Some quite Interesting Facts about rainbows

Until the 17th century, no one had the faintest idea what a rainbow was, how it got there, or what it was made of. Homer thought that the rainbow had only one colour – purple. Xenophanes moved things on a bit and gave it three: purple, yellow-green, and red. In his Meteorologica, Aristotle insists: “The rainbow has three colours, and these three, and no others”; and this idea persisted until the Renaissance, when four colours (red, blue, green, yellow) were fixed upon.
By the time Isaac Newton turned his attention to them, pretty much everyone in the West agreed there were definitely five colours (red, yellow, green, blue, purple). To the average Chinese, even today, rainbows have five colours. And in the Baltic countries, for most of history people believed that rainbows had only two significant colours – red and blue. “Rainbow” in Estonian is vikerkaar.

Seven

It was Newton’s great rival, René Descartes (1596-1650), who first realised in 1637 that rainbows were caused by light from the sun being split into different colours by rain. But it is to Newton we owe the notion that rainbows consist of seven colours. Newton came up with this perception in 1666, holed up alone in his remote Lincolnshire farmhouse having escaped Cambridge, which was being ravaged by the plague.
The idea that the universe runs to the rule of seven goes back at least to Pythagoras – seven notes in the musical scale, seven days of the week, and, until the discovery of Uranus by the German-born, British-based William Herschel (the German for “rainbow” is Regenbogen) in 1781, seven planets – which is why Newton warmed to the idea. He added the colour indigo, along with orange, to give us a neat spectrum of seven colours.

Not seven

Of course, there aren’t really seven colours in a rainbow, nor any particular number. Each “colour” shades imperceptibly into the next with no hard boundary other than those imposed by human observers of different cultures.

Iris

The Greeks and Romans thought a rainbow was the path made by Iris, the goddess of the rainbow, between heaven and earth, linking gods with humans. The iris of the eye is named after her, because of its colour. “Rainbow” in Latin is arcus iris or arcus pluvius, a “rainy arch”.
The Greeks used the word “iris” to describe any coloured circle, such as the “eye” of a peacock’s tail. The flower called iris gets its name from the Greek, as does the chemical iridium (Ir), compounds of which are highly coloured. Iris is also the root of “iridescent”.

Here comes the science part

A rainbow doesn’t actually exist, so it doesn’t really have an end where you can find a pot of gold. Optically speaking, it is just a distorted “virtual image” of the sun. Each raindrop acts as a tiny, imperfect mirror. When the sun is right behind you its light passes through the raindrops in front of you, reflects off their rear surface and bounces back at you. The light is refracted or “bent” slightly as it passes from the air into the water; and again as it bounces back into the air again. The different wavelengths that combine to make daylight are “bent” by different amounts (42º for the red end of the spectrum, a shade less for the violet). Each raindrop acts as both prism (refraction) and mirror (reflection).
The world’s longest-lasting (or longest-observed) rainbow was seen over Sheffield from 9am to 3pm on 14 March 1994.

Bow

A rainbow is a portion or arc (the Italian for “rainbow” is arcobaleno) of a circle whose centre is the shadow of your head. In some situations, for example skydiving, you can see the whole ring. From a mountaintop you might see a half-circle or more; usually, the 42° rule means the sun has to be fairly low in the sky before you can see a rainbow at all.

Secondary to supernumerary

As well as the “primary arc” of a rainbow, it’s quite common to see a second bow outside it, with the spectrum reversed. These happen when light bounces around inside a raindrop more than once before making it out. Third and fourth and even multiple bows can sometimes be seen. So, occasionally, can “supernumerary” bows: extra bands of green and violet caused by complex interference patterns.
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