Fresnel number

Summary

The Fresnel number is the dimensionless number $a^2 / (L \lambda)$. Here $a$ is the size of an aperture, $L$ the distance to the observation plane and $\lambda$ the wavelength. It tells which approximation of diffraction applies. For a Fresnel number much smaller than $1$ the pattern is in the Fraunhofer regime. For a Fresnel number of order $1$ or more it is in the Fresnel regime. For very large values geometrical optics is accurate.