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light: Definition, Synonyms and Much More from Answers.com

️Wed Jul 01 2015

Concept

Light exists along a relatively narrow bandwidth of the electromagnetic spectrum, and the region of visible light is more narrow still. Yet, within that realm are an almost infinite array of hues that quite literally give color to the entire world of human experience. Light, of course, is more than color: it is energy, which travels at incredible speeds throughout the universe. From prehistoric times, humans harnessed light's power through fire, and later, through the invention of illumination devices such as candles and gas lamps. In the late nineteenth century, the first electric-powered forms of light were invented, which created a revolution in human existence. Today, the power of lasers, highly focused beams of high-intensity light, make possible a number of technologies used in everything from surgery to entertainment.

How It Works

Early Progress in Understanding of Light

The first useful observations concerning light came from ancient Greece. The Greeks recognized that light travels through air in rays, a term from geometry describing that part of a straight line that extends in one direction only. Upon entering some denser medium, such as glass or water, as Greek scientists noticed, the ray experiences refraction, or bending. Another type of incidence, or contact, between a light ray and any surface, is reflection, whereby a light ray returns, rather than being absorbed at the interface.

The Greeks worked out the basic laws governing reflection and refraction, observing, for instance, that in reflection, the angle of incidence is approximately equal to the angle of reflection. Unfortunately, they also subscribed to the erroneous concept of intromission—the belief that light rays originate in the eye and travel toward objects, making them visible. Some 1,500 years after the high point of Greek civilization, Arab physicist Alhasen (Ibn al-Haytham; c. 965-1039), sometimes called the greatest scientist of the Middle Ages, showed that light comes from a source such as the Sun, and reflects from an object to the eyes.

The next great era of progress in studies of light began with the Renaissance (c. 1300-c. 1600.) However, the most profound scientific achievements in this area belonged not to scientists, but to painters, who were fascinated by color, shading, shadows, and other properties of light. During the early seventeenth century, Galileo Galilei (1564-1642) and German astronomer Johannes Kepler (1571-1630) built the first refracting telescopes, while Dutch physicist and mathematician Willebrord Snell (1580-1626) further refined the laws of refraction.

The Spectrum

Sir Isaac Newton (1642-1727) was as intrigued with light as he was with gravity and the other concepts associated with his work. Though it was not as epochal as his contributions to mechanics, Newton's work in optics, an area of physics that studies the production and propagation of light, was certainly significant.

In Newton's time, physicists understood that a prism could be used for the diffusion of light rays—in particular, to produce an array of colors from a beam of white light. The prevailing belief was that white was a single color like the others, but Newton maintained that it was a combination of all other colors. To prove this, he directed a beam of white light through a prism, then allowed the diffused colors to enter another prism, at which point they recombined as white light.

Newton gave to the array of colors in visible light the term spectrum, (plural, "spectra") meaning the continuous distribution of properties in an ordered arrangement across an unbroken range. The term can be used for any set of characteristics for which there is a gradation, as opposed to an excluded middle. An ordinary light switch provides an example of a situation in which there is an excluded middle: there is nothing between "on" and "off." A dimmer switch, on the other hand, is a spectrum, because a very large number of gradations exist between the two extremes represented by a light switch.

Seven Colors…or Six?

The distribution of colors across the spectrum is as follows: red-orange-yellow-green-blue-violet. The reasons for this arrangement, explained below in the context of the electromagnetic spectrum, were unknown to Newton. Not only did he live in an age that had almost no understanding of electromagnetism, but he was also a product of the era called the Enlightenment, when intellectuals (scientists included) viewed the world as a highly rational, ordered mechanism. His Enlightenment viewpoint undoubtedly influenced his interpretation of the spectrum as a set of seven colors, just as there are seven notes on the musical scale.

In addition to the six basic colors listed above, Newton identified a seventh, indigo, between blue and violet. In fact, there is a noticeable band of color between blue and violet, but this is because one color fades into another. With a spectrum, there is a blurring of lines between one color and the next: for instance, orange exists at a certain point along the spectrum, as does yellow, but between them is a nearly unlimited number of orange-yellow and yellow-orange gradations.

Indigo itself is not really a distinct color—just a deep, purplish blue. But its inclusion in the listing of colors on the spectrum has given generations of students a handy mnemonic (memorization) device: the name "ROY G. BIV." These letters form an acrostic (a word constructed from the first letters of other words) for the colors of the spectrum. Incidentally, there is something arbitrary even in the idea of six colors, or for that matter seven musical notes: in both cases, there exists a very large gradation of shades, yet also in both cases, the divisions used were chosen for practical purposes.

Waves, Particles, and Other Questions Concerning Light

The Wave-Particle Controversy Begins

Newton subscribed to the corpuscular theory of light: the idea that light travels as a stream of particles. On the other hand, Dutch physicist and astronomer Christiaan Huygens (1629-1695) maintained that light travels in waves. During the century that followed, adherents of particle theory did intellectual battle with proponents of wave theory. "Battle" is not too strong a word, because the conflict was heated, and had a nationalistic element. Reflecting both the burgeoning awareness of the nation-state among Europeans, as well as Britons' sense of their own island as an entity separate from the European continent, particle theory had its strongest defenders in Newton's homeland, while continental scientists generally accepted wave theory.

According to Huygens, the appearance of the spectrum, as well as the phenomena of reflection and refraction, indicated that light was a wave. Newton responded by furnishing complex mathematical calculations which showed that particles could exhibit the behaviors of reflection and refraction as well. Furthermore, Newton challenged, if light were really a wave, it should be able to bend around corners. Yet, in 1660, an experiment by Italian physicist Francesco Grimaldi (1618-1663) proved that light could do just that. Passing a beam of light through a narrow aperture, or opening, Grimaldi observed a phenomenon called diffraction, or the bending of light.

In view of the nationalistic character that the wave-particle debate assumed, it was ironic that the physicist whose work struck a particularly forceful blow against corpuscular theory was himself an Englishman: Thomas Young (1773-1829), who in 1801 demonstrated interference in light. Directing a light beam through two closely spaced pinholes onto a screen, Young reasoned that if light truly were made of particles, the beams would project two distinct points onto the screen. Instead, what he saw was a pattern of interference—a wave phenomenon.

