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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.


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 intromissionthe 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 raysin 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.


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 colorjust 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


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 interferencea wave phenomenon.


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 pendulumshowed 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.


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 themselvesreally, 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.


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 wouldas he proved mathematicallyresult 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 anothermeaning 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 quantizedthat 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 101 to 103 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 Hzin other words, oscillations between 1 and 10 quadrillion times a second. Wavelengths in this region are from just above 106 to about 107 centimeters. These are often measured in terms of a nanometer (nm)equal to one-millionth of a millimetermeaning 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.



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, redwith a smaller frequency and a longer wavelength than blueactually 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 blueor 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.


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 wavelengthsthat is, toward the blue end of the spectrumtend 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.


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 realmpoliticsillustrating 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 imagesimages that people sometimes perceive as being all too realonto 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 seeand sometimes see things that are not there.


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.


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 colorsthe three colors which, when mixed, yield the remainder of the shades on the rainboware 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 colorchalk 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 spectrumspecifically, 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 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 rocksfor example, soap-stone or steatiteinto 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.


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 beingsthough, 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 lightingbefore 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 animalsfor example, the storm petrel, a bird heavy in fatto 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.


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 lightthat 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 lampin 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, howevera 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 heatwhich, 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 spectrumcloser to infraredbut 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.


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" itselfthe chamber which holds the laseris 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.


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 funas in the case of laser light shows held at parks in the summertime.


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 informationa set of encyclopedias or the New York metropolitan phone bookonto 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.


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

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An opening.


The bending of waves around obstacles, or the spreading of waves by passing them through an aperture.


A process by which the concentration or density of something isdecreased.


The complete range of electromagnetic waves on a continuous distribution from a very low range of frequencies and energylevels, 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 long-wave and short-wave radio; microwaves; infrared, visible, and ultraviolet light; x rays, and gamma rays.


A transverse wave with electric and magnetic fields that emanate from it. The directions of these fields are perpendicular to one another, and both are perpendicular to the line of propagation for the wave itself.


The number of waves passing through a given point during the interval of one second. The higher the frequency, the shorter the wavelength.


A unit for measuring frequency, named after nineteenthcentury German physicist Heinrich Rudolf Hertz (1857-1894).


Contact between a rayfor example, a light rayand a surface. Types of incidence include reflection and refraction.


A substance through which light travels, such as air, water, or glass. Because light moves by radiation, it does not require a medium, and, in fact, movement through a medium slows the speed of light somewhat.


An area of physics that studies the production and propagation of light.


The phenomenon whereby certain metalsacquire an electrical potential when exposed to light.


A particle of electromagnetic radiationfor example, lightcarryinga specific amount of energy, measured in electron volts (eV).


A three-dimensional glassshape used for the diffusion of light rays.


The act or state of traveling from one place to another.


The transfer of energy by means of electromagnetic waves, which require no physical medium (for example, water or air) for the transfer. Earth receives the Sun's energy (including its light), via the electromagnetic spectrum, by means of radiation.


In geometry, a ray is that part of a straight line that extends in one directiononly. The term "ray" is used to describe the directed line made by light as it moves through space.


A type of incidence whereby a light ray is returned toward its source rather than being absorbed at the interface.


The bending of a lightray that occurs when it passes through a dense medium, such as water or glass.


The continuous distribution of properties in an ordered arrangement across an unbroken range. Examples of spectra (the plural of "spectrum") include the colors of visible light, or the electromagnetic spectrum of which visiblelight is a part.


A wave in which the vibration or motion is perpendicular to the direction in which the wave is moving.


An area of space devoid of matter, including air.


The distance between a crest and the adjacent crest, or the trough and an adjacent trough, of a wave. The shorter the wavelength, the higher the frequency.

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Light is generally defined as that portion of the electromagnetic spectrum with wavelengths between 400 and 700 nanometers (billionths of a meter). Like all forms of electromagnetic radiation, light travels with a speed of 186,282 miles (299,728 kilometers) per second in a vacuum. It is perhaps the swiftest and most delicate form of energy found in nature.

Historical concepts

Considering how important light is in our daily lives, it is hardly surprising that philosophers and scientists have been trying to understand its fundamental nature for centuries. The ancient Greeks, for example, worked out some of the basic laws involving light, including the laws of reflection (bouncing off an object) and refraction (bending through an object). They did so in spite of the fact that they started with only an incorrect concept of light. They believed that light beams started out in the human eye and traveled to an object.

Words to Know

Corpuscle: A particle.

Diffraction: The bending of light or another form of electromagnetic radiation as it passes through a tiny hole or around a sharp edge.

Duality: The tendency of something to behave in two very different ways, for example, as both energy and matter.

Electromagnetic spectrum: The whole range of radiation that travels through a vacuum with a speed of about 3 × 108 meters per second.

Ether: Also spelled aether; medium that was hypothesized by physicists to explain the wave behavior of light.

Photoelectric effect: The production of an electric current when a beam of light is shined on a metal.

Photon: A tiny package of light energy.

Wave: A regular pattern of motion that involves some kind of disturbance in a medium.

Wavelength: The distance between two successive identical parts of a wave, such as two crests or two troughs.

