Arago spot

An Arago spot is a bright point which, owing to diffraction, appears at the center of the shadow of a circular object in light from a point source. The diffraction pattern of a circular object has considerable interest due to the historical part it played in the wave theory of light. Simeon 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 this 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 (Arago's) spot".