The Question of a Medium

As the nineteenth century progressed, evidence in favor of wave theory grew. Experiments in 1850 by Jean Bernard Leon Foucault (1819-1868)—famous for his pendulum—showed that light traveled faster in air than through water. Based on studies of wave motion up to that time, Foucault's work added substance to the view of light as a wave.

Foucault also measured the speed of light in a vacuum, a speed which he calculated to within 1% of its value as it is known today: 186,000 mi (299,339 km) per second. An understanding of just how fast light traveled, however, caused a nagging question dating back to the days of Newton and Huygens to resurface: how did light travel?

All types of waves known to that time traveled through some sort of medium: for instance, sound waves were propagated through air, water, or some other type of matter. If light was a wave, as Huygens said, then it, too, must have some medium. Huygens and his followers proposed a weak theory by suggesting the existence of an invisible substance called ether, which existed throughout the universe and which carried light.

Ether, of course, was really no answer at all. There was no evidence that it existed, and to many scientists, it was merely a concept invented to shore up an otherwise convincing argument. Then, in 1872, Scottish physicist James Clerk Maxwell (1831-1879) proposed a solution that must have surprised many scientists. The "medium" through which light travels, Maxwell proposed, was no medium at all; rather, the energy in light is transferred by means of radiation, which requires no medium.

Electromagnetism

Maxwell brought together a number of concepts developed by his predecessors, sorting these out and adding to them. His work led to the identification of a "new" fundamental interaction, in addition to that associated with gravity. This was the mode of particle interaction associated with electromagnetic force.

The particulars of electromagnetic force, waves, and radiation are a subject unto themselves—really, many subjects. As for the electromagnetic spectrum, it is treated at some length in an essay elsewhere in this volume, and the reader is encouraged to review that essay to gain a greater understanding of light and its place in the spectrum.

In addition, some awareness of wave motion and related phenomena would also be of great value, and, for this purpose, other essays are recommended. In the present context, a number of topics relating to these larger subjects will be handled in short order, with a minimum of explanation, to enable a more speedy transition to the subject of principal importance here: light.

Electromagnetic Waves

There is, of course, no obvious connection between light and the electromagnetic force observed in electrical and magnetic interactions. Yet, light is an example of an electromagnetic wave, and is part of the electromagnetic spectrum. The breakthrough in establishing the electromagnetic quality of light can be attributed both to Maxwell and German physicist Heinrich Rudolf Hertz (1857-1894).

In his Electricity and Magnetism (1873), Maxwell suggested that electromagnetic force might have aspects of a wave phenomenon, and his experiments indicated that electromagnetic waves should travel at exactly the same speed as light. This appeared to be more than just a coincidence, and his findings led him to theorize that the electromagnetic interaction included not only electricity and magnetism, but light as well. Some time later, Hertz proved Maxwell's hypothesis by showing that electromagnetic waves obeyed the same laws of reflection, refraction, and diffraction as light.

Hertz also discovered the photoelectric effect, the process by which certain metals acquire an electrical potential when exposed to light. He could not explain this behavior, and, indeed, there was nothing in wave theory that could account for it. Strangely, after more than a century in which acceptance of wave theory had grown, he had encountered something that apparently supported what Newton had said long before: that light traveled in particles rather than waves.

The Wave-Particle Debate Revisited

One of the modern physicists whose name is most closely associated with the subject of light is Albert Einstein (1879-1955). In the course of proving that matter is convertible to energy, as he did with the theory of relativity, Einstein predicted that this could be illustrated by accelerating to speeds close to that of light. (Conversely, he also showed that it is impossible for matter to reach the speed of light, because to do so would—as he proved mathematically—result in the matter acquiring an infinite amount of mass, which, of course, is impossible.)

Much of Einstein's work was influenced by that of German physicist Max Planck (1858-1947), father of quantum theory. Quantum theory and quantum mechanics are, of course, far too complicated to explain in any depth here. It is enough to say that they called into question everything physicists thought they knew, based on Newton's theories of classical mechanics. In particular, quantum mechanics showed that, at the subatomic level, particles behave in ways not just different from, but opposite to, the behavior of larger physical objects in the observable world. When a quantity is "quantized," its values or properties at the atomic or subatomic level are separate from one another—meaning that something can both be one thing and its opposite, depending on how it is viewed.

Interpreting Planck's observations, Einstein in a 1905 paper on the photoelectric effect maintained that light is quantized—that it appears in "bundles" of energy that have characteristics both of waves and of particles. Though light travels in waves, as Einstein showed, these waves sometimes behave as particles, which is the case with the photoelectric effect. Nearly two decades later, American physicist Arthur Holly Compton (1892-1962) confirmed Einstein's findings and gave a name to the "particles" of light: photons.

Light's Place in the Electromagnetic Spectrum

The electromagnetic spectrum is the complete range of electromagnetic waves on a continuous distribution from a very low range of frequencies and energy levels, with a correspondingly long wavelength, to a very high range of frequencies and energy levels, with a correspondingly short wavelength. Included on the electromagnetic spectrum are radio waves and microwaves; infrared, visible, and ultraviolet light; x rays, and gamma rays. As discussed earlier, concerning the visible color spectrum, each of these occupies a definite place on the spectrum, but the divisions between them are not firm: in keeping with the nature of a spectrum, one band simply "blurs" into another.

Of principal concern here is an area near the middle of the electromagnetic spectrum. Actually, the very middle of the spectrum lies within the broad area of infrared light, which has frequencies ranging from 10¹² to just over 10¹⁴ Hz, with wavelengths of approximately 10⁻¹ to 10⁻³ centimeters. Even at this point, the light waves are oscillating at a rate between 1 and 100 trillion times a second, and the wavelengths are from 1 millimeter to 0.01 millimeters. Yet, over the breadth of the electromagnetic spectrum, wavelengths get much shorter, and frequencies much greater.

Infrared lies just below visible light in frequency, which is easy to remember because of the name: red is the lowest in frequency of all the colors, as discussed below. Similarly, ultraviolet lies beyond the highest-frequency color, violet. Neither infrared nor ultraviolet can be seen, yet we experience them as heat. In the case of ultraviolet (UV) light, the rays are so powerful that exposure to even the minuscule levels of UV radiation that enter Earth's atmosphere can cause skin cancer.

