8/12/2023 0 Comments Order of diffraction grating![]() This convention is shown graphically in Figure 2-4. β 0 if the diffracted ray lies to the left (the counter-clockwise side) of the zero order (m = 0), and m Explicitly, spectra of all orders m exist for which In most cases, the grating equation allows light of wavelength λ to be diffracted into both negative and positive orders as well. Specular reflection, for which m = 0, is always possible that is, the zero order always exists (it simply requires β = – α). This restriction prevents light of wavelength λ from being diffracted in more than a finite number of orders. The grating equation reveals that only those spectral orders for which |m λ/d| 2, which is physically meaningless. Similarly, the second order (m = 2) and negative second order (m = –2) are those for which the path difference between rays diffracted from adjacent grooves equals two wavelengths. This happens, for example, when the path difference is one wavelength, in which case we speak of the positive first diffraction order (m = 1) or the negative first diffraction order (m = –1), depending on whether the rays are advanced or retarded as we move from groove to groove. The physical significance of this is that the constructive reinforcement of wavelets diffracted by successive grooves merely requires that each ray be retarded (or advanced) in phase with every other this phase difference must therefore correspond to a real distance (path difference) which equals an integral multiple of the wavelength. In fact, subject to restrictions discussed below, there will be several discrete angles at which the condition for constructive interference is satisfied. For a particular groove spacing d, wavelength λ and incidence angle α, the grating equation is generally satisfied by more than one diffraction angle β. ![]()
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