Light

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 (1642–1727). 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 (1629–1695).

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 material—perhaps like air, but much less dense—that 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 (1852–1931) and Edward W. Morley (1838–1923), 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 (1857–1894) 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 (1879–1955). 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 earlier—and 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 (1904–1990), 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 ]

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 × 10 ⁸ 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.

Bibliography

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

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 light.in (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.

Light

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

Bibliography

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.

245. Light

See also 110. DARKNESS ; 387. SUN

actinology

the study of the chemical effects of light in the violet and ultraviolet wavelengths. —actinologic, actinological , adj.

actinometry

the measurement of the heating power of light in the violet and ultraviolet range. —actinometrist , n. —actinometric, actinometrical , adj.

albedo

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.

birefringence

double refraction; the separation of light into two unequally refracted, polarized rays, as by some crystals. —birefringent , adj.

catadioptrics

the study of the reflection and refraction of light. —catadioptric, catadioptrical , adj.

catoptrics

the study of light reflection. —catoptric, catoptrical , adj. —catoptrically , adv.

chatoyancy

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.

dichroism

a property, peculiar to certain crystals, of reflecting light in two different colors when viewed from two different directions. —dichroic , adj.

dioptrics

the study of light refraction. —dioptric , adj.

iridescence

the state or condition of being colored like a rainbow or like the light shining through a prism. —iridescent , adj.

iriscope

a polished black glass, the surface of which becomes iridescent when it is breathed upon through a tube.

levorotation

rotation toward the left; counterclockwise rotation, a characteristic of the plane of polarization of light. —levorotatory , adj.

lithophany

the process of impressing porcelain objects, as lamp bases, with figures that become translucent when light is placed within or behind them. —lithophanic , adj.

noctiluca

any thing or creature that shines or glows in the dark, especially a phosphorescent or bioluminescent marine or other organism. —noctilucine , adj.

opties

the study of the properties of light. Also called photology . —optic, optical , adj.

pharology

the study of signal lights, especially lighthouses.

phengophobia

an abnormal fear of daylight.

photalgia

pain in the eyes caused by light.

photangiophobia

an abnormal fear of photalgia.

photics

the study of light.

photodrome

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.

photodynamics

the science or study of light in relation to the movement of plants. —photodynamic, photodynamical , adj.

photography

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.

photokinesis

movement of bodies, organisms, etc., in response to the stimulus of light. —photokinetic , adj.

photology

optics.

photolysis

the breakdown of matter or materials under the influence of light. —photolytic , adj.

photomania

an abnormal love of light.

photometry

the measurement of the intensity of light. —photometrician, photometrist , n. —photometric , adj.

photopathy

a pathologic effect produced by light. —photopathic , adj.

photophily

the tendency to thrive in strong light, as plants. —photophilic , adj.

photophobia

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.

photosynthesis

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.

phototropism

motion in a particular direction under the stimulus of light, as manifested by certain plants, organisms, etc. —phototropic , adj.

polarimetry

the measurement of the polarization of light, as with a polarimeter.

selaphobia

an abnormal fear or dislike of flashes of light.

spectrogram

a photograph of a spectrum. Also called spectrograph .

spectrograph

1. an optical device for breaking light down into a spectrum and recording the results photographically.

2. spectrogram. —spectrographic , adj.

spectrography

the technique of using a spectrograph and producing spectrograms.

triboluminescence

a form of Iuminescence created by friction. —triboluminescent , adj.

Light

Light influences fish’s 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 sunlight’s 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 fish’s 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 fish’s 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 insect’s 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 water’s 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 insect’s and the fish’s activities; it’s an important factor in finding actively feeding fish.

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

light A powerful symbol mentioned at the beginning (Gen. 1: 3) and the end (Rev. 22: 5) of the Bible for goodness and truth. Hence the Law is described as ‘a light for my path’ (Ps. 119: 105); ‘the day of the Lord’ was expected to be light (Amos 5: 18), and people were disconcerted when the prophet predicted darkness. Light and darkness are commonly encountered as antitheses in the Dead Sea scrolls, and this is a feature which they share with the gospel of John; Christ is described as the light of the world (John 8: 12), sent by God who himself is light (1 John 1: 5), and those who are sent by Christ become in turn the light of the world (Matt. 5: 14).

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.

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.

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.

light Electromagnetic radiation that can be seen by the human eye. It lies between the ultraviolet and infrared regions of the electromagnetic spectrum. Different wavelengths of light appear as different colours. Visible radiation ranges from wavelengths of about 750 nm at the red (long-wavelength) end to around 380 nm at the violet (short-wavelength) end.

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.

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.

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.

light adj.
1. carrying only light armaments: light infantry.

2. (of a vehicle, ship, or aircraft) traveling unladen or with less than a full load.

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

lights Butchers' term for the lungs of an animal.

Light: see NŪR.

light •affright, 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