With the rise of modern physics in the seventeenth century, scientists argued over two fundamental explanations of the nature of light: wave versus particle. According to the particle theory of light, light consists of a stream of particles that come from a source (such as the Sun or a lamp), travel to an object, and are then reflected to an observer. This view of light was first proposed in some detail by Isaac Newton (16421727). Newton's theory is sometimes known as the corpuscular theory of light.

At about the same time, the wave theory of light was being developed. According to the wave theory, light travels through space in the form of a wave, similar in some ways to water waves. The primary spokesperson for this concept was Dutch physicist Christiaan Huygens (16291695).

Over time, the wave theory became more popular among physicists. One of the main reasons for the triumph of the wave theory was that many typical wave properties were detected for light. For example, when light passes through a tiny pinhole or around a sharp edge, it exhibits a property known as diffraction. Diffraction is well known as a property of waves among physicists. Almost anyone can witness the diffraction of water waves as they enter a bay or harbor, for example. If light exhibits diffraction, scientists thought, then it must be transmitted by waves.

Today, scientists usually talk about light as if it were transmitted by waves. They talk about the wavelength and frequency of light, both properties of waves, not particles.

The mysterious ether

One of the serious problems arising out of the wave theory of light is the problem of medium. Wave motion is the regular up-and-down motion of some material. For water waves, that material (or medium) is water. If light is a form of wave motion, scientists asked, what is the medium through which it travels?

The obvious answer, of course, is that light travels through air as a medium. But that answer is contradicted by the fact that light also travels through a vacuum, a region of space that contains no air or anything else.

To resolve this problem, scientists developed the concept of an ether (or aether). The ether was defined as a very thin materialperhaps like air, but much less densethat permeates all of space. Light could be explained, then, as a wave motion in the ether.

Unfortunately, efforts to locate the ether were unsuccessful. In one of the most famous negative experiments of all time, two American physicists, Albert A. Michelson (18521931) and Edward W. Morley (18381923), devised a very precise method for detecting the ether. No matter how carefully they searched, they found no ether. Their experiments were so carefully designed and carried out that physicists were convinced that the ether did not exist.

Today, a somewhat simpler view of light as a wave phenomenon exists. Light is a form of radiation that needs no medium through which to travel. It consists of electric and magnetic fields that pulsate up and down as they travel through space.

Return of the corpuscular theory

By the early 1900s, most physicists had accepted the idea that light is a form of wave motion. But they did so somewhat reluctantly because some facts about light could not really be explained by the wave theory. The most important of these was the photoelectric effect.

The photoelectric effect was first observed by German physicist Heinrich Hertz (18571894) in about 1888. He noticed that when light is shined on a piece of metal, an electric current (a flow of electrons) is produced. Later experiments showed a rather peculiar property of the photoelectric effect. It doesn't make any difference how intense the light is that is shined on the metal. A bright light and a dim light both produce the same current. What does make a difference is the color of the light. Red light, for example, produces more of a current than blue light.

Unfortunately, there is no way for the wave theory of light to explain this effect. In fact, it was not until 1905 that a satisfactory explanation of the photoelectric effect was announced. That explanation came from German-born American physicist Albert Einstein (18791955). Einstein showed that the photoelectric effect could be explained provided that light were thought of not as a wave but as a bundle of tiny particles.

But the concept of light-as-particles is just what Isaac Newton had proposed more than 200 years earlierand what physicists had largely rejected. The important point about Einstein's explanation, however, was that it worked. It explained a property of light that wave theory could not explain.

Duality of light and matter

The conflict between light-as-waves and light-as-particles has had an interesting resolution. Today, physicists say that light sometimes acts like a wave and sometimes acts like a collection of particles. Perhaps it is a wave consisting of tiny particles. Those particles are now called photons. They are different from other kinds of particles we know of since they have no mass. They are just tiny packages of energy that act like particles of matter.

Two sets of laws are used to describe light. One set is based on the idea that light is a wave. Those laws are used when they work. The second set is based on the idea that light consists of particles. Those laws are also used when they work.

The philosophy of using wave or particle explanations for light is an example of duality. The term duality means that some natural phenomenon can be understood in two very different ways. Interestingly enough, other forms of duality have been discovered. For example, scientists have traditionally thought of electrons as a form of matter. They have mass and charge, which are characteristics of matter. But it happens that some properties of electrons can best be explained if they are thought of as waves. So, like light, electrons also have a dual character.

Čerenkov Effect

The Čerenkov effect (pronounced che-REN-kof) is the emission of light from something transparent when a charged particle travels through the material with a speed faster than the speed of light in that material. The effect is named for Russian physicist Pavel A. Čerenkov (19041990), who first observed it in 1934.

Many people have seen the Čerenkov effect without realizing it. In photographs of a nuclear power plant, the water surrounding the reactor core often seems to glow with an eery blue light. That light is Čerenkov radiation produced when rapidly moving particles produced in the core travel through the cooling water around it.

The definition of the Čerenkov effect often puzzles students because it includes references to charged particles traveling faster than the speed of light. Of course, nothing can travel faster than the speed of light in a vacuum. In a fluid such as air, water, plastic, or glass, however, it is possible for objects to travel faster than the speed of light. When they do so, they produce the bluish glow seen in a nuclear reactor.

