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Color and light

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an article on Light and Color

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Light & Color

Light

Light is taken for granted. Without it: 1. No sight 2. No food 3. No air 4. No warmth 5. No fuel

People have found ways of making and controlling light in order to see when there is no sunlight. First, they produced light with campfires and torches; Next, they developed candles, oil lamps, gaslights, and electric lights. People make and use light for many other purposes than to see by. The pictures on a television screen consist of spots of light. Using scientific instruments, people can study light itself and learn much about the universe; The light from distant stars can tell scientists what the stars are made of. It can also tell them if the stars are moving toward or away from the earth and how fast they are moving.

Light's Nature

People once thought light was something that traveled from a person's eyes to an object and then back again. If anything blocked the rays from the eyes, the object could not be seen. Since the 1600's, scientists have made many discoveries about light. They have learned that light is a form of energy that can travel freely through space. The energy of light is called radiant energy. There are many kinds of radiant energy, including infrared rays, radio waves, ultraviolet rays, and X rays. We can see only a tiny part of all the different kinds of radiant energy. This part is called visible light or simply light.

Newton

Isaac Newton's work on Optics explained why bodies appear to be colored. The discoveries also laid the foundation for the science of spectrum analysis. This science allows us to determine the chemical composition, temperature, and even the speed of such hot, glowing bodies as a distant star or an object heated in a laboratory.

Newton discovered that sunlight is a mixture of light of all colors. He passed a beam of sunlight through a glass prism and studied the colors that were produced. A green sweater illuminated by sunlight looks green because it largely reflects the green light in the sun and absorbs most of the other colors. If the green sweater were lighted by a red light or any color light not containing green, it would not appear green.

The study of light led Newton to consider constructing a new type of telescope in which a reflecting mirror was used instead of a combination of lenses. Newton's first reflecting telescope was 15 centimeters long, and, through it, Newton saw the satellites of Jupiter.

After Newton

Throughout the 1800's, scientists thought of light as a wave that travels much like a water wave. This idea of light as a wave was popular because it explained experiments in which light created a series of bright and dark lines called an interference pattern. Scientists could explain such interference patterns only by describing light as a wave.

The kind of wave needed to be descibed; Water waves were the easiest known example. They travel across the surface of the water while the water itself only moves up and down. To scientists of the 1800's, light seemed stranger than water waves because it travels through space from the sun and other stars to the earth. They assumed that light waves must also travel through some kind of material, just as water waves travel through water. Although scientists had no evidence of this material, they called it the ether. By the late 1800's, scientists had concluded that light waves consist of regions of force known as electric fields and magnetic fields.

A simple model of a light wave begins with a ray (a straight line) that shows the direction of the light's travel. Along the ray and perpendicular (at right angles) to it, short arrows represent the electric field. Some arrows point upward from the ray and other arrows point downward from it. They vary in length so that the overall pattern of the tips of the arrows looks like a wave. Arrows representing the magnetic field also resemble a wave, but these arrows make right angles to the arrows that represent the electric field. These patterns move along the ray. They are the light.

By the early 1900's, experiments had shown that scientists finally had to give up the idea of an ether. Many scientists realized that a wave of light, as a regularly varying pattern of electric and magnetic fields, can travel through empty space.

Light waves resemble other types of waves in some features, including wavelength, frequency, and amplitude. The wavelength is the distance along a straight line from one crest (peak) of the wave to the next. The frequency of a wave is the number of times each second that crests pass a stationary checkpoint. The amplitude of a wave is the greatest distance of a crest or trough (low point) from the ray.

A simple relation exists between a wave's frequency and wavelength: the higher the frequency, the shorter the wavelength. A wave's energy corresponds to its amplitude. The greater the amplitude, the more energy the wave has. The energy of a light wave also corresponds to its frequency. The wavelength determines the color of the light.

Photons

In 1905, Albert Einstein proposed a model of light just as useful as the wave model. In some experiments, light behaves as though it is a particle. We now call this type of particle a photon. In Einstein's model, a ray of light is the path taken by a photon. For example, when a flashlight sends a beam of light across a dark room, the beam of light consists of a great many photons, each traveling in a straight line.

