The Fresnel number F, named after the physicist Augustin-Jean Fresnel, is a dimensionsless 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

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

λ 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:

• Fraunhofer diffraction for $F\; \backslash ll\; 1$
• Fresnel diffraction for $F\; \backslash ge\; 1$

Values in between require a more detailed analysis based on the scalar diffraction theory.

See also

• Fresnel integral
• Fresnel zone

Diffraction