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Vector PEC scattering, dipole limits and controlled scalar diffraction — Examples

1 · Prerequisites

2 · Summary

Objective. The companion collects the single leaf example of the pair: the uniform rectangular aperture evaluated exactly inside the adopted scalar diffraction model, followed by the sinc-squared Fraunhofer pattern with its zeros and its central maximum, all up to the inherited Fraunhofer error.

Uniform rectangular aperture. For the rectangle R=[−a,a]×[−b,b] embedded in the plane y3=0 and a constant complex amplitude A0≠0, the example takes the plane-embedding pushforward α(E)=A0λ2(R∩ȷ−1(E)) as the aperture distribution and verifies that it is a finite complex measure of total variation 4ab∣A0∣, with the pairing identity ∫f dα=A0∫Rf∘ȷ dλ2 for bounded Borel f (Uniform rectangular aperture: the sinc-squared Fraunhofer pattern and its exact zeros and maxima). The Fraunhofer amplitude factor is exactly 4abA0sinc⁡(ka n1)sinc⁡(kb n2) for every direction n∈S2, so the model intensity in the Fraunhofer regime with aperture radius a′=a2+b2 is IF=a2b2∣A0∣22π2r2sinc⁡2(ka n1)sinc⁡2(kb n2), up to the Fraunhofer absolute error of the parent theorem transferred to I by its intensity clause.

Pattern structure. The example proves the exact zero set ka n1∈πZ∖{0} or kb n2∈πZ∖{0} and, using ∣sin⁡x∣≤∣x∣ with equality only at x=0, that the mathematical pattern on all of S2 attains its global maximum exactly at the two poles n=(0,0,±1). The model field is defined for x3>0, so its observable central direction is the single pole n=(0,0,1); the zeros and the lower pole belong to the ideal Fraunhofer factor, not to any measured record.

Scope. The computation is exact inside the adopted ideal scalar model. It inherits the Fraunhofer and Fresnel error bounds rather than re-estimating them, and it asserts no absolute radiometric calibration, polarization content, screen interaction, apparatus model, sampling law or empirical comparison.

3 · Logical flowchart

4 · Definitions, postulates, theorems and proofs

None yet.

5 · Examples, experiments and counterexamples

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Uniform rectangular aperture: the sinc-squared Fraunhofer pattern and its exact zeros and maxima

Physical scope

Exact computation in the adopted scalar aperture-diffraction model for a uniform rectangular aperture: the Fraunhofer amplitude 4abA0sinc⁡(kan1)sinc⁡(kbn2) and the sinc-squared model intensity pattern with its zeros and central maximum, up to the explicit Fraunhofer error of the parent theorem. Excluded: measured diffraction records, absolute radiometric calibration, polarization and vector effects, screen interaction and aperture back-reaction, apparatus models, sampling laws and every empirical or statistical claim.

Example

Assume Countable Choice (ACω) as carried by Controlled Fraunhofer and Fresnel estimates for the scalar aperture model, with intensity-error transfer and by the Riemann-Lebesgue comparison and Fubini suppliers used in the integral reduction below (The Axiom of Countable Choice (ACω)). Work in the scalar aperture model of The adopted scalar aperture-diffraction model and Controlled Fraunhofer and Fresnel estimates for the scalar aperture model, with intensity-error transfer, with frequency ω>0, k=ω/c, aperture plane y3=0, model field u=Gk∗α and I(x)=12∣u(x)∣2. Fix lengths a>0, b>0 in metres and A0∈C∖{0} in V m−2, so that α has the model unit V; identify the rectangle R:=[−a,a]×[−b,b]⊂R2 with the planar aperture {(y1,y2,0):(y1,y2)∈R}, let ȷ:R2→R3 be ȷ(y1,y2)=(y1,y2,0), and define the uniform rectangular aperture distribution as the plane-embedding pushforward

α(E):=A0 λ2(R∩ȷ−1(E))(E⊆R3 Borel).

This is a finite complex measure on R3, supported on the embedded compact rectangle S=ȷ(R) with ∣y∣≤a′:=a2+b2 and total variation ∥α∥TV=4ab∣A0∣ (Axis-parallel rectangles in Rm and their volume, The measure with density f relative to μ, Integration against a signed or complex measure, and the class L^1(nu) = L^1(|nu|)). Then: (a) [Fraunhofer amplitude] with sinc⁡ξ:=sin⁡ξ/ξ for ξ≠0 and sinc⁡0:=1, the Fraunhofer amplitude factor of Controlled Fraunhofer and Fresnel estimates for the scalar aperture model, with intensity-error transfer(a) is exactly

