(optics) A dimensionless quantity (denoted F), such that for an electromagnetic wave passing through an aperture and hitting a screen, F=(a²)/(Lλ) where a is the characteristic size (e.g. radius) of the aperture, L is the distance of the screen from the aperture, and λ is the incident wavelength.
The Fresnel equations (or Fresnel coefficients) describe the reflection and transmission of light (or electromagnetic radiation in general) when incident on an interface between different optical media.