Color

Color

Light and color

Rainbows

Refraction: the bending of light

Diffraction and interference

Transparent, translucent, and opaque

Mixing colors

Color vision

Color blindness

Color effects in nature

Characteristics of color

Mixing colorants, pigments, dyes, and printing

Additive and subtractive

Primary, secondary, and complementary

Colors are everywhere

Resources

Several scientific disciplines are involved in explaining the phenomenon of color. The physics of light, the chemistry of colorants, and the psychology and physiology of human emotion are all related to our experience of color.

Light and color

Colors are an aspect of light; strictly speaking, color is the form in which human beings and other color-perceiving animals detect differences in the wavelength of visible light. Thus, an object appears colored because of the way it interacts with light.

When we talk about light, we usually mean white light. When white light passes through a prism (a triangular transparent object) something very exciting happens. The colors that make up white light disperse into what human beings tend to perceive as seven bands of color. These bands of color are called the visible spectrum (from the Latin word for image). When a second prism is placed in just the right position in front of the bands of this spectrum, they merge to form invisible white light again. Isaac Newton (1642–1727) was a scientist who conducted research on the Sun, light, and color. Through his experiments with prisms, he was the first to demonstrate that white light is composed of the colors of the spectrum. White light is simply a mixture of colored lights.

Seven colors constitute white light: red, orange, yellow, green, blue, indigo, and violet. Students in school often memorize acronyms like ROY G BIV, to remember the seven colors of the spectrum and their order. Sometimes blue and indigo are treated as one color. In any spectrum the bands of color are always organized in this order from left to right. There are also wavelengths outside the visible spectrum, such as ultraviolet.

Rainbows

A rainbow is a spectrum naturally produced by millions of raindrops acting as prisms. One can often see rainbows after summer showers, early in the morning, or late in the afternoon, when the sun is low. Raindrops act as tiny prisms and disperse the white sunlight into the form of a large arch composed of visible colors. To see a rainbow, one must be located between the Sun and raindrops forming an arc in the sky. When sunlight enters the raindrops at the proper angle, it is refracted by the raindrops, then reflected back at an angle. This creates a rainbow. Artificial rainbows can be produced by spraying small droplets of water from a garden hose with one’s back to the sun.

Refraction: the bending of light

Refraction is the bending of a light ray as it passes at an angle from one transparent medium to another. As a beam of light enters glass at an angle, it is refracted or bent. The part of the light beam that strikes the glass is slowed down, causing the entire beam to bend. The more sharply the beam bends, the more it is slowed down.

Each color has a different wavelength, and it bends differently from all other colors. Short wavelengths are slowed more sharply upon entering glass from air than are long wavelengths. Red light has the longest wavelength and is bent the least. Violet light has the shortest wavelength and is bent the most. Thus violet light travels more slowly through glass than does any other color.

Like all other wave phenomena, the speed of light depends on the medium through which it travels. As an analogy, think of a wagon that is rolled off a sidewalk onto a lawn at an oblique angle. When the first wheel hits the lawn, it slows down, pulling the wagon toward the grass. The wagon changes direction when one of its wheels rolls off the pavement onto the grass. Similarly, when light passes from a substance of high density into one of low density, its speed increases, and it bends away from its original path. In another example, one finds that the speed and direction of a car will change when it comes upon an uneven surface like a bridge.

Diffraction and interference

Similar colors can be seen in a thin film of oil, in broken glass, and on the vivid wings of butterflies and other insects. Scientists explain this process by the terms, diffraction and interference. Diffraction and refraction both refer to the bending of light. Diffraction is the slight bending of light away from its straight line of travel when it encounters the edge of an object in its path. This bending is so slight that it is scarcely noticeable. The effects of diffraction become noticeable only when light passes through a narrow slit. When light waves pass through a small opening or around a small object, they are bent. They merge from the opening as almost circular, and they bend around the small object and continue as if the object were not there at all. Diffraction is the sorting out of bands of different wavelengths of a beam of light.

