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.
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The Arago spot, also known as the Poinsot spot, is a phenomenon observed in the diffraction pattern of light when it passes through a circular aperture or around a circular obstacle. It is named after the French physicist François Arago, who studied this effect in the early 19th century. When light waves encounter a circular obstacle, they bend around it and create a diffraction pattern.
