What is a zone plate? How does a positive zone plate differ from a negative zone plate? Show that each fresnel half period zone has nearly the same area?
Answers
Zone plate: A diffracting device used to experimentally demonstrate the diffraction effect.
Positive zone plate:
- Odd zones are kept transparent to the light
- Even zones are made opaque
Negative zone plate:
- Even zones are kept transparent to light
- Odd zones are made opaque
Proof:
Fresnel subdivided the wavefront into a number of circular zones I, II, III etc. Let PO = b. The area enclosed between O and O1, O1 and O2, O2 and O3 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 OO1 is called the first half period zone.
The area enclosed by the annular strip O1O2 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.
Zone plate: A diffracting device used to experimentally demonstrate the diffraction effect.
Positive zone plate:
Odd zones are kept transparent to the light
Even zones are made opaque
Negative zone plate:
Even zones are kept transparent to light
Odd zones are made opaque
Odd zones are made opaqueProof:
Fresnel subdivided the wavefront into a number of circular zones I, II, III etc. Let PO = b. The area enclosed between O and O1, O1 and O2, O2 and O3 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 OO1 is called the first half period zone.
The area enclosed by the annular strip O1O2 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.