewise, in mythology of Arabian Peninsula, the rainbow, called Qaus Quzaħ in Arabic, is the war bow of the god Quzaħ.

In Norse Mythology, a rainbow called the Bifröst Bridge connects the realms of Ásgard and Midgard, homes of the gods and humans, respectively. The Irish leprechaun's secret hiding place for his pot of gold is usually said to be at the end of the rainbow. This place is impossible to reach, because the rainbow is an optical effect which depends on the location of the viewer. When walking towards the end of a rainbow, it will appear to "move" further away (two people who simultaneously observe a rainbow at different locations will disagree about where a rainbow is).

Another ancient portrayal of the rainbow is given in the Epic of Gilgamesh: the rainbow is the "jewelled necklace of the Great Mother Ishtar" that she lifts into the sky as a promise that she "will never forget these days of the great flood" that destroyed her children. (The Epic of Gilgamesh, Tablet Eleven)

Then Ishtar arrived. She lifted up the necklace of great jewels that her father, Anu, had created to please her and said, "Heavenly gods, as surely as this jewelled necklace hangs upon my neck, I will never forget these days of the great flood. Let all of the gods except Enlil come to the offering. Enlil may not come, for without reason he brought forth the flood that destroyed my people."

According to Christian mythology and Judaic mythology, after Noah's flood the rainbow gained meaning as the sign of God's promise that terrestrial life would never again be destroyed by flood (Genesis 9:13–17):^[29]

I do set my bow in the cloud, and it shall be for a token of a covenant between me and the earth. And it shall come to pass, when I bring a cloud over the earth, that the bow shall be seen in the cloud: And I will remember my covenant, which is between me and you and every living creature of all flesh; and the waters shall no more become a flood to destroy all flesh. And the bow shall be in the cloud; and I will look upon it, that I may remember the everlasting covenant between God and every living creature of all flesh that is upon the earth. And God said unto Noah, This is the token of the covenant, which I have established between me and all flesh that is upon the earth.

In the Dreamtime of Australian Aboriginal mythology, the rainbow snake is the deity governing water. In Amazonian cultures, rainbows have long been associated with malign spirits that cause harm, such as miscarriages and (especially) skin problems. In the Amuesha language of central Peru, certain diseases are called ayona’achartan, meaning "the rainbow hurt my skin". A tradition of closing one's mouth at the sight of a rainbow in order to avoid disease appears to pre-date the Incan empire.^[30][31]

In New Age and Hindu philosophy, the seven colours of the rainbow represent the seven chakras, from the first chakra (red) to the seventh chakra (violet). 

http://en.wikipedia.org/wiki/Rainbow