Unravelling the mysteries of rainbows

We seldom spare a thought as to why and how rainbows are formed.

Throughout human history, rainbows have been a source of wonder and curiosity in mythology and folklore. In Australian aboriginal tales, passed down from generation to generation, they are recorded as snakelike creatures. Norse mythology tells of a rainbow as a connection from humans to the home of the gods.

In The Bible, in Genesis 9, Noah built an altar after the ‘Great Flood’ and a rainbow appeared as a sign of approval from God.

In Malay, a rainbow is called ‘pelangi’, meaning striped, and is seen as the pathway from which spirits return to the earth.

In 350 BC, Aristotle, a Greek, published the text ‘Meteorologica’, in which he suggested that a rainbow is a rare reflection of sunlight off water droplets held within rainclouds. Seventeen centuries later, in 1304, German monk Theodore of Freiburg proposed a different theory. Using experiments with glass prisms and spherical flasks of water, he concluded that the path of sunlight, via a water droplet to the human eye, produces a rainbow.

It was French mathematician Descartes, who in his 1637 essay ‘Les Meteores’, explained how sunlight is refracted as it enters a raindrop and then is reflected by the curved surface at the back of the raindrop only to be further refracted as it moves from the water globule back into the air. Through the use of a flask, he calculated from different viewpoints through the flask the angles of reflection.

Later, in the same century, Sir Isaac Newton and his throng provided various theories on how the different wavelengths of colours mix in what we perceive as daylight.

Today, meteorological physicists have proved that large raindrops are not spherical, but are flattened on their undersides as they fall owing to air resistance.

There are yet more rainbow mysteries yet to discover!

Why and where do rainbows appear?

The most usual form of this phenomenon is termed a primary rainbow with a radius of 46 degrees. Located at a point in the sky directly opposite to the sun, the higher the sun is in the sky, the lower the top of the rainbow. Thus, in the tropics, as here in Borneo, we never see a rainbow in the middle of the day and only in the early morning or towards sunset.

The late afternoon photo accompanying this article taken from my house in Somerset, England, illustrates this point.

This primary rainbow is created when sunlight is reflected back towards the observer’s eyes by raindrops. The reflection occurs on the back of the raindrops which then disperse light into the coloured spectrum with red always appearing on the outside of the rainbow and violet on the inside.

Secondary rainbows

Often light is reflected twice within the raindrops before reaching one’s eyes. This double reflection produces a secondary bow with a radius of about 51 degrees outside and above the primary bow.

This double rainbow is featured in the second photo taken in the early morning from Saradise towards Kuching Airport’s runway.

The colours of the secondary rainbow are reversed with red appearing on the inside of the arc and violet on the outside and is usually fainter than the primary bow beneath it. The area between the two arcs is usually much darker than the rest of the sky. This dark area is known as ‘Alexander’s dark band’, and is caused by raindrops in that area of the sky not returning light to the observer and instead directing it outside the field of view.

At sunrise and sunset, because short wavelengths (violet and blue) are scattered by the atmosphere, light from the sun usually consists of long wavelengths (yellow, orange, and red) and these only appear in rainbows.

Sometimes a completely red rainbow may be seen.

In areas where large lakes exist a lake’s flat reflecting surface between the sun and the observer produces its own rainbow. As the light travels upwards from the surface of the lake the centre of any reflected bow lies higher in the sky yet in the same direction as a normal bow with only a small part of such a bow visible to the naked eye appearing as an almost vertical band of colour.

Such a feature I momentarily observed in 1976 in the English Lake District National Park on a Geography fieldwork exercise.

Spray bows

Large and even small waterfalls with their larger water droplets created by spray provide spectacular rainbows as witnessed at the Niagara Falls (USA/Canada border) and the Victoria Falls on the Zambezi River (Zambia/Zimbabwe border) and may be even seen over large fountains.

The spray from such has the same angular dimensions as those created by raindrops. Very fine sprays from fountains produce colourless bows.

Dew bows

These provide similar colours to those of a rainbow and are often seen when short grass is covered by a network of spiders’ webs. Dewdrops, hanging from the network of spider webs, react like raindrops, reflecting and dispersing the light in a spectrum.

A dew bow is centred on the point opposite to the sun, and as dew drops are located on a horizontal plane, thus the dew bow is not a circular arc but appears as an ellipse. Such a sight I have witnessed on the village’s well-kept cricket pitch very near my house in the early morning in summertime.

Fogbows

They take the form of a white arc in the sky, with a similar radius of 42 degrees as a primary rainbow. The water droplets in the fog, mist or very low cloud are very small and thus the sunlight is no longer reflected back and refracted from within them and, instead, is scattered back towards the observer. Sometimes you may see very faint blue tinges on the inner edge and red tinges on the outer edge.

Likewise, a cloud bow can be seen from an aircraft. If flying within the cloud it appears circular but above the cloud the bow is projected onto the top of the cloud with its arms stretching away from one’s eyes. Both a fogbow and cloud bow I have seen; the former while driving to school in Kuching in the early morning an hour after sunrise, and the latter when flying from Kuala Lumpur to Kuching in the afternoon.

Do, please, lookout for such picturesque features of our ever-wonderful world and try to capture such phenomena with your camera.

I often sing, while walking in the countryside, the famous song from the Wizard of Oz entitled, ‘Somewhere Over the Rainbow’, and think of the late Bishop Desmond Tutu who, in 1994, described South Africa as the ‘Rainbow Nation’ when apartheid was abolished, and that country held its first democratic election.