In optics, the Arago spot, Poisson spot, or Fresnel 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 and is a common way to demonstrate that light behaves as a wave.
Arago spot experiment. A point source illuminates a circular object, casting a shadow on a screen. At the shadow's center a bright spot appears due to diffraction, contradicting the prediction of geometric optics.
Numerical simulation of the intensity of monochromatic light of wavelength λ = 0.5 μm behind a circular obstacle of radius R = 5 μm = 10λ.
Notation for calculating the wave amplitude at point P1 from a spherical point source at P0.
The on-axis intensity at the center of the shadow of a small circular obstacle converges to the unobstructed intensity.
In optics, the Fresnel diffraction equation for near-field diffraction is an approximation of the Kirchhoff–Fresnel diffraction that can be applied to the propagation of waves in the near field. It is used to calculate the diffraction pattern created by waves passing through an aperture or around an object, when viewed from relatively close to the object. In contrast the diffraction pattern in the far field region is given by the Fraunhofer diffraction equation.
Fresnel diffraction showing central Arago spot
Fresnel diffraction of circular aperture, plotted with Lommel functions
