This Demonstration shows the interference pattern produced by light diffracted through a small disk in a plane perpendicular to the propagation direction of the light waves. You can adjust the wavelength of the light and set the observed domain to examine details of the interference pattern. The bright spot in the center of the concentric interference rings is called the Poisson (or Arago) spot. This phenomenon cannot be explained by geometric optics, in which the obstacle is opaque. But, using wave optics, the observed phenomenon can be described accurately. Light penetrates behind the object because of diffraction of light waves. The spot at the center is the consequence of the constructive interference of light waves diffracted on the edge of the disk, its central position being determined by the symmetry of the disk. The Poisson spot is further evidence for the validity of wave optics.
The formula used in this Demonstration gives a very good approximation for the intensity of light, especially in the direction of the optical axis through the center of the pattern. The radius of the disk in this simulation is 20 micrometers. In a real experiment, except for the central bright spot, interference rings are hardly visible to the naked eye because the intensity difference between the Poisson spot (peak) and the external rings is relatively large. This large difference is eliminated in this Demonstration so that the outer interference rings can be seen.