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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The Huygens–Fresnel principle is a fundamental concept in the field of wave optics that describes how waves propagate and interfere. Named after Dutch physicist Christiaan Huygens and later expanded by the French physicist Augustin-Jean Fresnel, the principle provides a way to analyze the propagation of wavefronts, such as light waves.