Making light stand still

In January 2001, scientists at two separate laboratories in Cambridge, Massachusetts, conducted landmark experiments in which they brought light particles to a halt and then sped them back up to their normal speed. In the experiments, the scientists created chambers that held a gas. One research team used sodium gas, the other used the gas form of rubidium, an alkaline metal element. The gases in both chambers were chilled magnetically to within a few millionths of a degree of absolute zero, or 459°F (273°C). The scientists passed a light beam into the specially prepared chambers, and the light became fainter and fainter as it slowed and then eventually stopped. Even thought the light vanished, the information on its particles was still imprinted on the atoms of sodium and rubidium. That information was basically frozen or stored. The scientists then flashed a second light through the gas, which essentially reconstituted or revived the original beam. The light left each of the chambers with almost the same shape, intensity, and other properties it had when it entered the chambers.

Scientists believe the biggest impact of these experiments could come in futuristic technologies such as ultra-fast quantum computers. The light could be made to carry so-called quantum information, which involves particles that can exist in many places or states at once. Computers employing such technology could run through operations vastly faster than existing machines.

[See also Electromagnetic spectrum; Interference; Photoelectric effect; Wave motion ]

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light, visible electromagnetic radiation. Of the entire electromagnetic spectrum, the human eye is sensitive to only a tiny part, the part that is called light. The wavelengths of visible light range from about 350 or 400 nm to about 750 or 800 nm. The term "light" is often extended to adjacent wavelength ranges that the eye cannot detect—to infrared radiation, which has a frequency less than that of visible light, and to ultraviolet radiation and black light, which have a frequency greater than that of visible light.

If white light, which contains all visible wavelengths, is separated, or dispersed, into a spectrum, each wavelength is seen to correspond to a different color. Light that is all of the same wavelength and phase (all the waves are in step with one another) is called "coherent" ; one of the most important modern applications of light has been the development of a source of coherent light—the laser.

The Nature of Light

The scientific study of the behavior of light is called optics and covers reflection of light by a mirror or other object, refraction by a lens or prism, diffraction of light as it passes by the edge of an opaque object, and interference patterns resulting from diffraction. Also studied is the polarization of light. Any successful theory of the nature of light must be able to explain these and other optical phenomena.

The Wave, Particle, and Electromagnetic Theories of Light

The earliest scientific theories of the nature of light were proposed around the end of the 17th cent. In 1690, Christian Huygens proposed a theory that explained light as a wave phenomenon. However, a rival theory was offered by Sir Isaac Newton in 1704. Newton, who had discovered the visible spectrum in 1666, held that light is composed of tiny particles, or corpuscles, emitted by luminous bodies. By combining this corpuscular theory with his laws of mechanics, he was able to explain many optical phenomena.

For more than 100 years, Newton's corpuscular theory of light was favored over the wave theory, partly because of Newton's great prestige and partly because not enough experimental evidence existed to provide an adequate basis of comparison between the two theories. Finally, important experiments were done on the diffraction and interference of light by Thomas Young (1801) and A. J. Fresnel (1814–15) that could only be interpreted in terms of the wave theory. The polarization of light was still another phenomenon that could only be explained by the wave theory. Thus, in the 19th cent. the wave theory became the dominant theory of the nature of light.

The wave theory received additional support from the electromagnetic theory of James Clerk Maxwell (1864), who showed that electric and magnetic fields were propagated together and that their speed was identical with the speed of light. It thus became clear that visible light is a form of electromagnetic radiation, constituting only a small part of the electromagnetic spectrum. Maxwell's theory was confirmed experimentally with the discovery of radio waves by Heinrich Hertz in 1886.

Modern Theory of the Nature of Light

With the acceptance of the electromagnetic theory of light, only two general problems remained. One of these was that of the luminiferous ether, a hypothetical medium suggested as the carrier of light waves, just as air or water carries sound waves. The ether was assumed to have some very unusual properties, e.g., being massless but having high elasticity. A number of experiments performed to give evidence of the ether, most notably by A. A. Michelson in 1881 and by Michelson and E. W. Morley in 1887, failed to support the ether hypothesis. With the publication of the special theory of relativity in 1905 by Albert Einstein, the ether was shown to be unnecessary to the electromagnetic theory.

The second main problem, and the more serious of the two, was the explanation of various phenomena, such as the photoelectric effect, that involved the interaction of light with matter. Again the solution to the problem was proposed by Einstein, also in 1905. Einstein extended the quantum theory of thermal radiation proposed by Max Planck in 1900 to cover not only vibrations of the source of radiation but also vibrations of the radiation itself. He thus suggested that light, and other forms of electromagnetic radiation as well, travel as tiny bundles of energy called light quanta, or photons. The energy of each photon is directly proportional to its frequency.

With the development of the quantum theory of atomic and molecular structure by Niels Bohr and others, it became apparent that light and other forms of electromagnetic radiation are emitted and absorbed in connection with energy transitions of the particles of the substance radiating or absorbing the light. In these processes, the quantum, or particle, nature of light is more important than its wave nature. When the transmission of light is under consideration, however, the wave nature dominates over the particle nature. In 1924, Louis de Broglie showed that an analogous picture holds for particle behavior, with moving particles having certain wavelike properties that govern their motion, so that there exists a complementarity between particles and waves known as particle-wave duality (see also complementarity principle). The quantum theory of light has successfully explained all aspects of the behavior of light.

