Near field and far field

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In school, one often describes the diffraction pattern of a grating at a large distance. This is a phenomenon in the optical far field. In this case, the incoming wave fronts can, in a good approximation, be considered to be flat. At shorter distances to the grating the diffraction phenomena are more complex. In order to correctly describe phenomena in the optical near field one also has to take into account the curvature of the wave fronts.

nearfieldFarfield_ENIn order to compute the wave propagation behind an arbitrary aperture, the Kirchhoff-Fresnel diffraction integral is used. Its derivation and meaning is explained here:

Extra: Mathematical background