A bright spot at the centre of the shadow of a circular opaque obstacle. It was predicted to occur by Siméon-Denis Poisson as a consequence of the wave theory of light. The effect was observed by Dominique François Jean Arago (1786–1853), and is sometimes known as Arago’s spot. Arago also noted that the spot had been observed previously, in the early 18th century, but that the significance of this observation had not been appreciated. A quantum-mechanical analogue of Poisson’s spot has been observed for matter waves.