Fresnel number

Remarks

Example: $a = 1$ mm, $\lambda = 500$ nm and $L = 1$ m give $N_F = 10^{-6} / (5 \cdot 10^{-7}) = 2$. For a circular aperture of radius $a$ and a point on the axis, the $m$-th Fresnel zone has outer radius about $\sqrt{m \lambda L}$. So $N_F$ is the number of Fresnel zones that fit in the aperture. The regimes are: $N_F \ll 1$ for Fraunhofer diffraction, $N_F \gtrsim 1$ for Fresnel diffraction, and $N_F \gg 1$ for the geometrical shadow. The number depends on the choice of $a$ (radius or diameter), so only its order of magnitude matters.