[align=left][align=justify][align=right][align=justify]The simplicity of a laser can be understood by considering the light from a candle. Normally, a burning candle radiates light in all directions, and therefore illuminates various objects equally if they are equidistant from the candle. A laser takes light that would normally be emitted in all directions, such as from a candle, and concentrates that light into a single direction. Thus, if the light radiating in all directions from a candle were concentrated into a single beam of the diameter of the pupil of your eye (approximately 3 mm), and if you were standing a distance of 1m from the candle, then the light intensity would be 1,000,000 times as bright as the light that you normally see radiating from the candle! That is essentially the underlying concept of the operation of a laser. However, a candle is not the kind of medium that produces amplification, and thus there are no candle lasers. It takes relatively special conditions within the laser medium for amplification to occur, but it is that capability of taking light that would normally radiate from a source in all directions - and concentrating that light into a beam traveling in a single direction - that is involved in making a laser. The beam of light generated by a typical laser can have many properties that are unique. When comparing laser properties to those of other light sources, it can be readily recognized that the values of various parameters for laser light either greatly exceed or are much more restrictive than the values for many common light sources. We never use lasers for street illumination, or for illumination within our houses. We don’t use them for searchlights or flashlights or as headlights in our cars. Lasers generally have a narrower frequency distribution, or much higher intensity, or a much greater degree of collimation, or much shorter pulse duration, than that available from more common types of light sources. Therefore, we do use them in compact disc players, in supermarket check-out scanners, in surveying instruments, and in medical applications as a surgical knife or for welding detached retinas. We also use them in communications systems and in radar and military targeting applications, as well as many other areas. A laser is a specialized light source that should be used only when its unique properties are required.
MONOCHROMATICITY
All light consists of waves traveling through space. The color of the light is determined by the length of those waves. White light consists of a mixture of many different wavelengths. A prism can be used to disperse white light into its component wavelengths (colors). All common light sources emit light of many different wavelengths. White light contains all, or most, of the colors of the visible spectrum. Ordinary colored light consists of a broad range of wavelengths covering a particular portion of the visible-light spectrum. (Wavelength is the distance over which the wave repeats itself and is represented by the Greek letter l (lambda). Each color of visible light has its own characteristic wavelength).
Perfectly monochromatic light cannot be produced even by a laser, but laser light is many times more monochromatic than the light from any other source. In some applications, special techniques are employed to further narrow the range of wavelengths contained in the laser output and, thus, to increase the monochromaticity.
DIRECTIONALITY
All conventional light sources emit light in all directions(such as the light that emitted from a light bulb). "Directionality" is the characteristic of laser light that causes it to travel in a single direction within a narrow cone of divergence. But again, perfectly parallel beams of directional light cannot be produced. All light beams eventually spread (diverge) as they move through space. But laser light is far more directional than the light from any conventional source and thus less divergent.
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COHERENCE
Figure below depicts a parallel beam of light waves from an ordinary source traveling through space. None of these waves has any fixed relationship to any of the other waves within the beam. This light is said to be "incoherent".
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Figure below illustrates the light waves within a laser beam. All of these individual waves are in step, or "in phase," with one another at every point. "Coherence" is the term used to describe the in-phase property of light waves within a beam.
Just as laser light cannot be perfectly monochromatic or perfectly directional, it cannot have perfect coherence, yet laser light is far more coherent than light from any other source. Techniques currently in use greatly improve the coherence of light from many types of lasers.
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Coherence is the most fundamental property of laser light and distinguishes it from the light from other sources. Thus, a laser may be defined as a source of coherent light.[/align][/align][/align][/align]
