Two different regimes for diffraction are usually identified: Fresnel and Fraunhofer.
Fresnel diffraction is the regime in which the diffracted waves are observed close (as compared to the size of the object causing the diffraction) to the place where they are diffracted. Fresnel diffraction is usually very complicated to work with. The other regime, Fraunhofer diffraction, is much easier to deal with. Fraunhofer diffraction applies to situations in which the diffracted waves are observed far from the point of diffraction. This allows a number of simplifying approximations to be used, reducing diffraction to a very manageable problem.
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