When the sun shines onto the rain-darkened sky, nature's beautiful secret is revealed. In the arch that curves from the earth to the heavens we can read the origin of colors. Sunlight seems to 'take on' the color of anything it bounces off-a red rose or a green leaf-because all of these colors are already within the light, waiting to be sifted by an encounter with the tangible world. In the rainbow, raindrops do the sifting systematically, each band progressing through the visible spectrum from red to violet.

When Isaac Newton showed how this happened in the seventeenth century, he seemed to have solved at last the question that had puzzled and frustrated philosophers for centuries: what exactly is color? Yet Newton's answer was not the last word. Indeed, for some people it simply raised more questions. Painters struggled to understand what Newton's theory of light and color meant for the way they should apply their paints. The German Romantic philosopher Wolfgang von Goethe decided Newton's ideas were nonsense, and many were ready to agree with him. Even today it would be unwise to conclude that we fully understand color.

What did Newton say that created so much confusion and controversy? And why didn't his theory, brilliant though it was, tell the whole story? Why is color so hard to pin down?

Newton is often credited with 'explaining' (in John Keats' derogatory phrase, 'unweaving') the rainbow. But that is not quite what he did. Philosophers had known for centuries that light passing through glass, transparent minerals or water can generate a multitude of colors. The ancient Greeks speculated that rainbows are caused by sunbeams falling onto clouds, and in 1637 the French philosopher René Descartes showed that sunlight becomes focused into a circular arc when it bounces off raindrops.

It was Newton, however, who brought color to Descartes' rainbow. In1665 he split sunlight into the many-hued spectrum by passing it through a prism in a darkened room. He found that the individual colors could not be split further by a second prism. And if all the spectral colors were brought back together using a lens, they merged into a beam of white light. Newton deduced that the rays of different colors were being bent through different angles by the prism-and that the same thing happens in rainbows, where each raindrop acts like a tiny prism.

Newton decided that there were seven strands to this bow: red, orange, yellow, green, blue, indigo and violet. Schoolchildren are still taught the seven Newtonian colors, but in fact they are rather arbitrary: Newton's mystical thinking led him to imagine that the colors of the rainbow must mirror the seven notes of the musical scale. Most later color theorists chose to replace indigo and violet with just a single hue: purple or violet.

Color comes from plucking this rainbow. Objects absorb some of the colors, and reflect the rest. We see only the reflected rays, which determine the color we perceive. A red berry extracts green and blue from white sunlight; a yellow flower absorbs blue and red.

If color is just light, then what is light? Newton had a theory for this too, but it was not until another two centuries passed that the Scottish physicist James Clerk Maxwell gave us the modern answer. Light is a vibrating field of electrical and magnetic energy: an electromagnetic field, which can pass through empty space like a wave travelling across the sea. The frequency of the vibrations determines the color of the light: it gets progressively higher from the red to the violet end of the spectrum, while the wavelength gets steadily shorter.

Substances absorb light of a particular frequency because their clouds of electrons-the subatomic particles that bind one atom to another-have vibrations that resonate at the same frequency, like a guitar string humming in sympathy with a note sung loudly. These resonant frequencies depend on the chemical composition of the substance: which atoms it contains, and how they are joined together.

The pigments in flowers, animal skins and paintings derive their colors by absorbing light. But not all color is generated this way. The rainbow's variegated arc is not the result of light absorption by the raindrops, but of refraction: reflection of rays of different wavelengths at differing angles. This is an example of light 'scattering'. The sky is blue because blue light is scattered by molecules and dust in the atmosphere more strongly than red light: the blue rays from the sun bounce towards our eyes from all directions. Distant hills are blue-tinted for the same reason: the light reflected from the hills is augmented by blue from the atmosphere before reaching our eyes.

Some animal and plant colors are caused by light scattering. The blues on butterfly wings, for example, are produced by the microscopically ribbed surface of the tiny wing scales, the ridges spaced at just the right separation to reflect blue light but not red. The color of this scattered light can vary depending on which angle you view it from, giving rise to the iridescence of insect cuticle and the shimmering colors of a peacock's tail.

Artists and technologists interested in making colors have long recognized that there are two basic types of colored materials: those that come from the geological earth and those that come from the living world. The colored materials are respectively classed as inorganic ('non-living') and organic. When chemists today speak of 'organic' substances, however, they don't necessarily mean ones that originated in living organisms. Rather, they mean materials whose building blocks are carbon-based molecules. Many organic materials today are synthetic, made using industrial chemical techniques from the carbon compounds in crude oil, alcohol and other raw sources.

Traditionally, inorganic materials furnished pigments whereas organic materials provided dyes. The colors of dyes would usually fade when exposed to sunlight, because light breaks down the delicate light-absorbing carbon molecules.

Brightly colored inorganic substances usually contain metal elements. Some metals are apt to lend particular colors to their compounds: copper minerals are often green or blue, iron minerals red or yellow, cobalt minerals deep blue. Chromium is something of a chameleon, offering colors ranging from bright yellow to deep green and rich red. Its very name comes from the Greek word for color.

While rose quartz acquires its color from impurities of titanium or manganese, no such metals tint the rose itself: flowers and other living organisms are colored by organic compounds. Tyrian purple, the famous imperial dye of ancient Rome, was squeezed from shellfish; blue indigo was the frothy extract of a weed.

Nature owes its verdancy to chlorophyll, an organic molecule studded with a magnesium atom which imbibes the red and blue of the sun's rays. Chlorophyll channels this energy into the metabolic processes of plant cells. The light-absorbing heart of the hemoglobin in our blood is similar to that of chlorophyll, except that iron in all its ruddiness substitutes for magnesium. No longer do John Donne's words reflect our state of ignorance: 'Why grass is green, or why our blood is red/Are mysteries which none have reach'd into.'

Why roses are red and daffodils yellow is a question of the same order. The yellows, oranges and reds of many flowers, as well as of carrots, tomatoes and sweetcorn, are produced by substances called carotenoids. These so-called auxiliary pigments broaden the light-absorbing abilities of leaves, though their presence is usually masked by the strong absorption from chlorophyll. When in autumn the chlorophyll decays as the leaf dies, the golden colors of the auxiliary pigments shine through.

In his book Opticks, where Isaac Newton set out his theory of color in 1704, he did a curious thing. He bent the spectrum into a circle, marrying up red against violet so that the progression between all the colors became continuous. Newton invented the 'color wheel'.

This prismatic mandala organizes color into a pleasingly symmetrical pattern. Subsequent color theorists made the wheel even more symmetrical by cutting it up into six equal slices: red, orange, yellow, green, blue and violet.

The color wheel has come along way since then. Its modern incarnation is less pleasing to the eye but a lot more infomative: a figure drawn up by the Commission Internationale de l'Eclairage (CIE), called the CIE chromaticity diagram. The 'pure' colors of Newton's spectrum lie around the tongue-shaped edge, while the colors inside it are made by mixing these spectral rays. The artificiality of uniting red and violet in the color wheel is emphasized by the flat base of the tongue. Along here the purples and magentas are not found in even the finest unweaving of the rainbow's strands.

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