A double rainbow over Tullaghan Bay

Keats accused Newton of destroying all the poetry of the rainbow ‘by reducing it to the prismatic colours’. When I first started to notice the rainbows of Tullaghanbaun, part of me didn’t want to see the poetry drowned in a storm of science and angles of reflection and refraction. Isn’t it enough to know that the rainbow comes and goes, I thought, not a thing in itself but a fleeting relationship between me and the sun?

No, it’s not enough. Familiarity breeds not contempt, but curiosity. I began to ask myself why so many bridges to nowhere here, in this land of no promise?

The physics is fairly straightforward. It needs to be, for me to understand it. When light moves from one medium into another where its speed is different, the light ray is bent, or refracted. Because different wavelengths are bent to different extents, the light separates: red bends less sharply than violet, and this refraction is what creates a spectrum of colours.

As light hits the ‘front’ of a droplet of rain it is refracted and separated: instead of passing straight through, some of this refracted light is then reflected from the ‘back’ of the droplet, which acts as a mirror of sorts. And as the light bounces in the general direction of its source, it is refracted once more as it leaves the medium of the water and re-enters the air, and so it separates further. This process of refraction, reflection and refraction is what creates a primary rainbow. The secondary bow, outside the primary and always dimmer than its companion, is caused by light reflecting not once, but twice, bouncing across the back of the droplet before exiting. That second reflection is the reason why the colours are reversed in the secondary.

A secondary bow is always there in a theoretical sort of way, but it is usually too faint to be visible unless the primary is bright. Even more theoretical are the tertiary rainbow, the product of three internal reflections; and the quaternary, the product of four. Because light is lost with each reflection, it is rare for these to be visible with the naked eye. Both were photographed for the first time in history in 2011, and three years later a quinary rainbow was observed and photographed in New Mexico appearing in Alexander’s dark band, between the primary and secondary, so faint that even with enhanced imaging you’d be hard put to see it.

In normal circumstances, the most you can hope for is a double rainbow, and the only way to see either a single or a double is to have the sun behind you and rain in front of you. The best time of day is early or late, because the lower the sun is in the sky, the more rainbow you see. Think of a rainbow as the edge of a disc of light, the centre of which is diametrically opposed to the sun in the sky. So as the sun rises, more of this disc sinks below the observer’s horizon, and when the sun reaches a certain height it vanishes altogether. ‘The noonday-bow is therefore best seen “smiling in a winter’s day”,’ said Constable, ‘as in the summer after the sun has passed a certain altitude, a rainbow cannot appear.’

So you need sun for a rainbow. You also need rain: the clue is in the name. And there is plenty of rain here. In most months of the year, it rains on most days in Erris. But the curious thing is that the rain rarely lasts for more than a few minutes, and the showers are so localised that I watch them coming, sweeping in across the bog with that striped, jagged look that the sky has as clouds break under the weight of water. Or they creep over the Nephin Begs, covering their peaks in white to tumble down the mountainside and race towards me across the bay.

These localised showers bring Tullaghan its rainbows. The sun shines at my back and the sky turns dark and suddenly, for a moment there is Bifröst linking earth and heaven, there is God’s covenant. There is the poetry.

A rain shower moving down the bay

[For my explanation of how rainbows work, I’ve drawn on The Rainbow Bridge: Rainbows in Art, Myth and Science, by Raymond L. Lee and Alistair B. Fraser (Pennsylvania State University Press, 2001). Needless to say, if I’ve got it wrong the fault is mine and not theirs.]