from The New Book of Knowledge®
Our world is filled with color. We see the brilliant blue of a summer sky, the vivid yellow of a bunch of daffodils, and the soft pink tint of a seashell. These are examples of the many ways in which color surrounds us in nature.
We also use color to add beauty and variety to our lives. This happens when we choose our clothing, decorate our homes, or paint a picture. We select colors that please us and create a particular mood or feeling.
The great variety of color that surrounds us is usually accepted as commonplace, an everyday fact of life. Yet color is anything but commonplace. Many of its qualities remain a mystery, even to today's scientists. But if we observe and think about color, we can begin to understand what it is.
Some objects are commonly called "colors"—paints and crayons, for instance. These objects may display color, but none of them actually are color. Color has neither body nor substance. You cannot touch or feel it. A rug may be both soft and red, but the two qualitites are not at all similar. Waking up at night and without turning on the light, you can still feel the softness of the rug. But you cannot see its red color. The rug appears to be black or dark gray. Colors can be seen only in the presence of light. Without light there is no color. In bright light, colors are more intense. When the light fades, the colors fade along with it. All this makes sense once you realize that color is, in fact, light.
In 1665 the English scientist Sir Isaac Newton performed a simple experiment that revealed the relationship between color and light. He caused a narrow beam of sunlight to pass through a solid, three-sided piece of glass called a prism. As the light made its way through the prism, it was bent, or refracted. This forced the narrow beam to spread out and change, until it emerged on the other side of the prism as a wide band, or spectrum. Across this spectrum, colors ranged from red, through yellow, green, and blue, to violet.
Newton then reversed the process. He guided the beam of colored light through another prism. This prism turned the beam of colored light back into a narrow beam of colorless light. With this experiment, he showed that colorless, or "white," light is actually made up of several bright colors.
In time, scientists determined that light is a series of waves of differing lengths. The different wavelengths are seen by us as different colors. An object looks colored if it is able to throw back, or reflect, one or more of the light waves while taking in, or absorbing, the rest. For example, a red ball appears red because it reflects the red light waves and absorbs the other light waves. And a green shirt looks green because it reflects the green light waves and absorbs the others. Light such as sunlight that contains all the wavelengths appears colorless or white.
Mixing the Colors of Light
White light is made up of only three basic colors—red, green, and violet. These are called primaries. Red has the longest wavelength that we can see and violet the shortest. All other colors are mixtures of the primaries. Yellow light is a combination of red and green light. Blue light is a combination of green and violet light. The third mixture, red and violet, is not visible in the spectrum. But it can be produced by combining a beam of red light with a beam of violet light. The result is a light of a deep pink hue known as magenta. (Remember that we are referring to mixtures of colored light. Mixing paint or crayon colors produces very different results and is discussed below.)
When a color created by a mixture of any two primaries is added to the third primary, white light is produced. Thus yellow (red + green) plus violet makes white light, as do blue (green + violet) plus red, and magenta (red + violet) plus green. Because they complete the number of primaries needed for white light, the mixed colors—which are lighter than the primaries because they contain twice as much light—are often called complementaries. This white light also proves that the colors of light are additive, that is, with each added primary, more light is produced.
Pigments are chemical coloring substances. They bring color to many things, including paints. And mixing paints is a convenient way of mixing pigments. But mixing different-colored pigments is not the same as mixing light of different colors. Pigment colorsare subtractive. Pigments can display color only by absorbing, or subtracting, portions of white light while reflecting the rest. Each of the pigment primaries--yellow, magenta, and blue--absorbs one-third of white light and reflects two-thirds. For example, yellow pigment absorbs the violet light and reflects the red and green light. (As we have seen, red and green light combine to produce yellow.)
Wherever two pigment primaries overlap and are mixed, darker secondary colors are produced because less light is reflected by the mixture. For example, adding blue pigment (which absorbs red but reflects green and violet) to yellow pigment (which absorbs violet but reflects green and red) leaves only green light to be reflected. This is why the mixture of blue and yellow pigment looks green. When all three pigment primaries are mixed, black results, not white. White results when the three primary colors of light are mixed.
Pigment colors are classified according to brilliance and saturation. Brilliance refers to the amount of reflected light. A color of high brilliance reflects a large portion of white light. A color of low brilliance reflects only a small part. Saturation is the degree of purity. A red of high saturation means it reflects most of the red light, and only red light reaches your eyes. Shades and tints are variations of a particular color. Shade usually designates the deeper qualities. Tint refers to the lighter qualities. A tint of yellow typically means a little yellow pigment mixed with a lot of white.
Understanding all these facts about color helps artists who create paintings using pigment colors. Knowledge of how colors interact is also essential for color printing. In color printing, just three colors plus black are used. And they produce a full range of colors in every shade and tint from light to dark.
Colors in Nature
Most man-made colored objects depend on chemical coloring substances such as pigments. Many of nature's colors are also produced by such chemical substances. Flower colors, for instance, are usually pigment colors. Perhaps the most familiar pigment in nature is chlorophyll, the green coloring matter of plants.
Many other colors in nature, however, are not caused by pigments. Scientists call them physical, or structural, colors, meaning they are pure light reflected by tiny structures in organic or inorganic matter. Most white hues in nature are structural. The white of snow, for example, results from total scattered reflection of all incoming light by ice crystals. The blue of the sky is another structural color, produced by air molecules and tiny impurities in the atmosphere that reflect and scatter the blue/violet part of the spectrum.