When a beam of light passes through a small slit or pin hole, it spreads out to produce an image larger than the size of the hole. The longer waves spread out more than the shorter waves. The rays break up into dark and light bands or into colors of the spectrum. When a ray is diffracted at the edge of an opaque object, or passes through a narrow slit, it can also create interference of one part of a beam with another.

Interference occurs when two light waves from the same source interact with each other. Interference is the reciprocal action of light waves. When two light waves meet, they may reinforce or cancel each other. The phenomenon called diffraction is basically an interference effect. There is no essential difference between the phenomena of interference and diffraction.

Light is a mixture of all colors. One cannot look across a light beam and see light waves, but when light waves are projected on a white screen, one can see light. The idea that different colors interfere at different angles implies that the wavelength of light is associated with its colors. A spectrum can often be seen on the edges of an aquarium, glass, mirrors, chandeliers, or other glass ornaments. These colored edges suggest that different colors are deflected at different angles in the interference pattern.

The color effects of interference also occur when two or more beams originating from the same source interact with each other. When the light waves are in phase, color intensities are reinforced; when they are out of phase, color intensities are reduced.

When light waves passing through two slits are in phase, there is constructive interference, and bright light will result. If the waves arrive at a point on the screen out of phase, the interference will be destructive, and a dark line will result. This explains why bubbles of a nearly colorless soap solution develop brilliant colors before they break. When seen in white light, a soap bubble presents the entire visible range of light, from red to violet. Since the wavelengths differ, the film of soap cannot cancel or reinforce all the colors at once. The colors are reinforced, and they remain visible as the soap film becomes thinner. A rainbow, a drop of oil on water, and soap bubbles are phenomena of light caused by diffraction, refraction, and interference. Colors found in birds such as the blue jay are formed by small air bubbles in its feathers. Bundles of white rays are scattered by suspended particles into their components’ colors. Interference colors seen in soap bubbles and oil on water are visible in the peacock feathers. Colors of the mallard duck are interference colors and are iridescent, meaning they change in hue when seen from different angles. Beetles, dragonflies, and butterflies are as varied as the rainbow and are produced in a number of ways, which are both physical and chemical. Here, the colors are separated by thin films, and they flash and change when seen from different angles.

Light diffraction has the most lustrous colors of mother of pearl. Light is scattered for the blue of the sky which breaks up blue rays of light more readily than red rays.

Transparent, translucent, and opaque

Materials like air, water, and clear glass, which pass visible light with little diminishment, are called transparent. When light encounters transparent materials, almost all of it passes directly through them. Glass, for example, is transparent to all visible light. The color of a transparent object depends on the color of light it transmits. If green light passes through a transparent object, the emerging light is green; similarly if red light passes through a transparent object, the emerging light is red.

Materials like frosted glass and some plastics are called translucent. When light strikes translucent materials, only some of the light passes through them. The light does not pass directly through the materials. It changes direction many times and is scattered as it passes through. Therefore, we cannot see clearly through them; objects on the other side of a translucent object appear fuzzy and unclear. Because translucent objects are semitransparent, some ultraviolet rays can go through them. This is why a person behind a translucent object can get a sunburn on a sunny day.

Most materials are opaque. When light strikes an opaque object none of it passes through. Most of the light is either reflected by the object or absorbed and converted to heat. Materials such as wood, stone, and metals are opaque to visible light.

Mixing colors

What we see as color is the effect of light shining on an object. When white light shines on an object it may be reflected, absorbed, or transmitted. Glass transmits most of the light that comes into contact with it; thus it appears colorless. Snow reflects all of the light and appears white. A black cloth absorbs all light, and so appears black. A red piece of paper reflects red light better than it reflects other colors. Most objects appear colored because their chemical structure absorbs certain wavelengths of light and reflects others.

