A splash of colour

More supermarket science for the rather sweet lifestyle magazine The Simple Things. This time it’s a little discourse on colour. Just in case you should happen to pick this up at the checkout and wonder about the first paragraph, this is, for the record, what the piece looked like at the outset.

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Every culture has been entranced by rainbows. The Babylonians kept records of the most spectacular ones, and in Judaeo-Christian tradition the rainbow symbolises the covenant between God and the world. Australian Aborigines honour the Rainbow Serpent; for the Vikings the coloured arch was a bridge to Asgard. All this reflects astonishment at a vision in the sky that seems to be made of pure colour.

It was suspected for a long time that the rainbow holds the key to what colour itself is. Islamic philosophers in the early Middle Ages knew that you could make a kind of artificial rainbow by passing sunlight through glass or water to produce a spectrum, with its sequence of bright colours from red and yellow to blue and violet. The connection was first fully explained by Isaac Newton in the seventeenth century, who showed that “white” sunlight actually contained all the colours of the spectrum and that a glass prism could tease them apart. He said that rainbows are made when water droplets in the atmosphere act like little prisms.

So for Newton, colour was all about light, which he imagined as a stream of tiny particles that strike our eye and cause vibrations of its nerves. Vibrations of different “bigness”, he said, create sensations of different colours. That’s not so different from the modern view, although we now regard light as a wave, not a particle. Little protein molecules in our retina absorb light waves of different wavelength, triggering signals along the optical nerve that our brain interprets as colours. The longer the wavelength, the further towards the red end of the spectrum the colour is.

But is colour really so simple? The odd thing about Newton’s theory is that it implied that, if you mix all the colours of the rainbow, you should get white, whereas painters knew very well that this just makes a murky brown. What’s more, it was well known that a colour can look different in different light (at dusk, say), or depending on what other colours are next to it.

The puzzle about mixing was solved in the nineteenth century, when the Scottish scientist James Clerk Maxwell showed that mixing light is not like mixing paint. Pigments and dyes are coloured because they absorb some parts of the spectrum – the colour we see is what’s left, which is reflected to our eyes. So if you mix them, you absorb more and more colours until there’s virtually none left, and the mixture looks black. But if you mix coloured light, you’re adding rather than taking away. As you can see from looking at television pixels close up, red, blue and green light are enough in combination to look white from far enough away.

Even then, colour – like taste, smell, and music – is ultimately something made in the mind. That’s why colours that are “the same” according to their wavelengths of light can look quite different depending on what’s around them. As the philosopher and writer Johann Wolfgang von Goethe stressed in the early nineteenth century, colour is partly a psychological thing too.

What’s more, there are lots of ways to produce it. Most of the colour we see in nature is made by light-absorbing pigments. Chlorophyll molecules in grass and leaves, for example, absorb red and blue light, reflecting the yellow and green. But the blue of the sky comes from the way light bounces off molecules in the air: the blue light is scattered most strongly, and so seems to come from all over the sky. And some of nature’s most wonderful colour displays are produced in a similar way – not by absorbing light but by scattering it.

Take the blue Morpho butterfly, which seems positively to glow in South American forests as if it is lit up, so that it can be seen from a quarter of a mile away. Its wing scales are covered with microscopic bristles of cuticle-like material, each bearing a stack of shelf-like corrugations. Light waves bouncing off this stack interfere with one another so that some colours disappear and others are enhanced. These interference effects from tiny stacks or layers of material also produce the bright hues of the peacock’s tail and other bird plumage, and the iridescent shells of beetles. The colours are iridescent because the precise wavelength of light picked out by the interference depends on the angle you’re viewing from.

You get a similar bright spectrum of “interference colour” when light is reflected from the tiny dimples in CDs. In fact, technologists are now borrowing such colour-making tricks from nature to control light for fibre-optic telecommunications or to make iridescent paints. We are learning that there are many ways to make colour – and many ways to enjoy it.