Is light a wave or a particle? Seemingly, it cannot be both because the two models are so different. The best answer is that light is strictly neither. In some experiments light behaves like a wave, and in others it behaves like a particle.

Unlike other kinds of waves, light waves in a vacuum have one speed, and that speed is the fastest that anything can travel. Scientists do not understand why this is true. The fact that light in a vacuum has only one speed forms one of the foundations of Einstein's theory of relativity.

When light enters a material, it continually runs into atoms that delay its travel. But between atoms, light travels at its normal speed.

Electromagnetic waves

Because light consists of electric and magnetic fields, it is called an electromagnetic wave. The term light commonly refers to just those electromagnetic waves that we can see. For light to be visible, it must have a wavelength within a certain narrow range of values called the visible spectrum. Violet light has the shortest wavelength that is visible. Red light has the longest. Between them lie all the other colors of the spectrum, each with its own wavelength. Seen together at the same time, the colors appear as white light. Sunlight is white because it has all the colors. However, when it passes through a specially shaped transparent solid called a prism, the different colors separate and can be seen.

In 1704 Sir Isaac Newton defined the spectrum as: 1. RED: A primary color possessing the longest light wave in the color spectrum. 2. ORANGE: A color with a reddish yellow hue. 3. YELLOW: The color of gold, butter, or ripe lemons. 4. GREEN: A color in the spectrum between yellow and blue. It is one of the psychological primary colors. 5. BLUE: is one of the primary colors. It lies in the color spectrum between green and violet. The color of the clear sky in daylight. 6. INDIGO: a deep violet-blue color, one of the seven prismatic or primary colors. 7. VIOLET: the shortest rays of the visible spectrum, having wavelengths of about 3,850 angstroms.

The visible spectrum forms only a small part of the full range of electromagnetic waves. Waves that have wavelengths slightly too short to be seen are called ultraviolet rays. They cause suntan, sunburn, and skin cancer. Waves with somewhat shorter wavelengths than ultraviolet rays are called X rays. These rays can penetrate a person's body. Doctors and dentists use them to "see" inside the body. Gamma rays have even shorter wavelengths than X rays. They result from nuclear reactions, such as those in the sun.

Waves with wavelengths slightly longer than those of red light are called infrared rays. When you stand in bright sunlight or in front of a fire, you feel warm largely because of the infrared light shining on you. Microwaves and radio waves have longer wavelengths than infrared waves. A microwave oven shines microwaves on food to warm it. Radio and television stations broadcast programs by sending radio waves.

Sunlight spread into its different colors by a prism creates a continuous spectrum. From violet to red, the spectrum blends smoothly from one color to the next. Many other sources of light do not produce a continuous spectrum. For example, a street lamp may produce bright yellow, blue, and a few dimmer colors, but it also has dark regions in its spectrum. The colors are produced by certain atoms in the gas inside the lamp. For example, the yellow comes from sodium atoms. Each type of atom can produce only certain colors.

Scientists can learn what kinds of atoms make up a light source by observing what colors are present in the light. They direct the light through an instrument called a spectrometer to separate the colors. The spectrometer may be a simple prism or it may be a more complicated device.

Sometimes a spectrum contains gaps because the light from a source has traveled through a gas that absorbed certain colors. For example, when sunlight is sent through a high-quality spectrometer, its spectrum has thousands of such gaps. The light produced within the sun must travel through the outer atmosphere of the sun to reach the earth. Each type of atom in the sun's atmosphere absorbs certain colors. By noting which colors are removed, scientists are able to determine what kinds of atoms are in the atmosphere of the sun.

Our Understanding of Light

Early ideas about light. The understanding of light has developed mainly since the 1600's. In 1666, Sir Isaac Newton discovered that white light is made up of all colors. Using a prism, he found that each color in a beam of white light could be separated. Newton proposed the theory that light consists of tiny particles that travel in straight lines through space. He called these particles corpuscles, and his theory became known as the corpuscular theory.

About the same time that Newton proposed his theory of light, the physicist and astronomer Christiaan Huygens suggested that light consists of waves. He proposed the wave theory to explain the behavior of light. The corpuscular and wave theories appear to be completely opposite, and scientists argued about them for about 100 years. Then, in the early 1800's, the physicist Thomas Young demonstrated the interference of light. He showed that two light beams cancel each other under certain conditions. Water waves also behave this way. Because it is hard to understand how interference could occur with particles, most scientists accepted Young's experiment as proof of the wave theory of light.

