Fresnel number

English

Etymology

Named after the physicist Augustin-Jean Fresnel.

Noun

Fresnel number (plural Fresnel numbers)

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