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 ) of the spherical wave.
Its width, , depends on both the wavelength and the
radius of the spherical wave.
Figure 7 The Fresnel zone. Its width
corresponds to the range within which another spherical wave will