n. In optics, the spreading of light or deflection of its rays, accompanied by phenomena of interference: occasioned by the neighborhood of an opaque body to the course of the light, as when it passes by the edge of an opaque body or through a small aperture, the luminous rays appearing to be bent or deflected from their straight course and mutually interfering with one another. See interference.n. Thus, if a beam of monochromatic light is passed through a narrow slit and received on a screen in a dark room, a series of alternately light and dark bands or fringes is seen, which diminish in intensity and distinctness on either side of the central line; if white light is employed, a series of colored spectra of different orders is obtained. Similar phenomena of diffraction are obtained from diffraction gratings, which consist of a band of equidistant parallel lines (from 10,000 to 30,000 or more to the inch), ruled on a surface of glass or of polished metal; the spectra obtained by this means are called interference or diffraction spectra. They differ from prismatic spectra, since in them the colors are uniformly distributed in their true order and extent according to their difference in wave-length; while in the latter the less refrangible (red) rays are crowded together, and the more refrangible (blue, violet) are dispersed. Diffraction gratings are now much used, especially in studying the solar spectrum. The best gratings are ruled on speculum metal with a concave surface (often called Rowland gratings, after Professor Henry A. Rowland of Baltimore), and give an image of the spectrum directly, without the intervention of a lens.n. Hence In acoustics, the analogous modification produced upon sound-waves when passing by the edge of a large body, as a building.