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# Fresnel number

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 F is defined as

$F = \frac{a^{2}}{L \lambda}$

where λ is wavelength, 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.