What is a zone plate? How does a positive zone plate differ from a negative zone
plate? Show that each Fresnel zone has nearly the same area.
Zone plate: A diffracting device used to experimentally demonstrate the diffraction effect.
Positive zone plate:
Negative zone plate:
Proof:
Fresnel subdivided the wavefront into a number of circular zones I, II, III etc. Let PO = b. The area enclosed between O and , and , and etc. are known as half period zones.
Each zone differs from its neighbor by a phase difference of π or a path difference of λ/2. The area enclosed by the first circle of radius is called the first half period zone.
The area enclosed by the annular strip is known as second half period zone and so on. Thus, the annular area between (n-1)th circle and nth is the nth hall period zone.
Sn ≅ πbλ
Hence, each Fresnel half period zone has nearly the same area.
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