Ultraviolet light occupies a much narrower band than infrared, in the area of about 10¹⁵ to 10¹⁶ Hz—in other words, oscillations between 1 and 10 quadrillion times a second. Wavelengths in this region are from just above 10⁻⁶ to about 10⁻⁷ centimeters. These are often measured in terms of a nanometer (nm)—equal to one-millionth of a millimeter—meaning that the wavelength range is from above 100 down to about 10 nm.

Between infrared and ultraviolet light is the region of visible light: the six colors that make up much of the world we know. Each has a specific range and frequency, and together they occupy an extremely narrow band of the electromagnetic spectrum: from 4.3 · 10¹⁴ to 7.5 · 10¹⁴ Hz in frequency, and from 700 down to 400 nm in wavelength. To compare its frequency range to that of the entire spectrum, for instance, is the same as comparing 3.2 to 100 billion.

Real-Life Applications

Colors

Unlike many of the topics addressed by physics, color is far from abstract. Numerous expressions in daily life describe the relationship between energy and color: "red hot," for instance, or "blue with cold." In fact, however, red—with a smaller frequency and a longer wavelength than blue—actually has less energy; therefore, blue objects should be hotter.

The phenomenon of the red shift, discovered in 1923 by American astronomer Edwin Hubble (1889-1953), provides a clue to this apparent contradiction. As Hubble observed, the light waves from distant galaxies are shifted to the red end, and he reasoned that this must mean those galaxies are moving away from the Milky Way, the galaxy in which Earth is located.

To generalize from what Hubble observed, when something shows red, it is moving away from the observer. The laws of thermodynamics state that where heat is involved, the movement is always away from an area of high temperature and toward an area of low temperature. Heated molecules that reflect red light are, thus, to use a colloquialism, "showing their tail end" as they move toward an area of low temperature. By contrast, molecules of low temperature reflect bluish or purple light because the tendency of heat is to move toward them.

There are other reasons, aside from heat, that some objects tend to be red and others blue—or another color. Chemical factors may be involved: atoms of neon, for example, can be made to vibrate at a particular wavelength, producing a specific color. In any case, the color that an object reflects is precisely the color that it does not absorb: thus, if something is red, that means it has absorbed every color of the spectrum but red.

Why Is the Sky Blue?

The placement of colors on the electromagnetic spectrum provides an answer to that age-old question posed by generations of children to their parents: "Why is the sky blue?" Electromagnetic radiation is scattered as it enters the atmosphere, but all forms of radiation are not scattered equally. Those having shorter wavelengths—that is, toward the blue end of the spectrum—tend to scatter more than those with longer wavelengths, on the red and orange end.

Yet the longer-wavelength light becomes visible at sunset, when the Sun's light enters the atmosphere at an angle. In addition, the dim quality of evening light means that it is easiest to see light of longer wavelengths. This effect is known as Rayleigh scattering, after English physicist John William Strutt, Lord Rayleigh (1842-1919), who discovered it in 1871. Thanks to Rayleigh's discovery, there is an explanation not only for the question of why the sky is blue, but why sunsets are red, orange, and gold.

Rainbows

On the subject of color as children perceive it, many a child has been fascinated by a rainbow, seeing in them something magical. It is easy to understand why children perceive these beautiful phenomena this way, and why people have invented stories such as that of the pot of gold at the end of a rainbow. In fact, a rainbow, like many other "magical" aspects of daily life, can be explained in terms of physics.

A rainbow, in fact, is simply an illustration of the visible light spectrum. Rain drops perform the role of tiny prisms, dispersing white sunlight, much as scientists before Newton had learned to do. But if there is a pot of gold at the end of the rainbow, it would be impossible to find. In order for a rainbow to be seen, it must be viewed from a specific perspective: the observer must be in a position between the sunlight and the raindrops.

Sunlight strikes raindrops in such a way that they are refracted, then reflected back at an angle so that they represent the entire visible light spectrum. Though they are beautiful to see, rainbows are neither magical nor impossible to reproduce artificially. Such rainbows can be produced, for instance, in the spiral of small water droplets emerging from a water hose, viewed when one's back is to the Sun.

Perception of Light and Color

People literally live and die for colors: the colors of a flag, for instance, present a rallying point for soldiers, and different colors are assigned specific political meanings. Blue, both in the American and French flags, typically stands for liberty. Red can symbolize the blood shed by patriots, or it can mean some version of fraternity or brotherhood. Such is the case with the red of the French tricolor (red-white-blue); likewise, the red in the flag of the former Soviet Union and other Communist countries stood for the alleged international brotherhood of all working peoples. In Islamic countries, by contrast, green stands for the unity of all Muslims.

These are just a few examples, drawn from a specific realm—politics—illustrating the meanings that people ascribe to colors. Similarly, people find meanings in images presented to them by light itself. In his Republic, the ancient Greek philosopher Plato (c. 427-347 B.C.) offered a complex parable, intended to illustrate the difference between reality and illusion, concerning a group of slaves who do not recognize the difference between sunlight and the light of a torch in a cave. Modern writers have noted the similarities between Plato's cave and a phenomenon which the ancient philosopher could hardly have imagined: a movie theatre, in which an artificial light projects images—images that people sometimes perceive as being all too real—onto a screen.

People refer to "tricks of the light," as, for instance, when one seems to see an image in a fire. One particularly well-known "trick of the light," a mirage, is discussed below, but there are also manmade illusions created by light, shapes, and images. An optical illusion is something that produces a false impression in the brain, causing one to believe that something is as it appears, when, in fact, it is not. When two lines of equal length are placed side by side, but one has arrows pointed outward at either end while the other line has arrows pointing inward, it appears that the line with the inward-pointing arrows are shorter.

This is an example of the ways in which human perception plays a role in what people see. That topic, of course, goes far beyond physics and into the realms of psychology and the social sciences. Nonetheless, it is worthwhile to consider, from a physical standpoint, how humans see what they see—and sometimes see things that are not there.

A Mirage

Because they can be demonstrated in light waves as well as in sound waves, diffraction and interference are discussed in separate essays. As for refraction, or the bending of light waves, this phenomenon can be seen in the familiar example of a mirage. While driving down a road on a hot day, one may observe that there are pools of water up ahead, but by the time one approaches them, they disappear.

