Diffraction Gratings
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A reflection grating is an array of fine, parallel, equally spaced
grooves on a reflective substrate. When polychromatic light is
incident on a diffraction grating, diffraction and interference effects
spread the various wavelengths in discrete directions called "orders"
or "special orders."
Because the angle of deviation of the diffracted beam
is wavelength dependent, a diffraction grating separates the incident
beam spatially into its constituent wavelength components. The groove
dimensions and spacings are on the order of the wavelength in question.
Order zero corresponds to direct transmission or specular reflection.
Higher orders result in deviation of the incident beam from the
direction predicted by geometric (ray) optics. With a normal angle of
incidence, the angle θ, the deviation of the diffracted ray from the
direction predicted by geometric optics is given by the following
equation where m is the spectral order, λ is the wavelength, and d is
the spacing between corresponding parts of adjacent grooves: mλ=dsinθ
By controlling the cross-sectional shape of the grooves, it is
possible to concentrate most of the diffracted energy in the order of
interest. This technique is called "blazing." The blaze angle is then
defined as the angle of incidence that allows maximum concentration in
the first order.