How are Fresnel Zones formed? What phenomena of light allow ellipsoid areas to be in phase? I've tried reading articles, but they more or less introduce me to characteristics of light, and then tell me that Fresnel Zones exist. How does the circular wave coming out of one antenna "bend back" to the other one in the shape of an ellipse?
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When you derive a Huygen-Fresnel Propagator (which is how actual wavefronts propagate according to Maxwell's equations) a Fresnel zone is really the difference (in phase) between surfaces of equal phase on the propagating wavefront and a plane slicing or tangent to that surface of equal phase.
These Fresnel zones are defined when propagating a plane wave incident on some circular aperture.
This picture shows the concept, except the Fresnel zones are shown for a finite conjugate (i.e. phase difference between a propagating wave and a point that we want to use the zone plate act as a "lens").
Basically, the Fresnel zone is the phase difference map between something like a spherical wavefront, and a plane tangent to that wave front. That is exactly what it is physically and conceptually. A zone plate (manufactured on a plane) can then be used to change the propagation properties of the light wave.
I say something like a spherical wavefront above because that is a good approximation, the actual Green's function for the propagator is the derivative of a spherical wavelet, which is required to satisfy the boundary conditions properly.
The figure above shows how canceling the additive and subtractive parts of the wavefront can result in "lens type" behavior.
