Noun
Fresnel number (plural Fresnel numbers)
(optics) A dimensionless quantity (denoted
F
{\displaystyle F}
), such that for an electromagnetic wave passing through an aperture and hitting a screen,
F
=
a
2
L
λ
{\displaystyle F={\frac {a^{2}}{L\lambda }}}
where
a
{\displaystyle a}
is the characteristic size (e.g. radius) of the aperture,
L
{\displaystyle L}
is the distance of the screen from the aperture, and
λ
{\displaystyle \lambda }
is the incident wavelength.
Physics and engineering * Fresnel number – wavenumber over distance * Reynolds number is commonly used in fluid mechanics to characterize flow, incorporating both properties of the fluid and the flow. Source: Internet