Daniel MacCannell - Rainbows: Nature and Culture

Rainbows are not objects. They cannot be approached or touched, and cannot be said to have any actual size. Every rainbow is a distorted image of the Sun, and like the Sun, it would appear as a full circle if the surface of the Earth did not get in the way. Each rainbow is very much in the eye of the beholder: multiple observers can see similar rainbows in the same approximate location at the same time, but no two people can actually see the same rainbow, even if they are standing directly beside each other. This is because the centre of each rainbow-circle is located on an imaginary line that could be drawn from the Sun into the back of the individual observers head, out again through his or her eye, and to the shadow of the head on the ground. As such, the commonplace childrens book and advertising image of a rainbow beside or in front of the Sun could never be seen in reality, except perhaps on a planet with more than one sun. Moreover, rainbows are created by the bending of sunlight through drops of water in the air, and such drops are always in motion generally, towards the ground. Thus, even an unmoving observer is not seeing the same rainbow from one instant to the next, but rather a lengthy series of very similar rainbows, created by a succession of new falling water particles as they pass through the portion of sky that is relevant to rainbow production from the standpoint of that individual person. Nevertheless, for reasons that will be explained below, the bow always measures 2 degrees in thickness/height, and always appears at an angle of 4042 degrees above the aforementioned imaginary line through the observers head.

It follows from all this that the rainbow appears largest when the Sun is lowest in the sky that is, passing the horizon at sunrise or sunset. A person at sea level can only see the rainbow as a half-circle when the Sun is at the horizon, while to see it as a full circle requires him or her to be at a very high vantage point, typically on a mountaintop or in an aircraft. It also follows that the higher in the sky the Sun is at the moment of observation, the smaller the rainbow will appear; and when the Sun moves more than 42 degrees above the horizon, the rainbow must necessarily disappear altogether. Consciously or not, it may have been for this reason that the best-selling British science fiction author Douglas Adams (19522001) chose 42 as his comedic Answer to the Ultimate Question of Life, the Universe and Everything.

It became accepted in European scientific circles in the seventeenth century though not before then that the rainbow has seven colours, with the innermost band being violet, followed as one looks upwards by indigo, blue, green, yellow, orange, and finally red at the top. In fact, its colours shade into one another continuously, as black-and-white photographs of rainbows reveal. It is human perception partly biological, and partly learned that groups these fine colour gradations into seven (or some other number), probably via a cognitive process known as Metamerism refers to the fact that the normal human eye contains only three types of colour receptors, known as cone cells, with each type especially sensitive to a particular range of wavelengths of light; these sensitivities are strongly overlapping, but are centred around 440 nanometres (nm) (blue), 540 nm (green) and 570 nm (yellow/orange). It follows that it is more difficult for us to perceive red light (700 nm) than the other aforementioned colours; and where the wavelength of light is beyond 810 nm, we have no ability to perceive its colour at all. Certainly, due to this biological structure some might say defect of the human eye, we are able to see the same colour as the product of quite different combinations of wavelengths of light, and are also tempted to see clumps of colour where objective measurement finds none. Nevertheless, the precise reasons for our perception of the rainbow as a set of coloured bands remain the subject of speculation, as Ari Ben-Menahem explains:

However distinct they seem, the separateness of the coloured bands that make up the rainbow is produced in the mind.

Diagram from Grays Anatomy (1918) illustrating the relation of the rods and cones to other neurons within the human retina.

All true rainbows are produced by the interaction of sunlight with water droplets often but not always raindrops in the atmosphere. As well as rain, bows are frequently observed in mist, fog, dew and spray. Our understanding, via computer simulation, of precisely how the shapes and sizes of these water particles affect a bows size, precise hues, visual sharpness and apparent luminosity is in its infancy. That being said, we do know that only relatively small raindrops contribute to the broadly horizontal crown of the rainbow arch, whereas all sizes of drops contribute to its legs, and this is what makes the bow so often appear brighter nearest to the ground. Rainbows produced by moonlight, known as moonbows, contain the same colours in the same order as regular rainbows, since moonlight is merely reflected sunlight. However, with each reflection, a considerable amount of light is lost, and this is especially so in the case of objects (including the Moon) that are not smooth. This renders moon-bows so faint in comparison to normal rainbows that their colours can easily pass unnoticed. Ice crystals in the atmosphere can also produce a range of rainbow-like phenomena that are described briefly at the end of this chapter.