High Quality Content by WIKIPEDIA articles! In optics, an Arago spot, Fresnel bright spot, or Poisson spot is a bright point that appears at the center of a circular object`s shadow due to Fresnel diffraction. This spot played an important role in the discovery of the wave nature of light (see history section below) and is a common way to demonstrate that light behaves as a wave for example in undergraduate physics laboratory exercises. The basic experimental setup is shown in the figure on the right. The wave source must be at least smaller in diameter than the circular object casting the shadow and the dimensions of the setup must comply with the requirements for Fresnel diffraction. Namely, the Fresnel number must satisfy