Fresnel number
The Fresnel number F , named after the physicist Augustin Jean Fresnel , is a dimensionless number that is used in optics , especially in diffraction . It describes how strong the diffraction of a light beam at a diaphragm is.
For an electromagnetic wave that passes through a diaphragm and hits a screen, the Fresnel number F is defined as
With
- a the characteristic size of the aperture (e.g. its radius )
- L is the distance from the screen to the diaphragm
- λ is the wavelength .
Depending on the value of F , one can differentiate between the following cases:
- : Fraunhofer diffraction (e.g. aperture a very small or beam path in the far field , i.e. distance L very large)
- : Fresnel diffraction
- : Geometric optics (e.g. aperture a very large or beam path in the near field , i.e. distance L very small).