Monday, 3 March 2014

Awesomosity of Rainbows - Part 2


A lovely rainbow
So, we can explain the basics but what about the fancy bits? Carrying on from my last post, here is the original picture of the rainbow I took, and what follows is advanced rainbology.

Alexander's Band is the space between the primary and the secondary rainbow and was first described by Alexander of Aphrodisias, an Ancient Greek famous for being a commentator on Aristotle. What Alexander noticed is that the sky in-between the primary and secondary rainbow is darker. The image on the right is cut from my initial photo and then I've fiddled with the contrast to show the difference in brightness on each side of the Alexander's band. Trust me, there is no trickery here!


Contrast Enhanced: Alexander's Band
The explanation for this is positively fiendish, and to do it justice really needs a dozen ray diagrams, but they can be found elsewhere on the internet. However, the key is to appreciate that the primary rainbow does not stop with violet, it effectively continues down below the coloured bands. However, in this area we are seeing all the colours of the rainbow, reflected from other parts of other raindrops. This combination of colours is perceived by us as white light and a lightening of the sky. The reason we see a rainbow at the top of this lightened region is because we hit the minimum angle of deviation. Above the rainbow, raindrops reflect light, but at an angle that means it can never reach our eyes, no matter how the light goes through the raindrop. The effect on the other side of the Alexander's band is the same, and is part of the secondary rainbow, but fainter and flipped upside down since the light bounces twice inside the raindrop. The net result of all this is the bands of sky above and below are brightened and the sky between the rainbows appears darker.


Contrast Enhanced: Primary and Supernumerary Rainbow
All of this can be understood with straight forward geometric, classical optics. To understand the supernumerary rainbow you need to invoke the wave nature of light. The doctored image above shows a chunk of the primary rainbow from the top of the original photo. Once again, all I've done is bump up the contrast. Just inside the final violet band of the primary rainbow is another strip of colour. I've convinced myself there is a smudge of yellow then green, followed by a clear blue and violet band. This is the supernumerary rainbow and it is caused by constructive interference of light reflected in the same way as the light that makes the Alexander's band. You can't explain this with geometry, it needs light to be a wave. It pretty rare, normally only seen it in little patches and indicates very small and very uniform raindrops. What's most interesting to those interested in the history of science is that it gave Thomas Young the first clues to the wave like nature of light. He spoke about it during a lecture in 1804 he gave at the Royal Society in London. The transcript can be found online and the interesting bit starts on page 8.

So there you have it, rainbows in all their complicated glory. If you want more, there are great pictures and explanations here and here (although this one is in German). I've always believed that an understanding of science, rather than taking the magic away, makes the world around us more exciting. When you see something as wonderful as a rainbow, with all the rarer, accompanying phenomena - and have a full understanding of what is going on - it takes the spectacle to a whole new level.

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Science TV Presenter, live show performer, writer, strange prop builder and all round Science Bloke. All opinions expressed are mine alone. Not the BBC's, just mine.