The sensation of white light is produced through a mixture of all visible colored light. While the entire spectrum is present, the eye deceives us into believing that only white light is present. White light results from the combination of the visible portions of the spectrum. When equal brightnesses of these are combined and projected on a screen, we see white. The screen appears yellow when red and green light alone overlap. The combination of red and blue light produces the bluish-red color of magenta. Green and blue produce the greenish blue color called cyan. Almost any color can be made by overlapping light in three colors and adjusting the brightness of each color.

Color vision

What we call color depends on the effects of light waves on receptors in the eye’s retina. The currently accepted scientific theory is that there are three types of cones in the eye. One of these is sensitive to the short blue light waves; it responds to blue light more than to light of any other color. A second type of cone responds to light from the green part of the spectrum; it is sensitive to medium wavelengths. The third type of light-sensitive cone responds to the longer red light waves. If all three types of cone are stimulated equally our brain interprets the light as white. If blue and red wavelengths enter the eye simultaneously we see magenta. Recent scientific research indicates that the brain is capable of comparing the long wavelengths it receives with the shorter wavelengths. The brain interprets electric signals that it receives from the eyes like a computer.

Nearly 1, 000 years ago, Alhazen, an Arab scholar recognized that vision is caused by the reflection of light from objects into our eyes. He stated that this reflected light forms optical images in the eyes. Alhazen believed that the colors we see in objects depend on both the light striking these objects and on some property of the objects themselves.

Color blindness

Some people are unable to see some colors. This is due to an inherited condition known as color blindness. John Dalton (1766–1844), a British chemist and physicist, was the first to discover color blindness in 1794. He was color blind and could not distinguish red from green. Many color blind people do not realize that they do not distinguish colors accurately. This is potentially dangerous, particularly if they cannot distinguish between the colors of traffic lights or other safety signals. Those people who perceive red as green and green as red are known as red-green color blind. Others are completely color blind; they only see black, gray, and white. It is estimated that 7% of men and 1% of women are born color blind.

Color effects in nature

We often wonder why the sky is blue, the water in the sea or swimming pools is blue or green, and why the sun in the twilight sky looks red. When light advances in a straight line from the sun to Earth, the light is refracted, and its colors are dispersed. The light of the dispersed colors depends on their wavelengths. Generally the sky looks blue because the short blue waves are scattered more than the longer waves of red light. The short waves of violet light (the shortest of all the light waves) disperse more than those of blue light. Yet the eye is less sensitive to violet than to blue. The sky looks red near the horizon because of the specific angle at which the long red wavelengths travel through the atmosphere. Impurities in the air may also make a difference in the colors that we see.

Characteristics of color

There are three main characteristics for understanding variations in color. These are hue, saturation, and intensity or brightness. Hue represents the observable visual difference between two wavelengths of color. Saturation refers to the richness or strength of color. When a beam of red light is projected from the spectrum onto a white screen, the color is seen as saturated. All of the light that comes to the eye from the screen is capable of exciting the sensation of red. If a beam of white light is then projected onto the same spot as the red, the red looks diluted. By varying the intensities of the white and red beams, one can achieve any degree of saturation. In handling pigments, adding white or gray to a hue is equivalent to adding white light. The result is a decrease in saturation.

A brightly colored object is one that reflects or transmits a large portion of the light falling on it, so that it appears brilliant or luminous. The brightness of the resulting color will vary according to the reflecting quality of the object. The greatest amount of light is reflected on a white screen, while a black screen would not reflect any light.

Mixing colorants, pigments, dyes, and printing

Color fills our world with beauty. We delight in the golden yellow leaves of autumn and the beauty of spring flowers. Color can serve as a means of communication, to indicate different teams in sports, or, as in traffic lights, to instruct drivers when to stop and go. Manufacturers, artists, and painters use different methods to produce colors in various objects and materials. The process of mixing colorants, paints, pigments, and dyes is entirely different from the mixing of colored light.