The Speed of Light

An important question in the history of the study of light has been the determination of its speed and of the relationship of this speed to other physical phenomena. At one time it was thought that light travels with infinite speed—i.e., it is propagated instantaneously from its source to an observer. Olaus Rømer showed that it was finite, however, and in 1675 estimated its value from differences in the time of eclipse of certain of Jupiter's satellites when observed from different points in the earth's orbit. More accurate measurements were made during the 19th cent. by A. H. L. Fizeau (1849), using a toothed wheel to interrupt the light, and by J. B. L. Foucault (1850), using a rotating mirror. The most accurate measurements of this type were made by Michelson. Modern electronic methods have improved this accuracy, yielding a value of 2.99792458 × 108 m (c.186,000 mi) per sec for the speed of light in a vacuum, and less for its speed in other media. The theory of relativity predicts that the speed of light in a vacuum is the limiting velocity for material particles; no particle can be accelerated from rest to the speed of light, although it may approach it very closely. Particles moving at less than the speed of light in a vacuum but greater than that of light in some other medium will emit a faint blue light known as Cherenkov radiation when they pass through the other medium. This phenomenon has been used in various applications involving elementary particles.

Luminous and Illuminated Bodies

In general, vision is due to the stimulation of the optic nerves in the eye by light either directly from its source or indirectly after reflection from other objects. A luminous body, such as the sun, another star, or a light bulb, is thus distinguished from an illuminated body, such as the moon and most of the other objects one sees. The amount and type of light given off by a luminous body or reflected by an illuminated body is of concern to the branch of physics known as photometry (see also lighting). Illuminated bodies not only reflect light but sometimes also transmit it. Transparent objects, such as glass, air, and some liquids, allow light to pass through them. Translucent objects, such as tissue paper and certain types of glass, also allow light to pass through them but diffuse (scatter) it in the process, so that an observer cannot see a clear image of whatever lies on the other side of the object. Opaque objects do not allow light to pass through them at all. Some transparent and translucent objects allow only light of certain wavelengths to pass through them and thus appear colored. The colors of opaque objects are caused by selective reflection of certain wavelengths and absorption of others.


See W. L. Bragg, The Universe of Light (1959); J. Rublowsky, Light (1964); H. Haken, Light (1981).

