In optics, an Arago spot is a bright point which appears at the center of the shadow of a circular object in light from a point source. It is also sometimes called a Poisson spot.^[1] It is of considerable interest due to the historical part it played in the wave theory of light. In 1818 Siméon Poisson deduced from Augustin Fresnel's theory the necessity of a bright spot at the centre of the shadow of a circular opaque obstacle. With his counterintuitive result Poisson hoped to disprove the wave theory; however Dominique Arago experimentally verifed the prediction and today the demonstration goes by the name "Poisson's (or Arago's) spot." Since the spot occurs within the geometrical shadow, no particle theory of light could account for it, and its discovery in fact provided weighty evidence for the wave nature of light, much to Poisson's chagrin.

The existence of the spot had previously been observed in 1723 by Giacomo F. Maraldi, but the work had been largely unrecognized.

The presence of Arago's spot can be easily understood. When light shines on a circular obstacle, Huygens' principle says that every point along the circumference acts as a new point source of light. But clearly, the light coming from each of these points to the center of the shadow travels exactly the same distance, and so all the light arrives in phase and constructively interferes. But this argument holds inside the object's shadow, where geometric optics and particle theories of light predict there should be no light at all.

Notice also that the above argument works specifically for a circular obstacle. Many objects have a central bright peak in their diffraction pattern, but few of those display that peak inside their shadow. The ring pattern of a Fresnel zone plate is one such arrangement.

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