Introductionlight, 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.
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