Far field approximation to Kirchhoff's diffraction formula, i.e. when the plane of observation is far from the object diffracting.
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Fraunhofer diffraction refers to a type of diffraction pattern produced when light waves pass through a slit or around an obstacle and are observed at a considerable distance from the aperture or obstacle, where the incoming and outgoing waves can be treated as parallel. This condition is typically satisfied when the source of light and the observation screen are placed at infinity or sufficiently far away compared to the size of the aperture.