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light1 / līt/ • n. 1. the natural agent that stimulates sight and makes things visible: the light of the sun [in sing.] the street lamps shed a faint light into the room. ∎  a source of illumination, esp. an electric lamp: a light came on in his room. ∎  (lights) decorative illuminations: Christmas lights. ∎  a traffic light: turn right at the light. ∎  [in sing.] an expression in someone's eyes indicating a particular emotion or mood: a shrewd light entered his eyes. ∎  the amount or quality of light in a place: the plant requires good light in some lights she could look beautiful. 2. understanding of a problem or mystery; enlightenment: she saw light dawn on the woman's face. ∎  spiritual illumination by divine truth. ∎  (lights) a person's opinions, standards, and abilities: leaving the police to do the job according to their lights. 3. an area of something that is brighter or paler than its surroundings: sunshine will brighten the natural lights in your hair. 4. a match or lighter that produces a flame or spark. ∎  the flame produced: he asked me for a light. 5. a window or opening in a wall to let light in. ∎  any of the perpendicular divisions of a mullioned window. ∎  any of the panes of glass forming the roof or side of a greenhouse or the top of a cold frame. 6. a person notable or eminent in a particular sphere of activity or place: such lights of Liberalism as the historian Goldwin Smith. • v. (past and past part. lit / lit/ or light·ed ) [tr.] 1. provide with light or lighting; illuminate: the room was lighted by a number of small lamps lightning suddenly lit up the house. ∎  switch on (an electric light): only one of the table lamps was lit. ∎  [intr.] (light up) become illuminated: the sign to fasten seat belts lit up. 2. make (something) start burning; ignite: Allen gathered sticks and lit a fire [as adj.] (lighted or lit) a lighted cigarette. ∎  [intr.] begin to burn; be ignited: the gas wouldn't light properly. ∎  (light something up) ignite a cigarette, cigar, or pipe and begin to smoke it: she lit up a cigarette and puffed on it serenely [intr.] workers who light up in prohibited areas face dismissal. • adj. 1. having a considerable or sufficient amount of natural light; not dark: the bedrooms are light and airy it was almost light outside. 2. (of a color) pale: her eyes were light blue. PHRASES: bring (or come) to light make (or become) widely known or evident: an investigation to bring to light examples of extravagant expenditure. go out like a light inf. fall asleep or lose consciousness suddenly. in a —— light in the way specified; so as to give a specified impression: the audit portrayed the company in a very favorable (the) light of drawing knowledge or information from; taking (something) into consideration: the exorbitant prices are explainable in the light of the facts. light a fire under someonesee fire. light at the end of the tunnel a long-awaited indication that a period of hardship or adversity is nearing an end. light the fusesee fuse2 . the light of day daylight. ∎  general public attention: bringing old family secrets into the light of day. the light of someone's life a much loved person. lights out bedtime in a school dormitory, military barracks, or other institution, when lights should be switched off. ∎  a bell, bugle call, or other signal announcing this. lit up inf., dated drunk. see the light understand or realize something after prolonged thought or doubt. ∎  undergo religious conversion. see the light of day be born. ∎ fig. come into existence; be made public, visible, or available: this software first saw the light of day back in 1993. shed (or throw or cast) light on help to explain (something) by providing further information about it.PHRASAL VERBS: light up (or light something up) (with reference to a person's face or eyes) suddenly become or cause to be animated with liveliness or joy: his eyes lit up and he smiled a smile of delight lit up her face.DERIVATIVES: light·ish adj. light·less adj. light·ness n. ORIGIN: Old English lēoht, līht (noun and adjective), līhtan (verb), of Germanic origin; related to Dutch licht and German Licht, from an Indo-European root shared by Greek leukos ‘white’ and Latin lux ‘light.’ light2 • adj. 1. of little weight; easy to lift: they are very light and portable you're as light as a feather. ∎  deficient in weight, esp. by a specified amount: the sack of potatoes is 5 pounds light. ∎  not strongly or heavily built or constructed; small of its kind: light, impractical clothes light armor. ∎  carrying or suitable for small loads: light commercial vehicles. ∎  carrying only light armaments: light infantry. ∎  (of a vehicle, ship, or aircraft) traveling unladen or with less than a full load. ∎  (of food or a meal) small in quantity and easy to digest: a light supper. ∎  (of a foodstuff) low in fat, cholesterol, sugar, or other rich ingredients: stick to a light diet. ∎  (of drink) not too sweet or rich in flavor or strongly alcoholic: a glass of light Hungarian wine. ∎  (of food, esp. pastry or sponge cake) fluffy or well aerated during cooking. ∎  (of soil) friable, porous, and workable. ∎  (of an isotope) having not more than the usual mass; (of a compound) containing such an isotope. 2. relatively low in density, amount, or intensity: passenger traffic was light light summer breezes trading was light for most of the day. ∎  (of sleep or a sleeper) easily disturbed. ∎  easily borne or done: he received a relatively light sentence some light housework. 3. gentle or delicate: she planted a light kiss on his cheek my breathing was steady and light. ∎  (of a building) having an appearance suggestive of lightness: the building is lofty and light in its tall nave and choir. ∎  (of type) having thin strokes; not bold. 4. (of entertainment) requiring little mental effort; not profound or serious: pop is thought of as light entertainment some light reading. ∎  not serious or solemn: his tone was light. ∎  free from worry or unhappiness; cheerful: I left the island with a light heart. 5. archaic (of a woman) unchaste; promiscuous. PHRASES: be light on be rather short of: light on hard news. be light on one's feet (of a person) be quick or nimble. a (or someone's) light touch the ability to deal with something delicately, tactfully, or in an understated way: a novel that handles its tricky subject with a light touch. make light of treat as unimportant: I didn't mean to make light of your problems. make light work of accomplish (a task) quickly and easily. travel light travel with a minimum load or minimum luggage.DERIVATIVES: light·ish adj. light·ly adv. light·ness n. light3 • v. (past and past part. lit / lit/ or light·ed ) [intr.] 1. (light on/upon) come upon or discover by chance: he lit on a possible solution. 2. archaic descend: from the horse he lit down. ∎  (light on) fall and settle or land on (a surface): a feather just lighted on the ground. PHRASAL VERBS: light into inf. criticize severely; attack: he lit into him for his indiscretion. light out inf. depart hurriedly.

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Light is a form of electromagnetic radiation. Other types of electromagnetic radiation include radio waves, microwaves, infrared, ultraviolet, x-rays, and gamma rays. All electromagnetic waves possess energy. Moreover, electro-magnetic waves (including light) are produced by accelerated electric charges (such as electrons). Light moves through space in a wave that has an electric part and a magnetic part. That is why it is called an electromagnetic wave.

Speed of Light

Light travels through empty space at a high speed, very close to 300,000 kilometers per second (km/s). This number is a universal constant: it never changes. Since all measurements of the speed of light in a vacuum always produce exactly the same answer, the distance light travels in a certain amount of time is now defined as the standard unit of length. For convenience, the speed of light is usually written as the symbol, c.

Characteristics of Waves

All waves, including light waves, share certain characteristics: They travel through space at a certain speed, they have frequency, and they have wavelength. The frequency of a wave is the number of waves that pass a point in one second. The wavelength of a wave is the distance between any two corresponding points on the wave. There is a simple mathematical relationship between these three quantities called the wave equation. If the frequency is denoted by the symbol f and the wavelength is denoted by the symbol λ, then the wave equation for electromagnetic waves is:

c = f λ

Since c is a constant, this equation requires that a light wave with a shorter wavelength have a higher frequency.

Waves also have amplitude. Amplitude is the "height" of the wave, or how "big" the wave is. The amplitude of a light wave determines how bright the light is.