Of course, the pools were never there; light itself has created an optical illusion of sorts. As light moves from one material to another, it bends with a different angle of refraction, and, though, in this instance, it is traveling entirely through air, it is moving through regions of differing temperature. Light waves travel faster through warm air than through cool air, and, thus, when the light enters the area over the heated surface of the asphalt, it experiences refraction. The waves are thus bent, creating the impression of a reflection, which suggests to the observer that there is water up ahead.

How the Eye Sees Color

White, as noted earlier, is the combination of all colors; black is the absence of color. Where ink, dye, or other forms of artificial pigmentation are concerned, of course, black is a "real" color, but in terms of light, it is not. In the same way, the experience of coldness is real, yet "cold" does not exist as a physical phenomenon: it is simply the absence of heat.

The mixture of pigmentation is an entirely different matter from the mixture of light. In artificial pigmentation, the primary colors—the three colors which, when mixed, yield the remainder of the shades on the rainbow—are red, blue, and yellow. Red mixed with blue creates purple, blue mixed with yellow makes green, and red mixed with yellow yields orange. Black and white are usually created by using natural substances of that color—chalk for white, for instance, or various oxides for black. For light, on the other hand, blue and red are primary colors, but the third primary color is green, not yellow. From these three primary colors, all other shades of the visible spectrum can be made.

The mechanism of the human eye responds to the three primary colors of the visible light spectrum: thus, the eye's retina is equipped with tiny cones that respond to red, blue, and green light. The cones respond to bright light; other structures called rods respond to dim light, and the pupil regulates the amount of light that enters the eye.

The eye responds with maximum sensitivity to light at the middle of the visible color spectrum—specifically, green light with a wavelength of about 555 nm. The optimal wavelength for maximum sensitivity in dim light is around 510 nm, on the blue end. It is difficult for the eye to recognize red light, at the far end of the spectrum, against a dark background. However, this can be an advantage in situations of relative darkness, which is why red light is often used to maintain vision for sailors, amateur astronomers, and the military on night maneuvers. Because there is not much difference between the darkness and the red light, the eye adjusts and is able to see beyond the red light into the darkness. A bright yellow or white light in such situations, on the other hand, would minimize visibility in areas beyond the light.

Artificial Light

Prehistoric Lighting Technology

Prehistoric humans did not know it, but they were making use of electromagnetic radiation when they lit and warmed their caves with light from a fire. Though it would seem that warmth was more essential to human survival than artificial light, in fact, it is likely that both functions emerged at about the same time: once humans began using fire for warmth, it would have been a relatively short time before they comprehended the power of fire to drive out both darkness and the fierce creatures (for instance, bears) that came with it.

These distant forebears advanced to the fashioning of portable lighting technology in the form of torches or rudimentary lamps. Torches were probably made by binding together resinous material from trees, while lamps were made either from stones with natural depressions, or from soft rocks—for example, soap-stone or steatite—into which depressions were carved by using harder material. Most of the many hundreds of lamps found by archaeologists at sites in southwestern France are made of either limestone or sandstone. Limestone was a particularly good choice, since it conducts heat poorly; lamps made of sandstone, a good conductor of heat, usually had carved handles to protect the hands of the user.

Artificial Light in Pre-Modern Times

The history of lighting is generally divided into four periods, each of which overlap, and which together illustrate the slow pace of change in illumination technology. First was the primitive, a period encompassing the torches and lamps of prehistoric human beings—though, in fact, French peasants used the same lighting methods depicted in nearby cave paintings until World War I.

Next came the classical stage, the world of Greece and Rome. Earlier civilizations, such as that of Egypt, belong to the primitive era in lighting—before the relatively widespread adoption of the candle and of vegetable oil as fuel. Third was the medieval stage, which saw the development of metal lamps. Last came the modern or invention stage, which began with the creation of the glass lantern chimney by Leonardo da Vinci (1452-1519) in 1490, culminated with Thomas Edison's (1847-1931) first practical incandescent bulb in 1879, and continues today.

At various times, ancient peoples used the fat of seals, horses, cattle, and fish as fuel for lamps. (Whale oil, by contrast, entered widespread use only during the nineteenth century.) Primitive humans sometimes used entire animals—for example, the storm petrel, a bird heavy in fat—to provide light. Even without such cruel excesses, however, animal fat made for a smoky, dangerous, foul-smelling fire.

The use of vegetable oils, a much more efficient medium for lighting, did not take hold until Greek, and especially, Roman times. Animal oils remained in use, however, among the poor, whose homes often reeked with the odor of castor oil or fish oil. Because virtually all fuels came from edible sources, times of famine usually meant times of darkness as well.

The candle, as well as the use of vegetable oils, dates back to earliest antiquity, but the use of candles only became common among the richest citizens of Rome. Because it used animal fat, the candle was apparently a return to an earlier stage, but its hardened tallow actually represented a much safer, more stable fuel than lamp oil.

Incandescent Light

Lighting technology in the period from about 1500 to the late nineteenth century involved a number of improvements, but in one respect, little had progressed since prehistoric times: people were still burning fuel to provide illumination. This all changed with the invention of the incandescent bulb, which, though it is credited to Edison, was the product of experimentation that took place throughout the nineteenth century. As early as 1802, British scientist Sir Humphry Davy (1778-1829) showed that electricity running through thin strips of metal could heat them enough to cause them to give off light—that is, electromagnetic radiation.

Edison, in fact, was just one of several inventors in the 1870s attempting to develop a practical incandescent lamp. His innovation lay in his understanding of the parameters necessary for developing such a lamp—in particular, decreasing the electrical resistance in the lamp filament (the part that is heated) so that less energy would be required to light it. On October 19, 1879, using low-resistance filaments of carbon or platinum, combined with a high-resistance carbon filament in a vacuum-sealed glass container, Edison produced the first practical lightbulb.

Much has changed in the design of light-bulbs during the decades following Edison's ingenious invention, of course, but his design provided the foundation. There is just one problem with incandescent light, however—a problem inherent in the definition and derivation of the word incandescent, which comes from a Latin root meaning "to become hot." The efficiency of a light is determined by the ratio of light, or usable energy, to heat—which, except in the case of a campfire, is typically not a desirable form of energy where lighting is concerned.

Amazingly, only about 10% of the energy output from a typical incandescent light bulb is in the form of visible light; the rest comes through the infrared region of the spectrum, producing heat rather than light that people can use. The visible light tends to be in the red and yellow end of the spectrum—closer to infrared—but a blue-tinted bulb helps to absorb some of the red and yellow, providing a color balance. This, however, only further diminishes the total light output, and, hence, in many applications today, fluorescent light takes the place of incandescent light.

