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:
* Fraunhofer diffraction for $F\; ll\; 1$
* Fresnel diffraction for $F\; gtrsim\; 1$

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

ee also

* Fresnel integral
* Fresnel zone