The Speed of Light

Although light seems to travel across a room the instant a window shade is raised, it actually takes some time to travel any distance. The speed of light in empty space--where atoms do not delay its travel--is 186,282 miles (299,792 kilometers) per second. This speed is said to be invariant because it does not depend on the motion of the light's source. For example, light that is emitted by a rapidly moving flashlight has the same speed as light that is emitted by a stationary flashlight. Scientists do not know why this is true, but the fact is one of the foundations of Einstein's theory of relativity.

From ancient times, people argued about whether the speed of light is limited or infinite. During the early 1600's, Galileo Galilei devised an experiment to measure the speed of light, and so settle the argument. Galileo sent an assistant to a distant hill with instructions that the assistant should open the shutter of a lantern when he saw Galileo on another hill open the shutter of his lantern. Galileo reasoned that because he knew the distance between the hills, he could find the velocity of light by measuring the time between opening his shutter and seeing the light of the second lantern. Galileo's thinking was sound, but the experiment failed. The velocity of light is so great that he could not measure the short time involved.

Electromagnetic Theory

In 1864, the British physicist James Clerk Maxwell proposed the mathematical theory of electromagnetism. According to this theory, the influence that changing electric fields and magnetic fields have on one another allows for the travel of waves. Maxwell's theoretical waves had the exact mathematical properties that had been measured for light. The vibrating electric charges that produce light are the electric charges in the atom. Atomic physicists had already shown that these vibrating electric charges exist. Maxwell's work gave the wave theory of light a solid foundation.

Maxwell's electromagnetic theory also did away with an idea that had stood in the way of scientists' acceptance of the wave theory for more than a century. Scientists felt they had to find the medium (material) through which light waves travel. They reasoned that if light travels as waves, there must be something for them to travel through, just as sound waves need air to travel through. But for light, this something could not be matter, because light can travel in a vacuum. To get around this difficulty, scientists suggested that the medium light traveled through was the ether.

All attempts to observe or measure the properties of the ether failed. Scientists became increasingly convinced that the ether did not exist. Experiments conducted by Albert Michelson and the American physicist Edward Morley in 1887 helped destroy the ether theory.

Quantum mechanics

In 1900, Max Planck discovered an equation that matched experimental data about the emission of light by a hot surface. Planck could not explain why the equation worked. But he realized that it predicted that the tiny emitters of light on the surface can have only certain values of energy. When energy is restricted to certain values, it is said to be quantized.

In 1905, Einstein revealed that light itself is quantized. Einstein reasoned that if light emitters can have only certain values of energy, then the energy they emit as light will retain its quantized character. The light comes in tiny packets of energy that are known as quanta. The concept of light as quantized energy explained how light behaves as a particle in certain experiments, instead of as a wave. These particles of light came to be called photons.

Niels Bohr, in 1913, proposed that the energy of atoms was also quantized. When energy is given to an atom, either by a collision or by shining light on it, the atom can accept only certain values of energy. In this way, the atom becomes excited. When it de-excites, it must get rid of the extra energy. One way it can do this is by emitting a photon that carries the energy away. Each type of atom accepts a different set of energies. Thus, when atoms emit light, the photons from one type of atom differ in energy from the photons from other types of atoms.

A field of physics known as quantum mechanics is the study of how atoms and light are quantized. It involves the fact that light and matter behave as waves in some experiments and as particles in other experiments.

Color

One does not actually see colors or object. The light that objects reflect or give off is what is actually seen. Our eyes absorb this light and change it into electrochemical signals. The signals travel through nerves to the brain, which interprets them as colored images. However, there is much that scientists still do not know about how our eyes and brain enable us to sense color.

The roles of the eyes and brain

Our ability to see color depends on many highly complicated workings of the eyes and brain. When we look at an object, light coming from the object enters our eyes. Each eye focuses the light, forming an image of the object on the retina. The retina is a thin layer of tissue covering the back and sides of the inside of the eyeball. It contains millions of light-sensitive cells. These cells absorb most of the light that falls on the retina and convert the light to electrical signals. These electrical signals then travel through nerves to the brain.

