Fresnel number

Named after the physicist Augustin-Jean Fresnel.

Fresnel number (plural Fresnel numbers)

1. (optics) A dimensionless quantity (denoted ${\displaystyle F}$), such that for an electromagnetic wave passing through an aperture and hitting a screen, ${\displaystyle F={\frac {a^{2}}{L\lambda }}}$ where ${\displaystyle a}$ is the characteristic size (e.g. radius) of the aperture, ${\displaystyle L}$ is the distance of the screen from the aperture, and ${\displaystyle \lambda }$ is the incident wavelength.