Editing Diffraction grating (section)

==Gratings as dispersive elements==
The wavelength dependence in the grating equation shows that the grating separates an incident [[polychromatic]] beam into its constituent wavelength components at different angles, i.e., it is angular [[dispersion (optics)|dispersive]].  Each wavelength of input beam [[electromagnetic spectrum|spectrum]] is sent into a different direction, producing a [[rainbow]] of colors under white light illumination.  This is visually similar to the operation of a [[triangular prism (optics)|prism]], although the mechanism is very different.  A prism refracts waves of different wavelengths at different angles due to their different refractive indices, while a grating diffracts different wavelengths at different angles due to interference at each wavelength.

[[Image:light-bulb-grating.png|thumb|right|300px|A light [[incandescent light bulb|bulb]] of a [[flashlight]] seen through a transmissive grating, showing two diffracted orders. The order ''m'' = 0 corresponds to a direct transmission of light through the grating.  In the first positive order (''m'' = +1), colors with increasing wavelengths (from blue to red) are diffracted at increasing angles.]]

The diffracted beams corresponding to consecutive orders may overlap, depending on the spectral content of the incident beam and the grating density. The higher the spectral order, the greater the overlap into the next order.

[[File:Argon laser beam and diffraction mirror.png|thumb|An argon laser beam consisting of multiple colors (wavelengths) strikes a silicon diffraction mirror grating and is separated into several beams, one for each wavelength. The wavelengths are (left to right) 458&nbsp;nm, 476&nbsp;nm, 488&nbsp;nm, 497&nbsp;nm, 502&nbsp;nm, and 515&nbsp;nm.]]

The grating equation shows that the angles of the diffracted orders only depend on the grooves' period, and not on their shape. By controlling the cross-sectional profile of the grooves, it is possible to concentrate most of the diffracted optical energy in a particular order for a given wavelength. A triangular profile is commonly used. This technique is called ''[[Blazed grating|blazing]].'' The incident angle and wavelength for which the diffraction is most efficient (the ratio of the diffracted optical energy to the incident energy is the highest) are often called ''blazing angle'' and ''blazing wavelength.'' The [[grating efficiency|efficiency]] of a grating may also depend on the [[Polarization (waves)|polarization]] of the incident light. Gratings are usually designated by their ''groove density'', the number of grooves per unit length, usually expressed in grooves per millimeter (g/mm), also equal to the inverse of the groove period. The groove period must be on the order of the [[wavelength]] of interest; the spectral range covered by a grating is dependent on groove spacing and is the same for ruled and holographic gratings with the same grating constant (meaning ''groove density'' or the groove period). The maximum wavelength that a grating can diffract is equal to twice the grating period, in which case the incident and diffracted light are at ninety degrees (90°) to the grating normal. To obtain frequency dispersion over a wider frequency one must use a [[Prism (optics)|prism]]. The optical regime, in which the use of gratings is most common, corresponds to wavelengths between 100 [[nanometer|nm]] and 10 [[micrometre|µm]]. In that case, the groove density can vary from a few tens of grooves per millimeter, as in [[Echelle grating|''echelle gratings'']], to a few thousands of grooves per millimeter.

When groove spacing is less than half the wavelength of light, the only present order is the ''m'' = 0 order. Gratings with such small periodicity (with respect to the incident light wavelength) are called [[subwavelength grating]]s and exhibit special optical properties. Made on an [[Isotropy|isotropic]] material the subwavelength gratings give rise to form [[birefringence]], in which the material behaves as if it were [[birefringent]].