Newton gave the rainbow seven colours to match the notes of a musical scale
The rainbow's seven colours are a choice, not a law of nature. Isaac Newton first counted five, then added orange and indigo so the spectrum would mirror the seven notes of a musical scale, a link inspired by ancient Greek ideas. What he called indigo, scholars note, we would simply call blue today.
A rainbow forms when sunlight enters water droplets, bends, reflects off the back of each drop and bends again on the way out, splitting into a continuous spectrum. It always appears directly opposite the sun, which is why rainbows show up in the western sky in the morning and the eastern sky in early evening. Rain is not required: mist, spray from waterfalls and fountains, and airborne dew all work. The best displays come when half the sky is still dark with rain and the observer stands in sunshine.
In the main bow, red sits on the outside and violet inside. A fainter second bow often appears about 10 degrees further out, with the order reversed. The sky inside the primary arc is noticeably brighter than outside, because each spherical drop scatters light across a whole disc; where the colours overlap they blend into white, and only at the edge does the separation by wavelength produce the bands. The light is also strongly polarised, about 96 percent in the primary arc and 90 percent in the secondary.
From the ground a rainbow looks like an arch, but from an aircraft, with enough mist, it can be a complete circle, sometimes confused with the much smaller optical effect called a glory. Photographing the full half circle requires an 84-degree field of view, meaning a 19-millimetre lens or wider on a 35-millimetre camera, though panorama software now makes it easy. On bright moonlit nights a moonbow can form, usually looking white because human colour vision fades in dim light.
Newton admitted his eyes were not especially good at telling colours apart. His sevenfold sequence survives in the mnemonic Richard Of York Gave Battle In Vain, and the modern version often swaps indigo for cyan. The distinct bands are really an artefact of perception. For endurance, the Guinness record belongs to a rainbow that lasted 8 hours 58 minutes over Taipei's Yangmingshan range in November 2017.
Source: Rainbow
