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Seeing red…and yellow…and green…and: we owe our appreciation of color—what it is and how we perceive it—to scientists and artists. Do we also have some hungry primate ancestors to thank for the great pleasure it brings us?
Natural History, March, 2002 by Philip Ball
When the sun shines through a rain-darkened sky, one of nature s most celebrated wonders 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 these colors lie within the light, waiting to be sifted out by an encounter with the tangible world. In the rainbow, raindrops do the sifting systematically; each band is part of a progression through the visible spectrum, from red to violet.
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In the seventeenth century, when Isaac Newton showed how this happened, he seemed at last to have answered the question that had frustrated philosophers for centuries: What exactly is color? Yet Newton's was not the last word. Indeed, for some people it simply raised more questions. Painters struggled to understand how Newton's theory of light and color applied to pigments. The German Romantic literary figure Johann Wolfgang von Goethe decided that Newton's ideas about color were nonsense, and some scientists 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 (or, in English poet John Keats's derogatory phrase, "unweaving") the rainbow. But that is not quite what he did. The ancient Greeks speculated that rainbows are caused by sunbeams falling onto clouds, and philosophers had known for centuries that light passing through glass, transparent minerals, or water can generate a multitude of colors. Then, in 1637, French philosopher Rene Descartes showed that sunlight becomes focused into a circular arc when it bounces off raindrops.
What Newton did was to bring color to Descartes's rainbow. In 1665 he split a sunbeam into a many-hued spectrum by passing it through a prism in a darkened room. And he found that if all the spectral colors were brought back together with a lens, they merged into a beam of white light. (By definition, white light is composed of rays of all the wavelengths from red to violet.) Newton deduced that the prism caused rays of different colors to bend through different angles and that the same thing happens in rainbows, wherein each raindrop acts like a tiny prism. More than three centuries later, schoolchildren are still taught that the spectrum's "bow" as Newton declared, has seven strands: red, orange, yellow, green, blue, indigo, and violet. In fact, this list of colors is rather arbitrary. A man of his time, Newton saw concordances throughout nature, leading him to imagine that the colors of the rainbow must mirror the seven notes of the heptatonic scale, the dominant scale used in Western music. Later color theorists generally replaced indigo and violet with just a single hue: purple or violet.
Newton had discovered that color comes from plucking this rainbow of light. But what is light? The modern answer to that question came two centuries after Newton, when Scottish physicist James Clerk Maxwell declared that light is a vibrating field of electrical and magnetic energy: an electromagnetic field passing through empty space like a wave traveling across the sea. The frequency of the vibrations increases from the red to the violet end of the spectrum, thus determining the perceived color of the light. The wavelength of these light waves gets shorter as the frequency gets higher.
Most objects acquire their color by absorbing rays of certain frequencies and reflecting the rest. A substance absorbs light of a particular frequency because the vibrations of its cloud of electrons--negatively charged subatomic particles that bind one atom to another--resonate at that same frequency, like a guitar string humming in sympathy with a loudly sung note. These resonant frequencies depend on the chemical composition of the substance: which atoms it contains and how they are joined together.
We don't see absorbed rays, only reflected ones. So we ascribe to an object the color of the very rays it rejects. A red berry soaks up green and blue from white sunlight; a yellow flower pulls in blue and red. The pigments on the painter's palette also derive their color by absorbing light.
But not all color is generated this way. The rainbow's variegated arc results from refraction, the bending of lights rays as they pass from one medium to another (in this case, from air to the water in raindrops). Another physical color-producing process is scattering: the dispersal of light in all directions by particles in the atmosphere that are about as big as a single wavelength. Light scattering is what makes the sky blue. Rays from the sun are scattered by atmospheric dust, sending the light bouncing in all directions. These dust particles scatter more high-frequency light than low-frequency light, so blue light bounces around in the atmosphere and reaches our eyes from all parts of the sky. Distant hills have a bluish tint because the light reflected from the hills is mixed with blue light from the atmosphere.
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