The Fresnel zone

Several grains generate different diffracted waves that interfere and may raise a new wave. Huygen's principle asserts that a plane wave results from the superposition of a set of spherical diffractions. However, each spherical pattern contributes to the plane wave reconstruction only to a limited extent. Figure (7) represents the zone where the spherical waves add constructively, the Fresnel zone. The length of this zone depends on the frequency (related to the wavelength $\bf \lambda$) of the spherical wave. Its width, $\bf f$, depends on both the wavelength and the radius $\bf R$ of the spherical wave.

 

Fresnel Figure 7 The Fresnel zone. Its width corresponds to the range within which another spherical wave will add constructively.