Lasers

A laser is an extremely focused, extremely narrow, and extremely powerful beam of light. Actually, the term laser is an acronym, standing for L ight A mplification by S imulated E mission of R adiation. Simulated emission involves bringing a large number of atoms into what is called an "excited state." Generally, most atoms are in a ground state, and are less active in their movements, but the energy source that activates a laser brings about population inversion, a reversal of the ratios, such that the majority of atoms within the active medium are in an excited rather than a ground state. To visualize this, picture a popcorn popper, with the excited atoms being the popping kernels, and the ground-state atoms the ones remaining unpopped. As the atoms become excited, and the excited atoms outnumber the ground ones, they start to cause a multiplication of the resident photons. This is simulated emission.

A laser consists of three components: an optical cavity, an energy source, and an active medium. To continue the popcorn analogy, the "popper" itself—the chamber which holds the laser—is the optical cavity, which, in the case of a laser, involves two mirrors facing one another. One of these mirrors fully reflects light, whereas the other is a partly reflecting mirror. The light not reflected by the second mirror escapes as a highly focused beam. As with the popcorn popper, the power source involves electricity, and the active medium is analogous to the oil in a conventional popper.

Types of Lasers

There are four types of lasers: solid-state, semiconductor, gas, and dye. Solid-state lasers are generally very large and extremely powerful. Having a crystal or glass housing, they have been implemented in nuclear energy research, and in various areas of industry. Whereas solid-state lasers can be as long as a city block, semiconductor lasers can be smaller than the head of a pin. Semiconductor lasers (involving materials such as arsenic that conduct electricity, but do not do so as efficiently as the metals typically used as conductors) are applied for the intricate work of making compact discs and computer microchips.

Gas lasers contain carbon dioxide or other gases, activated by electricity in much the same way the gas in a neon sign is activated. Among their applications are eye surgery, printing, and scanning. Finally, dye lasers, as their name suggests, use different colored dyes. (Laser light itself, unlike ordinary light, is monochromatic.) Dye lasers can be used for medical research, or for fun—as in the case of laser light shows held at parks in the summertime.

Laser Applications

Laser beams have a number of other useful functions, for instance, the production of compact discs (CDs). Lasers etch information onto a surface, and because of the light beam's qualities, can record far more information in much less space than the old-fashioned ways of producing phonograph records.

Lasers used in the production of CD-ROM (Read-Only Memory) disks are able to condense huge amounts of information—a set of encyclopedias or the New York metropolitan phone book—onto a disk one can hold in the palm of one's hand. Laser etching is also used to create digital videodiscs (DVDs) and holograms. Another way that lasers affect everyday life is in the field of fiber optics, which uses pulses of laser light to send information on glass strands.

Before the advent of fiber-optic communications, telephone calls were relayed on thick bundles of copper wire; with the appearance of this new technology, a glass wire no thicker than a human hair now carries thousands of conversations. Lasers are also used in scanners, such as the price-code checkers at supermarkets and various kinds of tags that prevent thefts of books from libraries or clothing items from stores. In an industrial setting, heating lasers can drill through solid metal, or in an operating room, lasers can remove gallstones or cataracts. Lasers are also used for guiding missiles, and to help building contractors ensure that walls and floors and ceilings are in proper alignment.

Where to Learn More

Burton, Jane and Kim Taylor. The Nature and Science of Colors. Milwaukee, WI: Gareth Stevens Publishing, 1998.

Glover, David. Color and Light. New York: Dorling Kindersley Publishing, 2001.

Kalman, Bobbie and April Fast. Cosmic Light Shows. New York: Crabtree Publishing, 1999.

Kurtus, Ron. "Visible Light" (Web site). <http://www.school-for-champions.com/science/light.html> (May 2, 2001).

"Light Waves and Color." The Physics Classroom (Web site). <http://www.glenbrook.k12.il.us/gbssci/phys/Class/light/lighttoc.html> (May 2, 2001).

Miller-Schroeder, Patricia. The Science and Light of Color. Milwaukee, WI: Gareth Stevens Publishing, 2000.

Nassau, Kurt. Experimenting with Color. New York: F. Watts, 1997.

Riley, Peter D. Light and Color. New York: F. Watts, 1999.

Taylor, Helen Suzanne. A Rainbow Is a Circle: And Other Facts About Color. Brookfield, CT: Copper Beech Books, 1999.

"Visible Light Waves" NASA: National Aeronautics and Space Administration (Web site). <http://imagers.gsfc.nasa.gov/ems/visible.html> (May 2, 2001).

Dansk (Danish)
1.
n. - lys, skær, dag, dagslys, lysåbning, rude, fyr, fyrtårn, ild, tændstik
v. tr. - tænde, oplyse, lede, føre
v. intr. - tændes
adj. - lys, klar

idioms:

bring to light bringe frem for dagens lys, trække frem i lyset
come to light komme for dagen, komme for en dag
hide one's light under a bushel sætte sit lys under en skæppe
in a bad light i et dårligt lys
in the light of i lyset af, på baggrund af, i betragtning af, i relation til
light bulb elektrisk pære
light dawns on someone der går et lys op for nogen
light of one's life sit livs lys
light pen lyspen
light stylus lyspen
light up lyse op, oplyse
light woman letsindig kvinde, tomhjernet kvinde
light year lysår
out like a light gå ud som et lys
pedestrian-crossing lights lys i fodgængerovergang
set light to sætte ild til
the light at the end of the tunnel der er lys for enden af tunnellen, der er lys forude

2.
adj. - let, mild, ubetydelig, ringe, letsindig, tankeløs, overfladisk, tomhjernet, letbevæbnet
adv. - let, mildt, ringe, letsindigt, tankeløst

idioms:

light aircraft lille fly
light in the head ør i hovedet, svimmel, tankeløs, overgiven
light industry let industri
light into angribe
light on træffe på, støde på
light opera operette
light out bryde op, stikke af
light reading let læsning
light sleeper person der sover let
light upon falde over
make light of ikke tage alvorligt, bagatellisere
make light work of it gøre det let for sig selv

