New Narratives: Light

R. Dawkins, 2011. The Magic of Reality: How we know what’s really true.Bantam Press: London

Note: Hedgehogs and jaguars and many other mammals work by night and sleep by day.

Into Orbit

Why do the planets stay in orbit around the sun? Why does anything stay in orbit around anything else? This was first understood in the seventeenth century by Sir Isaac Newton, one of the greatest scientists who ever lived. Newton showed that all orbits were controlled by gravity – the same force of gravity that pulls falling apples towards the ground, but on a larger scale. (Alas, the story that Newton got the idea when an apple bounced off his head is probably not really true.)

Newton imagined a cannon on top of a very high mountain, with its barrel pointing horizontally out to sea (the mountain is o the coast). Each ball it fires seems to start off moving horizontally, but at the same time it is falling towards the sea results in a graceful downward curve, culminating in a splash. It is important to understand that the ball is falling all the time, even on the earlier, flatter part of the curve. It’s not that it travels flat horizontally for a while, then suddenly changes its mind like a cartoon character who realises he ought to be falling and therefore starts doing so!

Light of our lives

I want to end this chapter by talking about the importance of the sun for life. We don’t know whether there is life elsewhere in the universe (I’ll discuss that question in a later chapter), but we do know that, if there is life out there, it is almost certainly a star.

What light is made of

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 the 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 multi-coloured like a rainbow, and Newton gave a name to 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 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. So, light is bent when it hits glass or water. But now here’s the point. The angle of the bend is slightly different depending on what colour the light is. Red light bends at a shallower than blue light. So, if white light is a mixture of coloured lights, as Newton guessed, what’s going to happen when you bend white light through a prism?n 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 when they 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 and violet.




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