Thursday, 27 February 2014

Awesomosity of Rainbows - Part 1

Rainbow over Otley
A few days back I posted this picture snapped from my office window. I promptly tweeted the photo and blithely announced you could clearly see the the primary and secondary rainbow, the Alexander's band and a supernumerary rainbow. Later that day my son demanded an explanation of these effects and I was somewhat stumped. I could mostly answer him, but the enquiring mind of a nine year old saw through my subterfuge and the BS that I was trying to spin. I have rectified this situation and can now give a better answer, although it's a long one and to spare you I have split it over two blog posts

First the basics: the main rainbow in the picture is gloriously luminous and made when sunlight hit an area of rain drops, behind the houses in the foreground. The light going into the raindrops, bounces once off the back and comes out the front. Since different colours of light, or wavelengths are bent more or less by this process (of reflection and refraction), the colours are split up and you see a rainbow.

Over the years I have done a few films about rainbows and spoken to many people on the subject. There are a few things that constantly astonish folk. First, that what you see as a rainbow is actually built up from millions of tiny points of light, each from an individual rain drop. The rainbow is effectively pixelated. Secondly, people never cease to be amazed when you point out that since the position of a rainbow depends on where the viewer is standing, we each see our own, unique rainbow. To take this to its logical extreme, each eyeball sees a different rainbow. There is something deeply satisfying to think that the rainbow you see is just yours and yours alone.

As an aside, people are not particularly bothered to hear why you can never get to the end of a rainbow. Possibly this is just common knowledge, or maybe lacing nature with a touch of Leprechaun based greed prevents them from listening.

Contrast enhanced: primary (bottom) & secondary (top)  
The secondary rainbow on the day of my picture was unusually vivid. Normally a secondary rainbow, a little further out from the main rainbow, is more diffuse and very, very faint. So faint that people often fail to notice it, but it's often there. It's caused by the exact same process as the main rainbow, except the light bounces twice inside the raindrop. Which is why the colour sequence is reversed, red on the inside of the bow and violet on the outside.

Astonishingly, the explanation for both the primary and the secondary rainbow was laid out, pretty much in full at the end of the 13th Century. It was discovered independently at about the same time by both an Iranian scholar, Kamal al-Din al-Faris, and a German Dominican friar known as Theoderic of Frieburg. Both men used spherical glass flasks to show the path of light inside a raindrop, presumably making their own rainbows. The other name that often crops up with rainbows is that of Isaac Newton.

By the middle of the 1600s, the issue that remained unanswered was where do the colours seen in the rainbow spring from. If you take a glass prism or a water filled sphere you can make a rainbow. White light goes in, colours come out. There were two competing ideas, either the prism or sphere coloured the light some how, or white light was made up of all the colours. On the face of it, neither seems a particularly likely explanation. Then in 1666, while Newton was at home in Woolsthorpe, Lincolnshire, he performed his experimentum crucis or critical experiment (and weirdly, as I write this, I have just passed this building on my train down to London). He took a ray of sunlight, split it into a rainbow with a prism, used a lense to focus it back onto another prism and showed that white light came out the other end. Thus, white light is made of a mix of coloured light. We know he did the experiment in Woolsthorpe because of his journals, but also in his notes he lists the distance from the hole in his shutter through which the ray of sunlight shone to the far wall. I have measured his room in Woolsthorpe and it is spot on. At the time, I was told by the National Trust curators that I was the first to do this, but I don't believe it. Still it's a good story for the kids.

As an aside, Newton laid down the daft colours of the rainbow we have. I mean, can you tell the difference between indigo and violet? Initially he just had five (red, yellow, green, blue and violet) but later added orange and indigo to the list. Significantly, this gave him seven colours and that suited his penchant for alchemical numerology: seven colours for seven musical notes and seven planets (that he knew about).

That covers your basic, commonly seen rainbology, for Alexander's band and supernumerary rainbows take a peek at the next post...


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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.