The retina has two main types of light-sensitive cells---rods and cones. The cells are named after their shapes. Rods are extremely sensitive to dim light but cannot distinguish wavelengths. For this reason, we see only tones of gray in a dimly lit room. As the light becomes brighter, the cones begin to respond and the rods cease functioning. The retina of a person with normal color vision has three types of cones. One type responds most strongly to light of short wavelengths, which corresponds to the color blue. Another type reacts chiefly to light of middle wavelengths, or green. The third type is most sensitive to light of long wavelengths, or red.

The brain organizes nerve signals from the eye and interprets them as colored visual images. Exactly how the brain makes us aware of colors is still much of a mystery. Scientists have developed several theories to explain color vision.

Some people do not have full color vision. Such people are said to be color blind. There are different types and degrees of color blindness, depending on different abnormalities in the retina's cones. In severe cases, one type of cone may be absent or not functioning. People who have such an abnormality confuse certain colors with others. Very few people cannot see colors at all. Most color-vision problems are inherited and cannot be cured.

Methods of color production

Manufacturers, artists, and craftworkers produce objects in a tremendous variety of colors. To create so many different colors, they use one of two basic methods. These methods are: 1. mixing colorants 2. mixing colored lights.

Mixing colorants

A great variety of colors can be created by mixing colorants. 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 to make them easier to apply to objects. Colorants that dissolve in liquids are called dyes. Colorants that do not dissolve but spread through liquids or other substances as tiny solid particles are called pigments.

When two different colorants are mixed, a third color is produced. For example, when paint with blue pigment is mixed with paint that has yellow pigment, the resulting paint appears green. When light strikes the surface of this paint, much of it penetrates the paint layer and hits pigment particles. The blue pigment absorbs most of the light of long wavelengths -- light that appears red, orange, and yellow. The yellow pigment absorbs most of the light of short wavelengths--light that appears blue and violet. Most of the light of medium wavelengths is not absorbed but reflected through the surface of the paint. When this light reaches our eyes, we see the paint as green. In a colorant mixture, each colorant absorbs, or subtracts, some of the wavelengths of light that strike it. For this reason, colorant mixtures are sometimes referred to as subtractive color mixtures or color by subtraction.

===Four Color Printing (CYMK)==== Any three colorants that can be mixed in different combinations to produce nearly any other color are known as primary colorants or primary colors in paint. A common group of primary colorants consists of red, yellow, and blue. When primary colorants are mixed in pairs, the resulting colors are called secondary colorants or secondary colors in paint. Orange is formed by mixing red and yellow, green by mixing yellow and blue, and purple by mixing blue and red. Color experts have found that magenta (purplish-red), yellow, and cyan (blue-green) also make a good set of primary colorants. These three colorants can be mixed to produce an extremely wide range of colors.

Mixing equal amounts of three primary colorants results in a color that is almost black. However, special black colorants, such as a fine black powder called carbon black, provide better blacks. Mixing black with a color produces a shade. Primary colorants absorb much light, and so they cannot be mixed to produce very light colors. For such purposes, either a chemical compound called titanium dioxide or some other special white colorant must be added. Mixing white with a color produces a tint. The combination of black and white forms gray. Mixing gray with a color creates a tone.

Mixing colored lights

When lights of different colors are projected together onto a screen, they blend and form new colors. Mixing colored lights produces new colors differently from the way mixing colorants does. Mixing colorants results in new colors because each colorant subtracts some wavelengths of light. But mixing colored lights produces new colors by adding light of different wavelengths. For this reason, colored light mixtures are sometimes called additive color mixtures or color by addition.

In an additive color mixture, the primary colors differ from those in paint. The primary colors in light are red, green, and blue. When red and green lights are mixed, the result is yellow light. A mixture of blue and green lights forms blue-green light, and blue and red lights form purple light. Combining all three primary colors in light in the proper proportions results in white light.

The colors of any two lights are complementary if they form white light when mixed. Therefore, the complementary color of any primary color in light is the color formed by combining the two other primary colors. The complement of blue is yellow (red light plus green light). The complement of red is blue-green (blue light plus green light). The complement of green is purple (red light plus blue light).