3.
v. intr. - lande, dale ned, sætte sig

Nederlands (Dutch)
opsteken, verlichten, bijlichten, aansteken, ontsteken, uitstappen, aan wal gaan, licht, verlichting, schijnsel, vuurtje (voor sigaret), lamp, (mv) koplichten, (mv) verkeerslichten, venster/bovenlicht, inzicht, ster, knappe kop, zaklantaarn, licht-, weinig calorieën bevattend, lichtgewicht, lichtbewapend, gemakkelijk, luchtig, lichtzinnig, met weinig bagage, helder

Français (French)
1.
n. - (Phys) radiation électromagnétique, lumière, clarté, lueur (dans les yeux), réverbère, feu (sur un bateau), (Aut) phare, feu arrière, veilleuse, flamme, (fig) jour, angle, aspect, lumière de (d'un témoignage), vue (arch), (fig) découverte (d'une vérité), (Transp) feux (npl), illuminations (npl)
v. tr. - allumer, éclairer, illuminer
v. intr. - s'allumer, s'éclairer, s'illuminer
adj. - léger, faible, meuble (une terre)

idioms:

bring to light mettre en lumière, révéler
come to light être découvert, être dévoilé
hide one's light under a bushel cacher son talent
in a bad light sous un mauvais jour
in light of compte-tenu de, à cause de, en prenant considération de
in the light of à la lumière de, selon
light at the end of the tunnel la lumière au bout du tunnel
light bulb ampoule
light dawns on someone comprendre la vérité
light of one's life son rayon de soleil
light pen (Comput) crayon optique
light stylus (Comput) crayon optique
light up allumer, s'allumer, (fig) s'éclairer, briller de joie (les yeux)
light woman femme légère
light year (Astron) année-lumière
out like a light (s'endormir) tout de suite
pedestrian-crossing lights feu pour piétons
set light to mettre feu à

2.
adj. - clair, blond, léger, modéré (un buveur), peu actif (une affaire), léger (une sentence), délicat (un mouvement), peu fatigant (un travail), facile (à lire), pas sérieux (une liaison), enjoué (une humeur), (Culin) allégé, light
adv. - légèrement, sans chargement (une locomotive)

idioms:

be light on ne pas être sévère sur (une sanction)
go light on y aller mollo
light aircraft avion léger
light in the head léger dans sa tête
light industry industrie légère
light into tomber dessus, rentrer dedans (qn)
light on rencontrer (qn), trouver (qn) par hasard
light opera opérette
light out (US) décamper
light reading lecture facile
light sleeper (avoir) le sommeil léger
light upon tomber sur (qn) par hasard
make light of traiter (qch) à la légère, attacher peu d'importance à
make light work of faire qch sans peine

3.
v. intr. - descendre de (d'un cheval, d'une voiture), se poser sur (un regard), se poser sur (une branche), tomber sur, s'abattre sur, s'enfuir rapidement (arch)

Deutsch (German)
1.
n. - Licht, Lichtschein, Ampel, Leuchtfeuer, Feuer, Fenster, Erleuchtung
v. - beleuchten, erhellen, anzünden, brennen, (auf)leuchten
adj. - hell, licht

idioms:

bring to light aufdecken
come to light ans Licht kommen
hide one's light under a bushel sein Licht unter den Scheffel stellen
in a bad light in einem schlechten Licht
in light of angesichts
in the light of im Licht
light at the end of the tunnel Licht am Ende des Tunnels
light bulb Glühbirne
light dawns on someone jmdm. geht ein Licht auf
light of one's life Sonne des Lebens
light pen Lichtstift
light stylus Lichtstift
light up erhellen, aufleuchten, sich eine Zigarette anzünden
light woman leichtes Mädchen
light year Lichtjahr
out like a light sofort weg
pedestrian-crossing lights Fußgängerampel
set light to etwas anzünden

2.
adj. - leicht, mild, gering, leichtfertig, behende
adv. - leicht, nicht schwer

idioms:

be light on knapp
go light on [auf etw. (+Akk)] kommen od. stoßen
light aircraft leichtes Flugzeug
light in the head leicht benommen
light industry Leichtindustrie
light into jmdn. angreifen
light on auf etwas stoßen
light opera Operette
light out fortgehen
light reading leichte Lektüre
light sleeper jmd. der einen leichten Schlaf hat
light upon auf etwas stoßen
make light of auf die leichte Schulter nehmen
make light work of leicht damit fertigwerden

3.
v. - stoßen auf, absteigen, sich niederlassen

Ελληνική (Greek)
n. - φως, λάμψη, φωτιά, λάμπα, κερί, φωτιά για άναμμα τσιγάρου, φωτισμός, σκοπιά, άποψη, όψη, φεγγίτης, παράθυρο
v. - ανάβω, φωτίζω/-ομαι, αστράφτω, λάμπω, διαφωτίζω, ελαφρώνω, αστράφτω, αποβιβάζομαι, αφιππεύω
adj. - ελαφρός, ανάλαφρος, ανοιχτόχρωμος, απαλός, ευκολονόητος, εύθυμος
adv. - με ελάχιστες αποσκευές

idioms:

bring to light φέρνω στο φως
come to light έρχομαι στο φως
hide one's light under a bushel είμαι υπερβολικά μετριόφρων
in a bad light δυσμενώς
in the light of υπό το φως
light aircraft ελαφρά αεροσκάφη
light bulb λάμπα, λαμπτήρας
light dawns on someone (κάποτε επιτέλους) καταλαβαίνω ότι
light in the head ζαλισμένος
light industry ελαφρά βιομηχανία
light into επιτίθεμαι σε, καταφέρομαι εναντίον
light of one's life φως της ζωής μου
light on κάθομαι σε
light opera οπερέτα, ελαφρά όπερα
light out φεύγω βιαστικά
light pen (Η/Υ) φωτεινή πένα
light reading ελαφρό ανάγνωσμα
light sleeper αυτός που κοιμάται ελαφρά
light stylus (Η/Υ) φωτεινή πένα/γραφίδα
light up ανάβω, φωτίζω/-ομαι, λάμπω, ακτινοβολώ, αστράφτω, διαφωτίζω
light upon ανακαλύπτω, "πέφτω πάνω σε", βρίσκω τυχαία
light woman ελαφριά (γυναίκα)
light year έτος φωτός
make light of παίρνω αψήφιστα
make light work of it ξεμπερδεύω στα γρήγορα
out like a light (αποκοιμιέμαι) μονομιάς
pedestrian-crossing lights φανάρια για πεζούς
set light to βάζω φωτιά σε
the light at the end of the tunnel φως στην άκρη του τούνελ

