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
where
- is the characteristic size (e.g. radius) of the aperture
- is the distance of the screen from the aperture
- is the incident wavelength.
Depending on the value of F the diffraction theory can be simplified into two special cases:
- Fraunhofer diffraction for
- Fresnel diffraction for
In case of, laws of geometrical optics are applied.
The agreement on this limit does not seem universal, for elaboration on this controversy see this article's discussion page. Intermediate values of the Fresnel number necessitate a more detailed analysis based on the scalar diffraction theory.
Famous quotes containing the word number:
“No Government can be long secure without a formidable Opposition. It reduces their supporters to that tractable number which can be managed by the joint influences of fruition and hope. It offers vengeance to the discontented, and distinction to the ambitious; and employs the energies of aspiring spirits, who otherwise may prove traitors in a division or assassins in a debate.”
—Benjamin Disraeli (1804–1881)