Richard Dawkins - The Magic of Reality

This myth has a strange ending. As the people were crossing over the rainbow bridge, some of the noisy ones looked down – and they were so frightened by the drop that they got dizzy. They fell off the rainbow into the sea, where they turned into dolphins.

The idea of the rainbow as a bridge crops up in other mythologies, too. In old Norse (Viking) myths, rainbows were seen as fragile bridges used by the gods to travel from the sky world to Earth. Many peoples, for example in Persia, west Africa, Malaysia, Australia and the Americas, have seen the rainbow as a large snake which soars out of the ground to drink the rain.

How do all these legends start, I wonder? Who makes them up, and why do some people eventually come to believe these things really happened? These questions are fascinating and not easy to answer. But there’s one question we can answer: what is a rainbow really ?

When I was about ten, I was taken to London to see a children’s play called Where the Rainbow Ends . You almost certainly won’t have seen it because it is too unfashionably patriotic for modern theatres to perform. It is all about how exceptionally special it is to be English, and at the climax of the adventure the children are rescued by St George, the patron saint of England (not Britain, for Scotland, Wales and Ireland have their own patron saints). But what I most vividly remember is not St George but the rainbow itself. The children actually went to the place where the rainbow planted its foot, and we saw them walking about in the middle of the rainbow where it hit the ground. It was cleverly staged, with coloured spotlights beaming down through swirling mist, and the children stumbled about in a spellbound daze. I think it was at about this moment that the shining-armoured, silver-helmeted St George appeared, and we children gasped at the scene as the children on the stage shouted: ‘St George! St George! St George!’

But it was the rainbow itself that seized my imagination. Never mind St George: how wonderful it must be to stand right in the foot of a giant rainbow!

You can see where the author of the play got the idea. A rainbow really does look like a proper object, hanging out there, perhaps a few miles away. It seems to have its left foot planted, say, in a wheat field and its right foot (if you are lucky enough to see a complete rainbow) on a hilltop. You feel you ought to be able to go straight to it and stand right where the rainbow steps on the ground, like the children in the play. All the myths I have described to you have the same idea. The rainbow is seen as a definite thing, in a definite place, a definite distance away.

Well, you’ll probably have worked out that it isn’t really like that! First, if you try to approach the rainbow, no matter how fast you run, you’ll never get there: the rainbow will run away from you until it fades away altogether. You can’t catch it. But it isn’t really running away because it isn’t really in a particular place at all, ever. It’s an illusion – but a fascinating illusion, and understanding it leads on to all sorts of interesting things, some of which we’ll come to in the next chapter.

First, we need to understand about something called the spectrum. It was discovered in the time of King Charles II – that’s about 350 years ago – by Isaac Newton, who may well have been the greatest scientist ever (he discovered lots of other things besides the spectrum, as we saw in the chapter on night and day). Newton discovered that white light is really a mixture of all the different colours. To a scientist, that’s what white means .

How did Newton find this out? He set up an experiment. First he blacked out his room so that no light could get in, and then he opened a narrow chink in the curtain, so that a pencil-thin beam of white sunlight came in. He then let the beam of light pass through a prism, which is a sort of triangular chunk of glass.

What a prism does is splay the narrow white beam out; but the splayed-out beam that emerges from the prism is no longer white. It is multicoloured like a rainbow, and Newton gave a name to the rainbow he made: the spectrum. Here’s how it works.

When a beam of light travels through air and hits glass, it gets bent. The bending is called refraction. Refraction doesn’t have to be caused by glass: water does the trick too, and that will be important when we come back to the rainbow. It is refraction that makes an oar look bent when you stick it in the river. But now here’s the point. The angle at which light bends is slightly different depending on what colour the light is. Red light bends at a shallower angle than blue light. So, if white light really is a mixture of coloured lights, as Newton guessed, what’s going to happen when you bend white light through a prism? The blue light is going to bend further than the red light, so they will be separated from each other when they emerge from the other side of the prism. And the yellow and green lights will come out in between. The result is Newton’s spectrum: all the colours of the rainbow, arranged in the correct rainbow order – red, orange, yellow, green, blue, violet.

Newton wasn’t the first person to make a rainbow with a prism. Other people had already got the same result. But many of them thought the prism somehow ‘coloured’ the white light, like adding a dye. Newton’s idea was quite different. He thought that white light was a mixture of all the colours, and the prism was just separating them from each other. He was right, and he proved it with a pair of neat experiments. First, he took his prism, as before, and stuck a narrow slit in the way of the coloured beams coming out of it, so that only one of them, say the red beam, passed through the slit. Then he put another prism in the path of this narrow beam of red light. The second prism bent the light, as usual. But what came out of it was only red light. No extra colours were added, as they would have been if what prisms did was add colour like a dye. The result Newton got was exactly what he expected, supporting his theory that white light is a mixture of light of all colours.

The second experiment was more ingenious still, using three prisms. It was called Newton’s Experimentum Crucis, which is Latin for ‘critical experiment’ – or, as we might say, ‘experiment that really clinches the argument’.

White light passed through a slit in Newton’s curtain and through the first prism, which spread it out into all the colours of the rainbow. The spread-out rainbow colours then passed through a lens, which brought them all together before they passed through the second of Newton’s prisms. This second prism had the effect of merging the rainbow colours back into white light again. That already neatly proved Newton’s point. But just to make quite sure, he then passed the beam of white light through a third prism, which splayed the colours out into a rainbow again! As neat a demonstration as you could wish for, proving that white light is indeed a mixture of all the colours.

Prisms are all very well, but when you see a rainbow in the sky, there isn’t a great big prism hanging up there. No, but there are millions of raindrops. So, does each raindrop act as a tiny prism? It is a bit like that, but not quite.

If you want to see a rainbow you have to have the sun behind you when you look at a rainstorm. Each raindrop is more like a little ball than a prism, and light behaves differently when it hits a ball from how it behaves when it hits a prism. The difference is that the far side of a raindrop acts as a tiny mirror. And that is why you need the sun behind you if you want to see a rainbow. The light from the sun turns a somersault inside every raindrop and is reflected backwards and downwards, where it hits your eyes.