The principal result of Chapter 1 was the demonstration that the Fraunhofer diffraction pattern associated with the field distribution existing across an aperture is the Fourier transform of that field. To be precise, the Fraunhofer diffraction pattern of an aperture distribution is obtained when the point of observation is infinitely distant from a coherently illuminated aperture. In practice, of course, this condition never describes a physical situation. However, the Fraunhofer theory provides an adequate approximation in many physically significant experiments.
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