Color television pictures are created by additive mixtures of the three primary colors in light. A color TV screen has thousands of tiny areas that glow when struck by a beam of electrons. Some areas produce red light, others produce green light, and still others produce blue light. When we watch a color program, we do not see each red, green, or blue area. Instead, we see a range of many colors produced when the red, green, and blue lights blend in our vision. We see white light when certain amounts of red, green, and blue light are combined. The combining of the primary colors to produce white light makes it possible for a color TV to show black-and-white pictures.

Characteristics of color

Every color has three basic characteristics. They are (1) hue, (2) lightness, and (3) chroma. Color experts describe an object's color in terms of these characteristics.

Hue is the property that gives a color its name--for example, red, orange, yellow, green, blue, or violet or a combination of such names. The dramatic differences that we see among the colors in the spectrum are produced by very slight differences in the wavelengths of light. For example, the wavelengths that appear as yellow are only slightly shorter than those that appear as orange. But there is a great visual difference between orange and yellow. This difference is a difference in hue.

Lightness is a measurement of the amount of light reflected from a colored object. The lightness of a color may be expressed by comparing the color's level of reflected light with that of samples on a lightness scale. A lightness scale runs from black, through shades of gray, to white. Black reflects very little light. A color that reflects about the same amount of light as black has a very low lightness level. Gray reflects more light than black. Thus, a color that reflects about the same amount of light as a shade of gray may have an intermediate level of lightness. White reflects nearly all the light that strikes it. Therefore, a color that reflects about the same amount of light as white has a very high lightness level. Color experts use the term brightness to describe the lightness level of a colored light source.

Chroma is a measurement of the saturation (concentration) of a color. For example, a teaspoon of red poster paint powder mixed with a teaspoon of water produces paint of a deep red color. The paint has a high concentration of red colorant, and so it has a high chroma. If we dilute the paint with a cup of water, the resulting mixture will have a low concentration of red colorant and, therefore, a low chroma.

Glossary

  • Foot-candle - unit of measurement of illumination, the amount of light that falls on an object. The foot-candle is part of the inch-pound system of measurement customarily used in the United States. Two factors determine the amount of light an object receives: (1) the luminous intensity (the amount of light a light source produces) and (2) the distance between the light source and the object. As the luminous intensity increases, illumination also increases. As the distance increases, illumination decreases by the square of the distance.
  • Redshift - comes from the shifts first detected in wavelengths of light, but such shifts also occur at radio and other electromagnetic wavelengths. When a redshift occurs, all wavelengths are lengthened by the same fraction. A redshift is expressed as a percentage increase over the normal wavelength. Astronomers believe redshifts occur in cosmic objects because the earth and the objects are speeding away from each other. The amount of redshift of an object indicates its velocity.
  • Solar Energy - usually means the direct use of sunlight to produce heat or electric power. The sun's energy is plentiful, but it is thinly distributed over a large area and must be collected and concentrated to produce usable power. As a result, solar energy is a more expensive power source than fossil fuels for most applications. Solar technology is improving rapidly, though. Someday, it may provide a clean and abundant source of power. The two chief ways that sunlight may be converted into electric power: (1) directly, in a process called photovoltaic conversion, or (2) by solar thermal conversion, which converts light to heat and then to electric power. Most solar thermal devices heat water to produce steam, which drives a steam turbine.
  • Sun - The Sun is a huge, glowing ball at the center of our solar system. The sun provides light, heat, and other energy to Earth. The sun is made up entirely of gas. Most of it is a type of gas that is sensitive to magnetism. This sensitivity makes this type of gas so special that scientists sometimes give it a special name: plasma. Nine planets and their moons, tens of thousands of asteroids, and trillions of comets revolve around the sun. The sun and all these objects are in the solar system. Earth travels around the sun at an average distance of about 92,960,000 miles (149,600,000 kilometers) from it.
  • Opticks 1704
  • The scientific search to explain photons is a major consideration in Colin Bruce's (by way of Sherlock Holmes) explores Einstein in The Strange Case of Mrs. Hudson's Cat is a help to understand the end of the Newtonian Universe; now known under a new title -- The Einstein Paradox And Other Science Mysteries Solved By Sherlock Holmes.