Origins
The word 'rainbow' is a transparent compound: 'rain' + 'bow,' describing the arc of colors seen in the sky during or after rain. The compound descends from Old English 'regnboga,' itself from Proto-Germanic *regnabugô, and the word is shared in remarkably consistent form across every Germanic language: German 'Regenbogen,' Dutch 'regenboog,' Swedish 'regnbåge,' Norwegian 'regnbue,' Danish 'regnbue,' and Icelandic 'regnbogi.' This consistency indicates that the compound was formed in Proto-Germanic times, making it one of the oldest compound words in the family.
The first element, 'rain,' comes from Proto-Germanic *regną, from PIE *h₃reǵ- (to moisten, to rain). This root also produced Latin 'rigāre' (to water, to irrigate) — the source of English 'irrigate.' The second element, 'bow,' comes from Proto-Germanic *bugô (a bend, a curve), from PIE *bheugh- (to bend). This is the same 'bow' as in a weapon or a violin bow — a curved object.
The rainbow has been one of the most symbolically charged phenomena in human culture. In the Hebrew Bible, the rainbow is the sign of God's covenant with Noah after the Flood (Genesis 9:12–17): a promise that the earth will never again be destroyed by water. This covenant symbolism made the rainbow a powerful image in Jewish and Christian theology — a bridge between the divine and the human, a sign of mercy after judgment.
Development
In Norse mythology, the rainbow was 'Bifröst' (the 'trembling path' or 'shimmering bridge'), the bridge connecting Midgard (the world of humans) to Asgard (the realm of the gods). The god Heimdall stood guard at Bifröst, watching for the approach of enemies. At Ragnarök (the end of the world), the fire giants would cross Bifröst and it would be destroyed.
In Greek mythology, the rainbow was personified as Iris, the messenger goddess who traveled between the gods and humanity. She gave her name to the iris of the eye (the colored part, which displays various hues), to the iris flower (which comes in many colors), and to the chemical element iridium (whose compounds display many colors).
The scientific explanation of the rainbow was developed gradually. Aristotle knew that rainbows were caused by the reflection of sunlight in rain. The Islamic scientist Ibn al-Haytham (Alhazen, 965–1040) and his successor Kamāl al-Dīn al-Fārisī made significant advances. But the full explanation — that sunlight entering a water droplet is refracted, internally reflected, and refracted again, with each color refracted at a slightly different angle — was worked out by René Descartes in 1637 and extended by Isaac Newton, who demonstrated that white light is composed of a spectrum of colors (published in 'Opticks,' 1704). Newton's identification of seven colors (red, orange, yellow, green, blue, indigo, violet) was partly arbitrary — indigo is barely distinguishable from blue — and may have been influenced by his desire for seven colors to match the seven notes of the musical scale.
Later History
The 'secondary rainbow' — a fainter, wider arc sometimes visible outside the primary bow, with colors reversed — is caused by light reflecting twice inside each water droplet. The dark band between the two bows, where the sky appears darker than either side, is called 'Alexander's dark band,' named after Alexander of Aphrodisias, who first described it in 200 CE.
In modern culture, the rainbow flag (designed by Gilbert Baker in 1978) became a symbol of LGBTQ+ pride and diversity, and the phrase 'somewhere over the rainbow' (from the 1939 film 'The Wizard of Oz') has become one of the most recognizable expressions of hope and longing in English.