Wavelength and Color

There is a simple way to remember the order of wavelengths of light from longest to shortest: ROY G. BIV. The letters stand for red, orange, yellow, green, blue, indigo, and violet. (This violet is not the same as the crayon color called violet, which is a shade of purple.) The human eye perceives different wavelengths of light as different colors. Red is the color of the longest wavelength the human eye can detect; violet is the shortest. Red light has a wavelength of around 700 nanometers (nm). (A nanometer is one-billionth of a meter.) Light with a wavelength longer than 700 nm is called infrared. ("Infra" means "below.") Violet light is around 400 nm. Electromagnetic radiation with a shorter wavelength is called ultraviolet. ("Ultra" means "beyond.") It is best not to use the terms "ultraviolet light" or "infrared light," for instance, because the word "light" should be applied only to wavelengths that the human eye can detect.

Refraction and Lenses

What happens when light encounters matter depends on the type of material. Glass, water, quartz, and other similar materials are transparent. Light passes through them. However, light slows down as it passes through a transparent material. This happens because the light is absorbed and reemitted by the atoms of the material. It takes a small amount of time for the atom to reemit the light, so the light slows down. In water, light travels around0.75c or 225,000 km/s. In glass, the speed is even slower, 0.67c. In diamond, light travels at less than half its speed in vacuum, 0.41c.

When a beam of light passes from vacuum (or air) into glass, it slows down, but if the beam hits the glass at an angle, it does not all slow down at the same time. The edge of the beam that hits the glass first slows down first. This causes the beam to bend as it enters the glass. The change in direction of any wave as it passes from one material to another and speeds up or slows down is called refraction . Refraction causes water to appear to be shallower than it is in reality. Refraction causes a diamond to sparkle.

Refraction is also what creates a mirage. Sometimes the air a few centimeters above the ground is much warmer than air a few meters farther up. As light from the sky passes into this warmer air, it speeds up and bends away from the ground. An observer may see light from the sky and be fooled into thinking that it is a lake. Sometimes, even trees and houses can be seen in the mirage, but they will appear upside down.

Refraction of light allows a lens to perform its function. In a converging lens, the center of the beam reaches the lens first and slows down first. This causes the beam to be bent toward the center of the lens. A parallel beam of light passing through a good-quality lens will be bent so that all the light arrives at a single point called the focal point. The distance from the lens to the focal point is called the focal length, f. A diverging lens spreads the beam out so that it appears to be coming from the focal point.

In a slide projector, the lens projects an image of an object (the slide) onto a screen. The distance from the lens to the image and the distance of the lens to the object are related to the focal length by this strange-looking formula (d i is the image distance and d o is the object distance):

Interpreting this formula is a little difficult. Remember that the focal length of the lens does not change, so is a constant. If the image distance (d i) gets larger, the object distance (d o) must get smaller to make the fractions add to the same constant value.

Reflection and Mirrors

When light hits a surface, it can also be reflected . Sometimes light is both refracted and reflected. If the object is opaque, however, the light will just be reflected. When light is reflected from a surface, it bounces off at the same angle to the surface. The angle of incidence is equal to the angle of reflection.

see also Vision, measurement of.

Elliot Richmond


Epstein, Lewis Carroll. Thinking Physics. San Francisco: Insight Press, 1990.

Giancoli, Douglas C. Physics, 3rd ed. Englewood Cliffs, NJ: Prentice Hall, 1991.

Haber-Schaim, Uri, John A. Dodge, and James A. Walter. PSSC Physics, 7th ed. Dubuque, IA: Kendall/Hunt, 1990.

Hewitt, Paul G. Conceptual Physics. Menlo Park, CA: Addison Wesley, 1992.

