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
- <math>F = \frac{a^{2}}{L \lambda}</math>
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 <math>F \ll 1</math>
- Fresnel diffraction for <math>F \gtrsim 1</math>
Values in between require a more detailed analysis based on the scalar diffraction theory.
