7.2.3. Fresnel zone plate.

A Fresnel zone plate is a diffractive element consisting of a number of concentric rings which are alternating transparent and opaque. The opaque rings block the odd Fresnel zones while the transparent rings passes the even zones. Alternatively, the opaque rings block the even and the transparent rings pass the odd zones. When a plane wave is incident on the plate it acts as a focusing lens. Also it is possible to arrange the plate such that it can be used as an imaging element.

Here two point sources in phase illuminate the zone plate. In the focus you can study the resolving power of the zone plate by changing the distance between the two point sources. The resolving power equals the size of the outermost ring of the zone plate.

A nice introduction to the theory and the relevant equations can be found at The Center For X-Ray Optics, Lawrence Berkeley National Laboratory.

\(w=\frac{\lambda }{2sin\theta} =\frac{\lambda }{2}\frac{2\Delta r}{\lambda } =\Delta r\)

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