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245. Light

See also 110. DARKNESS ; 387. SUN

the study of the chemical effects of light in the violet and ultraviolet wavelengths. actinologic, actinological , adj.
the measurement of the heating power of light in the violet and ultraviolet range. actinometrist , n. actinometric, actinometrical , adj.
the ratio between the light reflected from a surf ace and the total light falling upon that surf ace, as the albedo of the moon.
double refraction; the separation of light into two unequally refracted, polarized rays, as by some crystals. birefringent , adj.
the study of the reflection and refraction of light. catadioptric, catadioptrical , adj.
the study of light reflection. catoptric, catoptrical , adj. catoptrically , adv.
the condition or quality of changing in color or luster depending on the angle of light, especially of a gemstone that reflects a single shaft of light when cut in cabochon form. chatoyant , adj.
a property, peculiar to certain crystals, of reflecting light in two different colors when viewed from two different directions. dichroic , adj.
the study of light refraction. dioptric , adj.
the state or condition of being colored like a rainbow or like the light shining through a prism. iridescent , adj.
a polished black glass, the surface of which becomes iridescent when it is breathed upon through a tube.
rotation toward the left; counterclockwise rotation, a characteristic of the plane of polarization of light. levorotatory , adj.
the process of impressing porcelain objects, as lamp bases, with figures that become translucent when light is placed within or behind them. lithophanic , adj.
any thing or creature that shines or glows in the dark, especially a phosphorescent or bioluminescent marine or other organism. noctilucine , adj.
the study of the properties of light. Also called photology . optic, optical , adj.
the study of signal lights, especially lighthouses.
an abnormal fear of daylight.
pain in the eyes caused by light.
an abnormal fear of photalgia.
the study of light.
1. an apparatus that regulates light flashes so that a rotating object appears to be stationary or moving in a direction opposite to its actual motion.
2. an apparatus for producing unusual optical effects by flashing light upon disks bearing various figures, patterns, etc.
the science or study of light in relation to the movement of plants. photodynamic, photodynamical , adj.
the process or art of creating and recording images of people, objects, and phenomena, essentially by means of reflected light or emanating radiation. photographer , n. photographic, photographical , adj.
movement of bodies, organisms, etc., in response to the stimulus of light. photokinetic , adj.
the breakdown of matter or materials under the influence of light. photolytic , adj.
an abnormal love of light.
the measurement of the intensity of light. photometrician, photometrist , n. photometric , adj.
a pathologic effect produced by light. photopathic , adj.
the tendency to thrive in strong light, as plants. photophilic , adj.
1. an abnormal fear of light.
2. Also called photodysphoria . a painful sensitivity to light, especially visually.
3. a tendency to thrive in reduced light, as certain plants.
the synthesis of complex organic substances from carbon dioxide, water, and inorganic salts, with sunlight as the energy source and a catalyst such as chlorophyll. photosynthetic , adj.
phototaxis, phototaxy
the movement of an organism away from or toward a source of light. phototactic , adj.
phototherapy, phototherapeutics
the treatment of disease, especially diseases of the skin, with light rays. phototherapeutic , adj.
motion in a particular direction under the stimulus of light, as manifested by certain plants, organisms, etc. phototropic , adj.
the measurement of the polarization of light, as with a polarimeter.
an abnormal fear or dislike of flashes of light.
a photograph of a spectrum. Also called spectrograph .
1. an optical device for breaking light down into a spectrum and recording the results photographically.
2. spectrogram. spectrographic , adj.
the technique of using a spectrograph and producing spectrograms.
a form of Iuminescence created by friction. triboluminescent , adj.

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Light influences fishs activities. A photoperiod is defined as the amount of daylight in a twenty four hour period. It is influenced by the amount of cloud cover on a daily basis. Seasonally, summer months have longer photoperiod days and the sunlights angle is more direct. The fall, winter, and spring months include both shorter daylight periods and lowered sunlight angulations. In addition, there are longer shadows than during the midsummer times. Seasonal variances influence the amount of light entering the water.

Fish are more alert during bright sunlight conditions because they are more visible to animals of prey. The fishs food supplies are most abundant in the shallow littoral zones which are located in areas of more intense light penetration necessary for

photosynthesis. Fish may only feel safe to be in these shallow zones during subdued lighting conditions. This is usually early and late in the day or at times of seasonal low light conditions which occur in the fall, winter, and spring. During midsummer, a fishs presence in the shallows may be restricted to times of dawn, dusk, or overcast days. Consequently fish collect during bright light conditions into the darker areas adjacent to the littoral zones.

With present lighting conditions taken into consideration, select your fishing site accordingly. During the winter, spring, and fall, you will most likely find fish spending more time in the shallows than they do during the summer season. Dark, overcast, and rainy days can draw an abundance of fish into the shallows to feed. Dusk and dawn are also prime times for fishing these shallows. As the light intensity increases, the fish converge into the darker depths of adjacent channels and drop-offs.

Light affects insects activities. Usually they are most dynamic during low light periods. During intense lighting periods insects search out shaded areas deep in protective cover. The evening rise happens

as insects lay their eggs upon the surface at dusk. Overcast days prolong surface feeding because both the insects are more active and the fish are safer feeding in the shallows. With little knowledge of optical physics, the seasonal and dusk/dawn light phenomenon can be explained. Light rays striking the waters surface at a right angle travel through it with little deviation.

The angulated sunlight is less illuminating underwater because some of it is reflected away at the surface as rays hitting the water are bent upwards; thus the net result is diminished light penetrating the aquatic environment.

There are other light physics phenomena such as infrared rays which are elongated and penetrate cloud cover more readily. This red light from the visible spectrum is noticed more by the fish. Adding the color red to a fly improves its effectiveness, especially in baitfish imitations. Red is more visible.

In conclusion, lighting affects both the insects and the fishs activities; its an important factor in finding actively feeding fish.

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light Electromagnetic radiation to which the human eye is sensitive. Visible light is in the wavelength range from c.400nm (violet) to 770nm (red). The speed (symbol c) at which electromagnetic radiation (including light) travels in a vacuum is 299,792,458ms−1. The amount of light in an area is measured in lumens. Light exhibits typical phenomena of wave motion, such as reflection, refraction, diffraction, light polarization, and interference. Sir Isaac Newton investigated the properties of light in the 17th century, and he was the first to split white light into its component colours (spectrum) with a prism. Newton believed in a corpuscular (particle) theory of light, but the wave theory was well established by the 1820s, thanks to the work of English physicist Thomas Young and French physicist Augustin Jean Fresnel. At the beginning of the 20th century, experiments on the photoelectric effect and the work of German physicist Max Planck revived the idea that light can behave like a stream of particles. Albert Einstein stated that this stream must be photons of electromagnetic energy. Quantum theory, according to which light consists of elementary particles called photons, resolved this dilemma. When light interacts with matter, as in the photoelectric effect, energy exchanges in the form of photons and so light seems to be particles. Otherwise, it behaves as a wave. See also holography; laser; relativity

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light1 light at the end of the tunnel a long-awaited indication that a period of hardship or adversity is nearing an end.
the Light of the World Jesus Christ, Lux Mundi; the phrase is biblical, as in John 8:12; it was famously used by the Pre-Raphaelite artist Holman Hunt as the title of his picture (1851–3) of Jesus Christ, illustrating Revelation 3:20.
light year a unit of astronomical distance equivalent to the distance that light travels in one year, which is 9.4607 × 1012 km (nearly 6 million million miles); often used figuratively.
see the light understand or realize something after prolonged thought or doubt.