Colorants are chemical substances that give color to such materials as ink, paint, crayons, and chalk. Most colorants consist of fine powders that are mixed with liquids, wax, or other substances that facilitate their application to objects. Dyes dissolve in water. Pigments do not dissolve in water, but they spread through liquids. They are made up of tiny, solid particles, and they do not absorb or reflect specific parts of the spectrum. Pigments reflect a mixture of colors.

When two different colorants are mixed, a third color is produced. When paint with a blue pigment is mixed with paint that has yellow pigments, the resulting paint appears green. When light strikes the surface of this paint, it penetrates the paint layer and hits pigment particles. The blue pigment absorbs most of the light. The same color looks different against different background colors. Each pigment subtracts different wavelengths.

Additive and subtractive

All color is derived from two types of light mixture, an additive and a subtractive process. Both additive and subtractive mixtures are equally important to color design and perception. The additive and subtractive elements are related but different. In a subtractive process, blended colors subtract from white light the colors that they cannot reflect. Subtractive light mixtures occur when there is a mixture of colored light caused by the transmittance of white light. The additive light mixture appears more than the white light.

In a subtractive light mixture, a great many colors can be created by mixing a variety of colors. Mixing colored light produces new colors different from the way colorants are mixed. Mixing colorants results in new colors because each colorant subtracts wavelengths of light. But mixing colored lights produces new colors by adding light of different wavelengths.

Both additive and subtractive mixtures of hues adjacent to each other in the spectrum produce intermediate hues. The additive mixture is slightly saturated or mixed with light, the subtractive mixture is slightly darkened. The complementary pairs mixed subtractively do not give white pigments with additive mixtures.

Additive light mixtures can be used in a number of slide projectors and color filters in order to place different colored light beams on a white screen. In this case, the colored areas of light are added to one another. Subtractive color mixing takes place when a beam of light passes through a colored filter. The filter can be a piece of colored glass or plastic or a liquid that is colored by a dye. The filter absorbs and changes part of the light. Filters and paints absorb certain colors and reflect others.

Subtractive color mixing is the basis of color printing. The color printer applies the colors one at a time. Usually the printer uses three colors: blue, yellow, and red, in addition to black for shading and emphasis. It is quite difficult for a painter to match colors. To produce the resulting colors, trial and error, as well as experimenting with the colors, is essential. It is difficult to know in advance what the resulting color will be.

People who use conventional methods to print books and magazines make colored illustrations by mixing together printing inks. The color is made of a large number of tiny dots of several different colors. The dots are so tiny that the human eye does not see them individually. It sees only the combined effects of all of them taken together. Thus, if half of the tiny dots are blue and half are yellow, the resulting color will appear green.

Dying fabrics is a prehistoric craft. In the past, most dyes were provided solely from plant and animal sources. For example, yellow came from the sap of a tree, from the bark of the birch tree, and onion skins. There were a variety of colors obtained from leaves, fruits, and flowers. In antiquity, the color purple or indigo, derived from plants, was a symbol of aristocracy. In ancient Rome, only the emperor was privileged to wear a purple robe.

Today, many dyes and pigments are made of synthetic material. Thousands of dyes and pigments have since been created from natural coal tar, from natural compounds, and from artificially produced organic chemicals. Modern chemistry is now able to combine various arrangements and thus produce a large variety of color.

The color of a dye is caused by the absorption of light of specific wavelengths. Dyes are made of either natural or synthetic material. Chemical dyes tend to be brighter than natural dyes. In 1991 a metamorphic color system was created in a new line of clothes. The color of these clothes changes with the wearer’s body temperature and environment. This color system could be used in a number of fabrics and designs.

Sometimes white clothes turn yellow from repeated washing. The clothes look yellow because they reflect more blue light than they did before. To improve the color of the faded clothes a blue dye is added to the wash water, thus helping them reflect all colors of the spectrum more evenly in order to appear white.

Most paints and crayons use a mixture of several colors. When two colors are mixed, the mixture will reflect the colors that both had in common.

