Arago spot

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 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.