See also hide one's light under a bushel, in the cold light of day, green light, red-light district, sweetness and light.

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"light." The Oxford Dictionary of Phrase and Fable. . 21 Oct. 2016 <>.

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light1 emanation from the sun, etc.; illumination; lighted body. OE. lēoht (Angl. līht) = OS., OHG. lioht (Du., G. licht) :- WGmc. *leuχta :- IE. *leuktom f. *leuk *louk *lŭk-, repr. in Gr. leukós white, L. lūx, lūmen (:- *leuksmen light, lŭna (:- *leuksnā) moon, OIr. luach shining, ON. logi flame, OSl. luc̆a beam, Skr. ruc shine.
So light adj. OE. lēoht, līht = OS. (Du.), (O)HG. licht. light vb. OE. līhtan = OS. liuhtian, etc., Goth. liuhtjan, largely superseded by lighten1 XIII. comp. lighthouse XVII.

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"light." The Concise Oxford Dictionary of English Etymology. . 21 Oct. 2016 <>.

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light (or lite) As applied to foods usually indicates: (1)a lower content of fat compared with the standard product (e.g. breadspreads, sausages);(2)Sodium chloride substitutes lower in sodium (see salt, light);(3)Low‐alcohol beer or wine. US legislation restricts the term ‘light’ to modified foods that contain one‐third less energy or half the fat of a reference unmodified food, or to indicate that the sodium content of a low‐fat, low‐calorie food has been reduced by half. See also fat‐free; free from; low in; reduced.

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light. Adjective applied somewhat patronizingly and vaguely to mus. which is supposed to need less concentration than ‘serious music’ (another objectionable term). Thus there are also ‘light’ orchs. and ‘light’ opera. ‘Light’ mus. can refer to Elgar's shorter pieces or to works by composers such as Ronald Binge. ‘Light opera’ probably means Merrie England rather than The Merry Widow, but such classification is imprecise and unhelpful.

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408. Light

  1. Apollo god of light. [Gk. Myth.: Espy, 28]
  2. Asvins twin gods of light. [Hindu Myth.: Bent, 60]
  3. Balder god of light and peace. [Norse Myth.: Leach, 106]
  4. Jesus Christ I am the light of the world. [N.T.: John 8:12]
  5. Mithras god of light. [Pers. Myth.: Wheeler, 246]
  6. patée cross four spear-headed arms; symbolizes solar light. [Christian Iconog.: Brewer Dictionary, 280; Jobes, 386]

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"Light." Allusions--Cultural, Literary, Biblical, and Historical: A Thematic Dictionary. . 21 Oct. 2016 <>.

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light2 of little weight. OE. lēoht, līht = OS. -līht (Du. licht), OHG. līht(i) (G. leicht easy), ON. léttr, Goth. leihts :- Gmc. *liŋχt(j)az, f. *liŋgw :- IE. *leng h, as in Lith. len̄gvas light.
Hence lighten2 XV.

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light2 light come, light go proverbial saying, late 14th century, meaning that something gained without effort can be lost without much regret; a less common variant of easy come, easy go.

See also many hands make light work.

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light. Aperture (called day) through which daylight passes, e.g. a pane of glass, an area around which are mullions or transoms, or an opening defined by tracery-bars.

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"light." A Dictionary of Architecture and Landscape Architecture. . 21 Oct. 2016 <>.

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lights lungs (now of slaughtered beasts). XII. ME. lihte, pl. of LIGHT2 used sb.; cf. LUNG.

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lights Butchers' term for the lungs of an animal.

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Light: see NŪR.

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"Light." The Concise Oxford Dictionary of World Religions. . 21 Oct. 2016 <>.

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lightaffright, alight, alright, aright, bedight, bight, bite, blight, bright, byte, cite, dight, Dwight, excite, fight, flight, fright, goodnight, height, ignite, impolite, indict, indite, invite, kite, knight, light, lite, might, mite, night, nite, outfight, outright, plight, polite, quite, right, rite, shite, sight, site, skintight, skite, sleight, slight, smite, Snow-white, spite, sprite, tight, tonight, trite, twite, underwrite, unite, uptight, white, wight, wright, write •Shiite • Trotskyite • McCarthyite •Vishnuite • Sivaite • albite •snakebite • frostbite • soundbite •kilobyte • columbite • love bite •Moabite • megabyte • gigabyte •Jacobite • Rechabite • jadeite •lyddite • expedite • cordite • erudite •Luddite • recondite • troglodyte •hermaphrodite • extradite

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