Primary, secondary, and complementary

Three colorants that can be mixed in different combinations to produce several other colors are the primary colorants. In mixing red, green, and blue paint the result will be a muddy dark brown. Red and green paint do not combine to form yellow as do red and green light. The mixing of paints and dyes is entirely different from the mixing of colored light.

By 1730, a German engraver named J.C. LeBlon discovered the primary colors red, yellow, and blue are primary in the mixture of pigments. Their combinations produce orange, green, and violet. Many different three-colored combinations can produce the sensation of white light when they are superimposed. When two primary colors such as red and green are combined, they produce a secondary color. A color wheel is used to show the relationship between primary and secondary colors. The colors in this primary and secondary pair are called complementary. Each primary color on the wheel is opposite the secondary color formed by the mixture of the two primary colors. And each secondary color produced by mixing two primary colors lies half-way between them on a color wheel. The complementary colors produce white light when they are combined.

Colors are everywhere

Color influences many of our daily decisions, consciously or unconsciously, from what we eat to what we wear. Color enhances the quality of our lives, it helps us to fully appreciate the beauty of colors. Colors are also an important function of the psychology and physiology of human sensation. Even before the ancient civilizations in prehistoric times, color symbolism was already in use.

Different colors have different meanings that are universal. Colors can express blue moods. On the other hand, these could be moods of tranquility or moods of conflict, sorrow or pleasure, warm and cold, boring or stimulating. In several parts of the world, people have specific meanings for different colors. An example is how Eskimos indicate the different numbers of snow conditions. They have seventeen separate words for white snow. In the west the bride wears white; in China and in the Middle East area white is worn for mourning. Of all colors, the most conspicuous is red.

KEY TERMS

Beams— Many rays of light.

Colorant— A chemical substance that gives color to such materials as ink, paint, crayons, and chalk.

Diffraction— The bending of light.

Hue— The observable visual difference between two wavelengths of color.

Light— A form of energy that travels in waves.

Mirage— An optical illusion.

Pigment— A substance which becomes paint or ink when it is mixed with a liquid.

Ray— A thin line of light.

Reflection— The change in direction of light when it strikes a substance but does not pass through it.

Refraction— The bending of light that occurs when traveling from one medium to another, such as air to glass or air to water.

Spectrum— A display of the intensity of radiation versus wavelength.

The color red can be violent, aggressive, and exciting. The expression “seeing red” indicates one’s anger in most parts of the world. Red appears in more national and international colors and red cars are more often used than any other color. Homes can be decorated to suit the personalities of the people living in them. Warm shades are often used in living rooms because it suggests sociability. Cool shades have a quieting effect, suitable for study areas. Hospitals use appropriate colors depending on those that appeal to patients in recovery, in surgery, or very sick patients. Children in schools are provided bright colored rooms. Safety and certain color codes are essential. The color red is for fire protection, green is for first aid, and red and green colors are for traffic lights.

Human beings owe their survival to plants. The function of color in the flowering plants is to attract bees and other insects in order to promote pollination. The color of fruits attract birds and other animals which eat the fruit and help to distribute the seeds. The utmost relationship between humans and animals and plants are the chlorophylls. The green coloring substance of leaves and the yellowish green chlorophyll is associated with the production of carbohydrates by photosynthesis in plants. Life and the quality of Earth’s atmosphere depends on photosynthesis.

Resources

BOOKS

Mausfield, Rainer, and Dieter Heyer. Colour Perception: Mind and the Physical World. New York: Oxford University Press, USA, 2004.

Parker, Steve. The Science of Light: Projects and Experiments With Light And Color. Oxford, UK: Heinemann, 2005.

Pinna, Baingio, ed. Color, Line, and Space: The Neuroscience of Spatio-Chromatic Vision. Leiden, Netherlands: Brill Academic Pub, 2006.

Valberg, Arne. Light, Vision, Color. Hoboken, NJ: John Wiley & Sons, 2005.

Nasrine Adibe