At the Centre of the Rainbow Light Circle

Double rainbow on our third summer roadtrip (August 2016) – as seen in the Masuria region of Northern Poland.

Double rainbow on our third summer roadtrip (August 2016) – as seen in the Masuria region of Northern Poland.

I’m writing these lines after a day full of sightseeing lying in my hostel bed in Saigon, Vietnam. In stead of coming up with Vietnamese travel tales, however, this is going to be a different sort of post – not really about traveling at all, but rather about the magic of reality (but traveling often enough covers both: magic and reality).

This post is largely inspired by the fabulous sixth book of British evolutionary biologist Richard Dawkins, called Unweaving the Rainbow. In fact, this post is about exactly that: unweaving rainbows, metaphorical so, but also quite literally. To begin with: Dawkins wrote the book as a response to the claim that science would “take away” from the magic of reality, that it would diminish the view of the world – wrapping it up with some sort of sober and overly rational scientific blanket. Dawkins argues quite the opposite: Unweaving the rainbow does not diminish the imagination, it actually enlarges the picture as a whole: To actually grasp what a rainbow really is and what else there is to it is not only marvellous, it can be quite mesmerising. And that is what this post is about.

A rainbow as seen from the windows of my flat in Berlin-Friedrichshain.

A rainbow as seen from the windows of my flat in Berlin-Friedrichshain.

So we all know how a rainbow comes into being, right? In short: It is caused by reflection, refraction and dispersion of light in droplets of water in the sky. When caused by the light of the sun a rainbow always appears in the opposite part of the sky. This is because the back surface of raindrops (which are roughly spherical) act as concave mirrors. The lower the sun, the higher the rainbow, but if the sun is higher in the sky than 42 degrees above the horizon we can’t see a rainbow at all. (This is why we see rainbows usually in early morning or late afternoon.)

So the light of the sun hits the rain droplets’ concave walls, then gets reflected, leaves the drops and will land at your eyes, but not before being refracted while moving from water back into air. We see the complete light spectrum (from red via yellow and green to violet) because there are so many raindrops, enough for all the colours to hit your eyes.

Steep rainbow curve as seen in Bergen, Norway in September 2018.

Steep rainbow curve as seen in Bergen, Norway in September 2018.

Now, the fascinating part (something I never really bothered pondering about, but it is really quite worth it). All those different raindrops being able to perform a complete spectrum for you to see from whatever angle means something else as well: The rainbow you are seeing is slightly different all the time, depending on your position. It is not only different for every single being who sees it, it is (consequently) also different for each of your eyes. You are always standing at the centre of your personal rainbow, piecing it together from different collections of raindrops. (So everyone around you is always positioned at the centre of their very own rainbow).

And when you are staring at a rainbow while sitting inside a car or train you do not actually see “the same one” rainbow, but a steady series of rainbows in quick succession.

“Observe the rays of the sun in the composition of the rainbow, the colours of which are generated by the falling rain, when each drop in its descent takes every colour of the bow.”

(Leonardo da Vinci, Treatise on Painting in the 1490s)

Every colour of the bow, Leonardo remarks – but a bow, really? The classic romantic portray of the rainbow is caused by the illusion that it is always pegged far away at the horizon, impossible to approach. But a rainbow only appears as a semicircle (or bow) because “the horizon gets in the way of the lower part of the circle” as Dawkins puts it.

Red rainbows (and sometimes even monochrome rainbows) are possible when the colours with shorter wavelengths like blue and green are scattered and thereby removed from the spectrum.

Red rainbows (and sometimes even monochrome rainbows) are possible when the colours with shorter wavelengths like blue and green are scattered and thereby removed from the spectrum.

So when we see a rainbow, we are in fact merely seeing a part of the whole. And the reason it appears to be so huge (and hence far away) is that our brain is playing tricks: It projects the image we receive with our eyes “outwards on to the sky” (Dawkins), an effect easily being imitated by staring into some bright light (e.g. a lamp) and stamping the after-image on your retina before projecting it onto the sky.

So how can we see an actual full-circle rainbow? The largest section of the rainbow we would be usually able to see is about 50 % at either sunrise or sunset. In order to see the rainbow’s lower half there would need to be raindrops below the observer’s horizon plus sunlight that reaches them. This can be achieved by watching a rainbow either from a (very) high building or an airplane.

It is easy to create a (little) full-circle rainbow yourself by creating a water mist e.g. with a garden hose while facing away from the sun.

Note the reversed colours of the fainter upper rainbow.

Note the reversed colours of the fainter upper rainbow.

Why are there double rainbows? One not seldomly sees an additional (yet fainter) rainbow (at 8 degrees higher than the first and with reversed colours) which happens when light enters the raindrops both through the upper and the lower quadrant – under the right conditions the light can then “be reflected twice round the inside of the sphere” (Dawkins).

So much for rainbows on blogposts from my side! Keep all that in mind the next time you happen to experience a double rainbow (and possibly a monochrome one).

If you want to get to know more about what else there is to be unwoven I can only recommend to get a copy of the book mentioned in the beginning (see picture below). I will end this post with the following (highly enchanting) words by Richard Dawkins while celebrating the birthday of a dear friend of mine. Thank you for inspiring so many of my days. Much love to you!

“We are going to die, and that makes us the lucky ones. Most people are never going to die because they are never going to be born. The potential people who could have been here in my place but who will in fact never see the light of day outnumber the sand grains of Arabia. Certainly those unborn ghosts include greater poets than Keats, scientists greater than Newton. We know this because the set of possible people allowed by our DNA so massively exceeds the set of actual people. In the teeth of these stupefying odds it is you and I, in our ordinariness, that are here.”

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