![]() ![]() Light waves diffracted from these lines interfere, and all wavelengths but one are canceled in any particular direction through destructive interference. Anywhere from one to 10,000 fine parallel lines per millimeter can be engraved. "The grooves are similar to the indentations made by a plow in soil," says John Hoose of Richardson Grating Laboratory (Rochester, NY), except that they are much closer together. Modern ruled gratings can be either reflective or transmissive and are fabricated with a single diamond point that burnishes grooves on flat or concave surfaces. Rowland invented the process of ruling, or scratching parallel notches into metal deposited onto the surface of a flat, clear glass plate-a method that produced gratings of exceptionally high quality. A wavefront that passed through the system was confronted by alternate opaque and transparent regions, so that it underwent a modulation in amplitude. Earliest devices were multiple-slit assemblies, consisting of a grid of fine wire or thread wound about and extending between two parallel screws, which served as spacers. Joseph Fraunhofer first used diffraction gratings in 1819 to observe the spectrum of the sun. Applications are expanding one of the fastest growing areas for gratings-laser pulse compression-didn’t even exist until a few years ago. Gratings are indispensable in helping physicists determine the structure of atoms or helping astronomers calculate the chemical composition of stars and the rotation of galaxies. These minute, periodic structures diffract, or disperse, incident light in such a way that the individual wavelengths making up the incident light can be differentiated. Diffraction gratings are fundamental optical elements that have a precise pattern of grooves superimposed on them. ![]()
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