Arago spot

Common card · No. 335155

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. The basic experimental setup requires a point source, such as an illuminated pinhole or a diverging laser beam. The dimensions of the setup must comply with the requirements for Fresnel diffraction. Namely, the Fresnel number must satisfy F = d 2 ℓ λ ≳ 1 , {\displaystyle F={\frac {d^{2}}{\ell \lambda }}\gtrsim 1,} where d is the diameter of the circular object, ℓ is the distance between the object and the screen, and λ is the wavelength...

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