Arago Spot

A bright spot that appears in the shadow of a uniform disc being backlit by monochromatic light emanating from a point source.

Poisson Spot

Poisson originally predicted the existence of such a spot, and used the prediction to demonstrate how the wave theory of light must be in error to produce such a counterintuitive result. Subsequent observation of the Arago spot provided a decisive confirmation of the wave nature of light.