Light is electromagnetic radiation in the wave length range extending from about 0.4 µ to about 0.7 µ; or perhaps more properly, the visual response to electromagnetic radiation in this range. By extension, the term is frequently applied to adjacent wavelength ranges that the eye cannot detect; Ultraviolet light, infrared light, and black light. In addition to wave length, frequency in hertz, and wave number, in inverse units of length, are also used to specify and designate the character and quality of the radiation. Associated with wavelength or frequency is the visual response of color. The term monochromatic is applied to the idealized situations in which the light in a beam is all of one wave length.
Characteristic of Light
Light is characteristic not only by wave-length, essentially a temporal quality, but also by state and degree of polarization, 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 evident 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 response to this quality of light.
Light is further characterized by its degree of coherence. Coherence, closely related to the degree of polarization and to the degree of monochromaticity, refers to the ability of beam of light to interfere with itself. Coherence therefore an interferometric property of light. By the use of a Michelson Interferometer, most light source 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 centimeter. On the other hand, light from continuous wave gas laser produced fringes at a distance of more than 100 meters.
Light is moving energy that travels at a speed of 300,000 km/sec (186,000 m/sec). It can be regarded both as a particulate flow and as a wave phenomenon.