Light
From Encyclopædia
[edit] CHARACTERIZATION OF LIGHT
Light is characterized not only by wavelength, essentially a temporal quality, but also by state and degree of polarization (see POLARIZED LIGHT), a geometric or directional quality, and by intensity, essentially a physical quality. The visual response to intensity is brightness. In the human visual system, at least, there is no counterpart response, to the state and degree of polarization, but ample evidence exists that certain arthropods--bees in particular--are sensitive to the state of polarization of sky light. There is some speculation that certain migrating birds may also respond to this quality of light.Light is further characterized by its degree of coherence (see COHERENT LIGHT). Coherence, closely related to the degree of polarization and to the degree of monochromaticity, refers to the ability of a beam of light to interfere (see INTERFERENCE) with itself. Coherence is therefore an interferometric property of light. By the use of a Michelson interferometer, most light sources can be made to produce interference fringes. These are clearest when the length of the two arms of the interferometer are equal. As one arm is lengthened, however, the contrast of the fringes is seen to decrease until they are no longer visible. Unfiltered light from an incandescent source will barely produce fringes under any circumstances. Light from a mercury arc lamp will produce fringes over a range of one or two centimeters. On the other hand, light from a continuous-wave gas laser has produced fringes at a distance of over 100 meters. Therefore, light can be characterized by its degree of coherence or coherence length.Light is a transport of energy. It can be regarded both as a particulate flow and as a wave phenomenon. These two apparently diametrically opposed views have been brought together in a theory that combines the best features of each. The particulate unit is the PHOTON, which has associated with it a Central frequency or wavelength that determines (or is determined by) the amount of energy it contains. In a so-called monochromatic beam, the photons are all of the same energy and therefore have the same frequency. They can be made to interfere, which indicates a high degree of coherence as well as a more or less uniform state of polarization. If the distribution of the energy in the photons is more random, however, the beam will be less coherent and will have a lower degree of polarization.It is also convenient to think of light as propagating as wavefronts (see HUYGENS's PRINCIPLE). These waves, like the crest of an ocean wave, are surfaces on which the phase relationship is constant. Unlike an ocean wave, a wavefront or surface of constant phase is unobservable and undetectable. Light may be considered as energy being transported in a train of wavefronts. The direction of propagation (except for anisotropic media) is in a direction perpendicular to the wavefront. Rays can be conceived as trajectories of photons.
[edit] LIGHT PRODUCTION
Light, like any other electromagnetic radiation, results from either an accelerating electric charge or a nuclear fusion or fission reaction. In nuclear reactions, a photon is created in the same manner as other elemental partial products of the reaction. With the exception of sunlight and starlight, however, light usually is the result of changes in the electronic structure of atoms and molecules as they absorb and readmit energy.The incandescent electric light has as its light source the heat that results from the ohmic resistance of the filament to the electric current. A red-hot poker absorbs heat directly from the fire resulting from the liberation of chemical energy. As the material in the filament or poker heats up, the atoms and molecules gain kinetic energy, which is realized by an increase in the number of collisions among the particles. Boiling off of some of the material is one mechanism that can be used to maintain an equilibrium temperature. Another mechanism is for the electrons associated with the various atoms in the metal to move to higher ENERGY LEVELS. When they drop back to lower energy levels they emit a photon, keeping the temperature of the material more or less constant despite the fact that energy is continually supplied. The excess energy is emitted as light.Thermal production of light is essentially random and is idealized as BLACKBODY radiation. The light produced contains a mixture of wavelengths skewed around a Central maximum, which is related to the temperature T of the material in degrees Kelvin. This relation, is known as the Wien displacement law. The spectrum produced by the light from such a source is continuous. Although there is a dominant wavelength, this light is not monochromatic. It is generally unpolarized and has a relatively short coherence length.Another type of light source is energized plasma such as a flame or the gas in a discharge tube such as a neon bulb. Although light is produced by a mechanism similar to thermal emission, the atoms are in a gaseous phase and less random. The energy levels reached by the electrons depend more on the electronic structure of the atoms themselves, and therefore the photons emitted tend to be clustered around specific wavelengths. The spectrum produced by such a source is not at all continuous but consists of lines or bands that are characteristic of the atoms or molecules in the gas. Highly monochromatic light can be obtained from this type of source, particularly if the light is filtered. The light has a much longer coherence length but is generally unpolarized.A third type of source is the laser. Two principles are involved in laser operation. First, the lasing material is composed of atoms, or mixtures of atoms, that have a peculiar energy level structure. As they absorb energy, their electrons move up to higher energy levels, tending to accumulate at certain metastable levels. This is called population inversion. There they remain until stimulated by a photon of the proper frequency. Then the electrons drop to a lower energy level, emitting a photon of the same frequency and traveling in the same direction as the incident, stimulating photon. Because a single photon may stimulate the release of a large number of additional photons, the total number of photons is increased, thus increasing the intensity of the light within the medium. The process is referred to as gain.The second principle is the geometry of the laser itself. The laser can be regarded as a hollow tube, much like an organ pipe, which is tuned to the wavelength of the emitted photons. The process can be visualized as a wavefront being reflected back and forth between the two ends of the laser, picking up more photons with each reflection. The portion of the light that is permitted to escape from the cavity is highly monochromatic, with a long coherence length. In some circumstances the laser output is highly polarized.
[edit] DUALISTIC NATURE OF LIGHT
The historical development of a theory of light, at least from the 17th century on, involved two apparently contradictory descriptions. One concept was the corpuscular theory, which envisioned light as a stream, or flow, of small particles. Rene DESCARTES modified this concept. He viewed light more as a pressure than as a flow--not as motion but as a tendency to motion. And since light was not motion it was not limited by a finite velocity. In other words, a beam of light required no time of transit. Pierre FERMAT held a different view. He believed not only that light propagated at a finite velocity, but also that its particles described trajectories or rays. Christiaan HUYGENS, on the other hand, was a believer that light was a wave phenomenon. Light propagated at a finite velocity in the form of a moving disturbance, just as a water wave moves as a ripple on a smooth pond.As a ray of light passes across a surface from one medium to another (for example, from air to glass), its direction is changed--a phenomenon known as REFRACTION. The law of refraction, discovered first empirically by Willebrord SNELL, then subsequently derived formally by Descartes and Fermat, states that sin r = K sin i, where i is called the angle of incidence, the angle between the incident ray and the normal (perpendicular) to the refracting surface. The angle of refraction, r, is the angle between the refracted ray and the surface normal.Fermat and Descartes agreed on the form of the refraction law, but they disagreed violently on the meaning of the constant K. Fermat saw K as being proportional to the reciprocal of the velocity of propagation. Descartes, even though he believed that the velocity of propagation was infinite, concluded, on a different level of logic, that K was proportional to a velocity. The distinction is important because whether light speeds up or slows Down as it passes into a denser medium determines the meaning of K.