Fresnel number
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The Fresnel number F, named after the physicist Augustin-Jean Fresnel, is a dimensionless number occurring in optics, in particular in diffraction theory.
For an electromagnetic wave passing through an aperture and hitting a screen, the Fresnel number is defined as
where λ is wavelength of the wave, a is the characteristic size (e.g. radius) of the aperture, and L is the distance of the screen from the aperture.
Depending on the value of F the diffraction theory can be simplified into two special cases:
Values in between require a more detailed analysis based on the scalar diffraction theory.
