Arago spot
= Arago spot
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The <Arago spot> is the bright axial point in the shadow of a circular obstruction under suitably coherent illumination. In ideal <scalar diffraction theory>, secondary fields from the circular edge arrive in phase on axis. <Fresnel diffraction by an opaque disk> gives an axial intensity equal to the unobstructed intensity in its paraxial model; this is not a claim that the whole geometric shadow is bright.