Italiano (Italian)
illuminare, accendere, schiarirsi, luce, lume, fanale, illuminazione, leggero, facile, chiaro

idioms:

bring to light svelare
come to light viene rivelato
in a bad light in cattiva luce
in the light of alla luce di
light aircraft aereo leggero
light bulb lampadina
light dawns on someone qualcuno comincia a capire
light in the head idea luminosa
light industry industria leggera
light into attaccare
light of one's life luce degli occhi
light on/upon imbattersi in
light opera operetta
light out partire
light reading letture leggere
light sleeper dal sonno leggero
light up ubriacarsi
light upon imbattersi
light woman donna leggera
light year anno luce
make light of dare poca importanza a
out like a light addormentato (o svenuto) di colpo
pedestrian-crossing lights semaforo pedonale
phased traffic lights semafori sincronizzati
rear light fanale posteriore
set light/fire to dar fuoco a
shed/throw/cast light on fare luce su
street light lampione
the light at the end of the tunnel un lumicino nel buio
unit of light unità di luce

Português (Portuguese)
n. - luz (f), iluminação (f), fogo (m) (para acender cigarro), clarabóia (f), compreensão (f), esclarecimento (m)
v. - iluminar, acender, animar
adj. - leve, suave, vivo, superficial
adv. - facilmente

idioms:

bring to light elucidar
come to light desvendar-se
in a bad light tornar-se uma pessoa ruim
in the light of à luz de
light aircraft avião leve
light bulb lâmpada incandescente
light dawns on someone alguém que finalmente entende alguma coisa
light industry indústria da energia elétrica
light into ofender alguém
light of one's life luz da vida (de alguém)
light on/upon topar com, encontrar-se por acaso
light opera ópera de natureza alegre ou cômica
light out (gír.) escapar, sumir
light reading leitura leve
light sleeper um sono leve
light up acender (cigarro), iluminar, alegrar
light upon uma luz sobre
light woman mulher tranqüila
light year ano-luz
make light of não levar alguém à sério
out like a light estar inconsciente
pedestrian-crossing lights farol de pedestres
set light to atear fogo em alguma coisa
the light at the end of the tunnel a luz no fim do túnel

Русский (Russian)
свет, огонь, информация, аспект, знаменитость, зажигать, светить, освещать, светлый, легкий, нетрудный, легко

idioms:

bring to light обнаружить, вывести на чистую воду
come to light обнаруживаться, выявляться
in a bad light в невыгодном свете
in the light of в свете чего-л.
light aircraft легкий самолет
light bulb электролампочка
light dawns on someone внезапно понять что-л.
light in the head ветер в голове
light industry легкая промышленность
light into нападать, набрасываться
light of one's life светочь жизни
light on/upon неожиданно натолкнуться на что-л.
light opera оперетта
light out убегать
light reading развлекательная литература
light sleeper чутко спящий человек
light up освещать, светиться
light upon обнаруживать
light woman женщина легкого поведения
light year световой год
make light of недооценивать что-л.
out like a light немедленно заснул
pedestrian-crossing lights знак пешеходного перехода
phased traffic lights система светофоров
set light/fire to зажечь, поджечь
shed/throw/cast light on пролить свет на что-л.
the light at the end of the tunnel свет в конце туннеля

Español (Spanish)
1.
n. - luz, lumbre, resplandor, lámpara, faro, iluminación, energía lumínica, energía radiante, encendedor, (fig) líder destacado, comprensión
v. tr. - alumbrar, iluminar, encender, clarear, dar brillo
v. intr. - encenderse, iluminarse, brillar
adj. - alumbrado, claro, de luz, luminoso

idioms:

bring to light sacar a luz, descubrir
come to light descubrirse, salir a luz
hide one's light under a bushel ser muy modesto
in a bad light aspecto negativo
in light of a la luz de, en vista de que
in the light of a la luz de
light at the end of the tunnel la luz al final del túnel, vislumbrar el fin de sus problemas, solución
light bulb bombilla, foco
light dawns on someone caer en la cuenta, comprender
light of one's life la luz de la vida de uno
light pen (comp) lápiz óptico
light stylus (comp) lápiz óptico
light up iluminar, iluminarse, prender, encender (cigarro, etc.)
light woman mujer ligera de cascos
light year año luz
out like a light dormirse en seguida, caerse redondo
pedestrian-crossing lights semáforo para peatones
set light to pegar fuego a, encender

2.
adj. - de poco peso, leve, liviano, ligero, superficial, suave
adv. - a la ligera, con poca carga

idioms:

be light on dar con, tropezar con, suceder por casualidad
go light on emborracharse
light aircraft avión ligero, avioneta
light in the head ligero de cascos
light industry industria ligera
light into embestir, atacar
light on dar con, tropezar con
light opera opereta, zarzuela
light out largarse, poner pies en polvorosa
light reading lecturas de entretenimiento
light sleeper de sueño ligero
light upon posarse, dar con, tropezar con
make light of no dar importancia a, no tomar en serio
make light work of despachar algo rápidamente, hacer algo sin mucho esfuerzo

3.
v. intr. - posarse, descender, caer sobre, arremeter contra, increpar duramente, salir de pronto, suceder inesperadamente

Svenska (Swedish)
n. - ljus, belysning, dagsljus, dager, lampa, låga, tändsticka, klarhet, (sjö.) fyr, lanterna, ljusöppning, fönster, glasruta (i drivhus), (konst.) ljusparti (på tavla), (sl.) krita, kredit
v. - tända, lysa upp, belysa, förse med belysning, lysa ngn (på väg), tändas, ta eld, (börj
adj. - lätt, för lätt, inte fullviktig, underviktig, lätt lastad, som är avsedd för lätt last, (sjö.) utan last, tom, (mil.) lätt, lös, sandig, oviktig, obetydlig, ringa, lindrig, tanklös, lättsinnig, ytlig, flyktig, lättledd, lätt (färdig), (typ.) mager
adv. - lätt

