Far field approximation to Kirchhoff's diffraction formula, i.e. when the plane of observation is far from the object diffracting.
Near field approximation to Kirchhoff's diffraction formula, i.e. when the plane of observation is near the object diffracting.
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Diffraction is interference produced when waves scatter from spatial structure. A periodic structure concentrates the scattered intensity at reciprocal-lattice conditions.
Diffraction is a phenomenon that occurs when a wave encounters an obstacle or a slit that is comparable in size to its wavelength. It results in the bending and spreading of waves as they pass around the edges of the obstacle or through the slit. This behavior is observed with various types of waves, including sound waves, light waves, and water waves.