Most beautiful of all structural colors in nature are the pure, incredibly brilliant iridescent colors. These are displayed by many birds and insects, including hummingbirds and some butterflies. A hummingbird's throat may look yellow from one angle, orange from another, and flame red from a third angle. Being pure light, these colors seem to glow with a life of their own. All are produced by a process called light interference. The scales or feather branches contain huge numbers of tiny structures that bend and reflect incoming light. This reinforces some colors while weakening or eliminating others. As the viewing angle or direction of incoming light changes, different colors appear and re-appear rapidly. They shine with a purity never found in pigment colors.
Uses of Color in Nature
The great variety of color in nature has led scientists to look for proof of its usefulness. This search is inspired by the idea that special qualities in nature often have a purpose. Many plant and animal colors are indeed useful to their owners. The colors of flowers attract insects that fertilize them. Some animal colors serve as a disguise. This allows the animal to blend with its natural surroundings so well that it cannot be seen. Other animal colors serve as warning signs to discourage attackers. Among many birds, reptiles, and fish, bright colors help the males attract a mate during the breeding season.
Many other colors, however, seem to be little more than ornamental. In other words, they are beautiful but have no apparent usefulness. Even those colors that do serve a purpose often are not necessary for an animal's survival.
There is one color—or rather coloring substance—in nature that is unusually important. It is essential not only to the plants that display it but also to most other living creatures. That color is the green pigment chlorophyll. It absorbs the red and violet parts of the spectrum while reflecting the green part. The plant gains energy from the absorbed light. And this energy is used to produce food for the plant in a process called photosynthesis. Because of that vital function, chlorophyll is indeed the essence and color of life itself.
The ability to see different wavelengths of light as colors is not shared equally by all animals. Many one-celled animals, such as amoebas, have no eyes and cannot see. Even ordinary vision does not automatically include color vision. Furthermore, the ability to see color may differ. Honeybees, for instance, cannot see red. But they do see yellow, green, blue, and violet, as well as ultraviolet, which is invisible to humans. Other animals seeem to see the same color range as humans. These animals include most fish, amphibians, reptiles, and birds.
Most mammals, even though their bodies are highly complex, have little or no color vision. Many of them are active at night or during the twilight hours, when the sense of smell and hearing are of more value than even ordinary vision. The ability to see color is not important to these animals. Only monkeys and apes share with humans the ability to see the colors of the spectrum.
How We See Color
All vision in higher animals depends on certain structures in the eye. The retina is a surface inside the eye that receives a light image. The optic nerve passes along that image in the form of impulses. And the optic lobe, which is a part of the brain, translates the impulses into the picture we see.
All of this happens with such speed that we see images the moment we look at them. Color vision exists in addition to ordinary vision if special color-receptive structures are present in the retina. Without these color receptors, an animal is color-blind, even though its ordinary vision may be good. A person may be color-blind if his or her brain cannot process the information from the color receptors. One in about every 30 humans is either partially or completely color-blind. Color blindness in humans is a genetic defect that is passed on from parents to their children. It rarely affects women.
What we see is largely determined by the eyes and the brain. But the eyes and the brain are not perfectly suited for every situation. In some situations, color illusions occur. We may see colors that are not there. For example, if we look at certain color combinations for a while and then look at a sheet of white paper, we may see an afterimage, a mental picture whose colors are different from those we saw in the real picture.
Another type of color illusion demonstrates that the same color can look different under different conditions. A color looks darker when seen against a light background than when viewed against a dark background. Colors can also look brighter or duller depending on the colors that surround them.
Color in Our Lives
Color is important to us in many ways. A bright, "warm" color such as yellow or red can have a cheerful or exciting effect. A "cool" color such as blue can produce feelings of calmness and tranquility. Colors help make our environment more interesting and attractive. We enjoy surrounding ourselves with beautiful colored objects and using color in painting and arts and crafts.
Over the centuries, certain colors have acquired symbolic meanings because people relate them to familiar objects or ideas. This is known as color association. For example, red, the color of blood, often signifies courage as well as excitement and danger. Green, the color of ever-renewing plant life, is widely considered the symbol of hope and life. Purple stands for royalty, because purple robes were once worn by kings and queens as a sign of their high rank. In many cultures black has come to stand for death, evil, and fear of the unknown. White most often stands for purity and innocence.
Although some color associations are almost universal, others differ from one culture to another. In some countries white stands for sorrow. In India red is a holy color, and in China red symbolizes joy.
Other Uses of Color
Color has many uses in arts and crafts, decorating, clothing, photography, and printing. It also has many other uses in business and industry. Our responses to and feelings about color are important considerations in product design. Colors are used in toys and games, appliances, and packaging for foods and other products. Color is also used in technology for everything from traffic signs and signals to color coding for electronic wiring.
Color televisions that contain picture tubes rely on a process called phosphorescence. The inside of the tube is coated with tiny dots of phosphorescent material. There are three kinds of dots. When stimulated by electrons, each kind of dot emits a different color of light. The three colors are the primaries: red, green, and violet. These primaries are mixed to form all the colors of the spectrum.
The ability to see color is not necessary for human survival. After all, color-blind persons lead perfectly normal lives. But seeing color is an enriching experience. The beauty of many sights would be lost if not for the ability to perceive the various wavelengths of white light as bright colors.
Reviewed by Peter Goodwin
Author, Physics Can Be Fun