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The Diffraction Grating
 
The simplest diffraction grating is an array of a large number of parallel slits that all share the same width, with equal distances between each other?s centers.
If a parallel beam of light illuminates the grating, after the contact with the grating the light travels in several different directions. A converging lens that is located behind the grating will produce diffracting maxima of different orders in its focal plane. These maxima are called principal. Directions that the light beams assume after passing though the grating are determined by the formula
dsinθm=mλ (m=0,+-1,+-2...)
Were d is the period of the grating, λ is the wavelength, and m is an integer called the order of diffraction maximum.
At small angles, the distance between the zero-order maximum (m=0) and the m'th order maximum in the focal plane of the lens of focal length f is
ym=m(λ/d)f.
Since the position of the maximum (except when m=0) depends on the wavelength, the grating can disperse a light beam into spectra, i.e. it is a spectral device.
With the use of diffraction grating, it is possible to carry out measurements of the wavelength with remarkable precision. Given the grating period, the task of finding the wavelength is reduced to measuring the angle θm corresponding to the direction of the m'th maximum.
If the beam consists of two monochromatic waves of wavelengths λ1 and λ2, the grating can separate the waves from each other, in any order (except for m=0).
 
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