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In optics, a diffraction grating is an optical component with a periodic structure, which splits and diffracts light into several beams travelling in different directions. The emerging coloration is a form of structural coloration. The directions of these beams depend on the spacing of the grating and the wavelength of the light so that the grating acts as the dispersive element. Because of this, gratings are commonly used in monochromators and spectrometers. For practical applications, gratings generally have ridges or ''rulings'' on their surface rather than dark lines. Such gratings can be either transmissive or reflective. Gratings which modulate the phase rather than the amplitude of the incident light are also produced, frequently using holography. The principles of diffraction gratings were discovered by James Gregory, about a year after Newton's prism experiments, initially with items such as bird feathers.〔Letter from James Gregory to John Collins, dated 13 May 1673. Reprinted in: especially (p. 254 )〕 The first man-made diffraction grating was made around 1785 by Philadelphia inventor David Rittenhouse, who strung hairs between two finely threaded screws.〔See: * * Thomas D. Cope (1932) ("The Rittenhouse diffraction grating" ). Reprinted in: (A reproduction of Rittenhouse's letter re his diffraction grating appears on pp. 369–374.)〕 This was similar to notable German physicist Joseph von Fraunhofer's wire diffraction grating in 1821.〔See: * Frauhofer. Jos. (1821) ("Neue Modifikation des Lichtes durch gegenseitige Einwirkung und Beugung der Strahlen, und Gesetze derselben" ) (New modification of light by the mutual influence and the diffraction of () rays, and the laws thereof), ''Denkschriften der Königlichen Akademie der Wissenschaften zu München'' (Memoirs of the Royal Academy of Science in Munich), 8: 3-76. * Fraunhofer, Jos. (1823) ("Kurzer Bericht von den Resultaten neuerer Versuche über die Gesetze des Lichtes, und die Theorie derselben" ) (Short account of the results of new experiments on the laws of light, and the theory thereof) ''Annalen der Physik'', 74(8): 337-378.〕 Diffraction can create "rainbow" colors when illuminated by a wide spectrum (e.g., continuous) light source. The sparkling effects from the closely spaced narrow tracks on optical storage disks such as CD's or DVDs are an example, while the similar rainbow effects caused by thin layers of oil (or gasoline, etc.) on water are not caused by a grating, but rather by interference effects in reflections from the closely spaced transmissive layers (see Examples, below). A grating has parallel lines, while a CD has a spiral of finely-spaced data tracks. Diffraction colors also appear when one looks at a bright point source through a translucent fine-pitch umbrella-fabric covering. Decorative patterned plastic films based on reflective grating patches are very inexpensive, and are commonplace. ==Theory of operation== (詳細はHuygens–Fresnel principle, each point on the wavefront of a propagating wave can be considered to act as a point source, and the wavefront at any subsequent point can be found by adding together the contributions from each of these individual point sources. Gratings may be of the 'reflective' or 'transmissive' type, analogous to a mirror or lens respectively. A grating has a 'zero-order mode' (where ''m'' = 0), in which there is no diffraction and a ray of light behaves according to the laws of reflection and refraction the same as with a mirror or lens respectively. An idealised grating is considered here which is made up of a set of slits of spacing ''d'', that must be wider than the wavelength of interest to cause diffraction. Assuming a plane wave of monochromatic light of wavelength ''λ'' with normal incidence (perpendicular to the grating), each slit in the grating acts as a quasi point-source from which light propagates in all directions (although this is typically limited to a hemisphere). After light interacts with the grating, the diffracted light is composed of the sum of interfering wave components emanating from each slit in the grating. At any given point in space through which diffracted light may pass, the path length to each slit in the grating will vary. Since the path length varies, generally, so will the phases of the waves at that point from each of the slits, and thus will add or subtract from one another to create peaks and valleys, through the phenomenon of additive and destructive interference. When the path difference between the light from adjacent slits is equal to half the wavelength, ''λ''/2, the waves will all be out of phase, and thus will cancel each other to create points of minimum intensity. Similarly, when the path difference is ''λ'', the phases will add together and maxima will occur. The maxima occur at angles ''θ''m, which satisfy the relationship ''d'' sin''θ''m/''λ'' = |''m''|, where ''θ''m is the angle between the diffracted ray and the grating's normal vector, and ''d'' is the distance from the center of one slit to the center of the adjacent slit, and ''m'' is an integer representing the propagation-mode of interest. Thus, when light is normally incident on the grating, the diffracted light will have maxima at angles ''θ''m given by: : It is straightforward to show that if a plane wave is incident at any arbitrary angle ''θ''i, the grating equation becomes: : When solved for the diffracted angle maxima, the equation is: : Please note that these equations assume that both sides of the grating are in contact with the same medium (e.g. air). The light that corresponds to direct transmission (or specular reflection in the case of a reflection grating) is called the zero order, and is denoted ''m'' = 0. The other maxima occur at angles which are represented by non-zero integers ''m''. Note that ''m'' can be positive or negative, resulting in diffracted orders on both sides of the zero order beam. This derivation of the grating equation is based on an idealised grating. However, the relationship between the angles of the diffracted beams, the grating spacing and the wavelength of the light apply to any regular structure of the same spacing, because the phase relationship between light scattered from adjacent elements of the grating remains the same. The detailed distribution of the diffracted light depends on the detailed structure of the grating elements as well as on the number of elements in the grating, but it will always give maxima in the directions given by the grating equation. Gratings can be made in which various properties of the incident light are modulated in a periodic pattern; these include *transparency (transmission amplitude diffraction gratings); *reflectance (reflection amplitude diffraction gratings); *refractive index or optical path length (phase diffraction gratings); *direction of optical axis (optical axis diffraction gratings). The grating equation applies in all these cases. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Diffraction grating」の詳細全文を読む スポンサード リンク
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