中文（简体） (Chinese (Simplified))
1. 光, 光亮, 光线, 光源, 点, 照亮, 点燃, 使容光焕发, 点着, 变亮, 明亮的, 浅色的

idioms:

bring to light 发现, 揭发
come to light 被发现, 众所周知
hide one's light under a bushel 不露锋芒
in a bad light 在暗处, 看...坏的方面
in the light of 根据, 依照
light aircraft 轻型飞机, 小型飞机
light bulb 灯泡
light dawns on someone 某人茅塞顿开
light in the head 头晕的, 愚蠢的
light industry 轻工业
light into 痛打, 大吃
light of one's life 心爱的人, 亲爱者, 生命之光
light on 停落于
light opera 光动电驿, 轻歌剧
light out 匆匆离去
light pen 光笔
light reading 轻松读物
light sleeper 睡眠中易被惊醒的人
light stylus 光笔
light up 点燃, 照亮
light upon 停落于, 想到
light woman 轻浮女人, 荡妇
light year 光年
make light of 轻视
make light work of it 轻松解决某事
out like a light 很快入睡, 立刻昏厥
pedestrian-crossing lights 人行横道灯
set light to 点燃
the light at the end of the tunnel 成功之望

2. 明亮的, 浅色的, 轻地, 轻装地, 点着, 变亮

中文（繁體） (Chinese (Traditional))
1.
n. - 光, 光亮, 光線, 光源
v. tr. - 點, 照亮, 點燃, 使容光煥發
v. intr. - 點著, 變亮
adj. - 明亮的, 淺色的

idioms:

bring to light 發現, 揭發
come to light 被發現, 眾所周知
hide one's light under a bushel 不露鋒芒
in a bad light 在暗處, 看...壞的方面
in the light of 根據, 依照
light aircraft 輕型飛機, 小型飛機
light bulb 燈泡
light dawns on someone 某人茅塞頓開
light in the head 頭暈的, 愚蠢的
light industry 輕工業
light into 痛打, 大吃
light of one's life 心愛的人, 親愛者, 生命之光
light on 停落於
light opera 光動電驛, 輕歌劇
light out 匆匆離去
light pen 光筆
light reading 輕鬆讀物
light sleeper 睡眠中易被驚醒的人
light stylus 光筆
light up 點燃, 照亮
light upon 停落於, 想到
light woman 輕浮女人, 蕩婦
light year 光年
make light of 輕視
make light work of it 輕鬆解決某事
out like a light 很快入睡, 立刻昏厥
pedestrian-crossing lights 人行橫道燈
set light to 點燃
the light at the end of the tunnel 成功之望

2.
adj. - 明亮的, 淺色的
adv. - 輕地, 輕裝地
v. intr. - 點著, 變亮

한국어 (Korean)
1.
n. - 빛, 밝기, 가시광선, 주간, 불꽃, 발견 , 대가
v. tr. - 점화하다, 조명하다, 밝게 하다, 등불을 밝히고 안내하다
v. intr. - 불이 붙다, 밝아지다, 빛나다
adj. - 밝은, 연한, 흰 빛을 띤

idioms:

bring to light 드러내다, 폭로하다
come to light 밝은 데로 나오다, 나타내다
in a bad light 잘 보이지 않는 곳에, 불리한 위치에
in the light of 관점, ~을 고려하여
light up 밝게 하다, 비추다
set light to 불을 일으키다
the light at the end of the tunnel 어려운 일의 종말의 기미

2.
adj. - 가벼운, 적은 , 불명확한, 너그러운, 경쾌한, 경솔한
adv. - 가볍게 , 경쾌하게, 짐을 가지지 않고

idioms:

light into 욕하다, 꾸짖다
light on 우연히 발견하다, ~과 맞닥뜨리다, 떨어지다
light out 총총히 떠나다, 급히 떠나다
light upon 우연히 발견하다
make light of ~을 경시하다, 얕보다
make light work of it 빠르고 쉽게 성취하다

3.
v. intr. - 내리다, 머물다

日本語 (Japanese)
n. - 光, 明かり, 明るさ, 日中, 日の明かり, 火, 灯火, 信号灯, 輝き, 見方, 光明, 日光, 知識, 脚光, マッチ
adj. - 明るい, 薄い, 軽い, 量目不足の, 容易な, 肩の凝らない, 少しの, もたれない, 弱い, 軽快な, 浮ついた
adv. - 軽く, 身軽に, 軽装で
v. - 火をつける, 火がつく, 明るくする, 明るくなる, 降りる

idioms:

come to light 明るみに出る
in the light of 照らして, …として
light aircraft 軽飛行機
light bulb 電球
light dawns on someone 人を叱る
light in the head めまいがする, ばかな
light industry 軽工業
light into 激しく攻撃する, 叱る
light of one's life 人生の光明
light on/upon 十分でなく
light opera 軽喜歌劇
light out 全速で走る, 急いで立ち去る, 逃げ出す
light reading 軽い読み物
light sleeper 眠りの浅い人
light the fuse 導火線に火をつける
light up 火入
light upon 出会う, 見付ける
light year 光年
make light of 軽んじる, 軽視する
out like a light すぐ寝てしまう
pedestrian-crossing lights 歩行者横断用信号
reversing light 後退灯
shed/throw/cast light on 光を当てる, 明らかにする
the light at the end of the tunnel トンネルを抜けると見える光

العربيه (Arabic)
‏(الاسم) ضياء, نور (فعل) أضاء, نور, أشعل, أوقد (صفه) يكشف, يعلن, خفيف الوزن, مضىء, غير معتم, رقيق, تافه, لطيف (ظرف) يجعله معلوما أو معروفا‏

עברית (Hebrew)
n. - ‮אור, מקור אור, אש, חלון, צוהר, אספקט, אור יום, איש מופת, מואר, שטוף-אור, בהיר, נאורות, תקווה, אושר, אמצעי להדלקת אש, שמשה, בעיקר בחממה‬
v. tr. - ‮האיר, הדליק, האיר דרך, (שמחה) האירה את פניו‬
v. intr. - ‮נדלק, הואר, הדליק סיגריה, זרח משמחה‬
adj. - ‮מואר, שטוף-אור, בהיר‬
adj. - ‮קל, קליל, קל-דעת, עליז‬
adv. - ‮בקלות, קלילות, ללא מיטען‬
v. intr. - ‮ירד מכלי-רכב, נחת, הגיע (במקרה) ל-, נפל על‬

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