∫e−ikn⋅y dα(y)=4abA0sinc⁡(ka n1)sinc⁡(kb n2)(n=(n1,n2,n3)∈S2),

an identity for every direction n (The Euclidean inner product ⟨x,y⟩=∑k<nxkyk on Rn, The complex exponential by its power series, exp⁡(x+iy)=ex(cos⁡y+isin⁡y), ∣exp⁡(x+iy)∣=ex, and eiπ+1=0, The second fundamental theorem: if G is differentiable on [a,b] with G′=f and f is integrable, then ∫abf=G(b)−G(a), The derivatives of sine and cosine are cosine and minus sine, Sums, scalar multiples, products and quotients: (f+g)′(c)=f′(c)+g′(c), (αf)′(c)=αf′(c), (fg)′(c)=f′(c)g(c)+f(c)g′(c), and (f/g)′(c)=(f′(c)g(c)−f(c)g′(c))/g(c)2 when g(c)≠0, Riemann--Fubini on product rectangles, with lower and upper section integrals and content-zero exceptional sections, Every continuous function on a closed nondegenerate rectangle in Rm is Riemann integrable, Riemann–Lebesgue comparison for distribution test integrands, The Lebesgue integral is linear on L1(μ)). (b) [Fraunhofer intensity] in the Fraunhofer regime of Controlled Fraunhofer and Fresnel estimates for the scalar aperture model, with intensity-error transfer(a) with aperture radius a′:=a2+b2, i.e. for r≥2a′, the model intensity is

IF(rn)=a2b2∣A0∣22π2r2sinc⁡2(ka n1)sinc⁡2(kb n2)

up to the Fraunhofer absolute error of that theorem, transferred to I by its clause (c). The pattern vanishes exactly where ka n1∈πZ∖{0} or kb n2∈πZ∖{0}, and, as a mathematical pattern on all S2, has its global maximum exactly at the two poles n=(0,0,±1). The model observes only x3>0; its allowed directions have n3>0, where the central maximum is only n=(0,0,1). (c) [Scope] this is an exact computation inside the adopted ideal scalar model, not a measured diffraction record: no absolute radiometric calibration, polarization content, screen interaction, apparatus model or sampling law is asserted, and the Fraunhofer/Fresnel error bounds of Controlled Fraunhofer and Fresnel estimates for the scalar aperture model, with intensity-error transfer are inherited rather than re-estimated.

Facts & Assumptions

Given: Countable Choice; the data a>0, b>0, A0∈C∖{0}, the rectangle R=[−a,a]×[−b,b], the embedding ȷ(y1,y2)=(y1,y2,0), the pushforward α of the Example, and the model field u=Gk∗α, I=12∣u∣2 of the scalar aperture model.

[F1]

Parent model and theorem: the model field and I=12∣u∣2 are those of The adopted scalar aperture-diffraction model, and Controlled Fraunhofer and Fresnel estimates for the scalar aperture model, with intensity-error transfer provides, for a finite complex measure supported in ∣y∣≤a0 with r≥2a0 and all n∈S2, the Fraunhofer estimate u(rn)=eikr(4πr)−1∫e−ikn⋅y dα(y)+error with ∣error∣≤∥α∥TV(πr)−1[a0/r+ka02/r], uniformly in direction, together with the intensity-error transfer ∣12∣uapprox∣2−I∣≤12δ(∣u∣+∣uapprox∣) whenever ∣uapprox−u∣≤δ and the period-average identity of Period averages of products of real parts of complex exponentials.

[F2]

Rectangle, area and density measure: R is a nondegenerate closed rectangle in R2 with two-dimensional Lebesgue measure λ2(R)=4ab, its embedded image S=ȷ(R) is compact with ∣y∣≤a′=a2+b2 and the corners attaining a′, by Axis-parallel rectangles in Rm and their volume and A box in Rn with parameters ai≤bi is Lebesgue measurable of measure ∏i<n(bi−ai), whichever of its faces are included. The nonnegative restricted measure λ2∣R has density 1R as in The measure with density f relative to μ; the complex pushforward itself is defined by the displayed formula in the Example and verified in step 1.1.

[F3]

Measure-theoretic toolbox: ȷ is continuous, hence the inverse image of every Borel set is Borel; a finite complex measure on the Borel sets is countably additive and its total variation is as in A complex measure is a finite-valued countably additive set function and The total variation |nu|(E) from countable measurable partitions; the integral against it is defined by Integration against a signed or complex measure, and the class L^1(nu) = L^1(|nu|), and for any bounded Borel function h on a finite-variation measure space, h is integrable and ∣∫h dα∣≤∫∣h∣ d∣α∣≤∥h∥∞∥α∥TV by Integrals against signed or complex measures are bounded by total variation; this includes the simple-function bound (A continuous map has Borel preimages of Borel sets, Simple integrals are bounded by total variation, The Axiom of Countable Choice (ACω)).

[F4]

Integral comparison: for a bounded Borel function on a nondegenerate rectangle that is Riemann integrable, the Lebesgue integral exists and agrees with the Riemann integral (Riemann–Lebesgue comparison for distribution test integrands); a continuous function on a compact rectangle is Riemann integrable (Every continuous function on a closed nondegenerate rectangle in Rm is Riemann integrable); the Riemann double integral factors through iterated one-dimensional integrals when the sections are integrable (Riemann--Fubini on product rectangles, with lower and upper section integrals and content-zero exceptional sections); and the Lebesgue integral is complex-linear on L1 (The Lebesgue integral is linear on L1(μ)).

Verification

technique · direct
1.1givenF2F3algebra

[The pushforward is a finite complex measure with the stated support and total variation] For disjoint Borel sets Em⊆R3 the inverse images ȷ−1(Em) are disjoint and Borel by [F3], and countable additivity of λ2 gives α(⋃mEm)=A0∑mλ2(R∩ȷ−1(Em))=∑mα(Em), while α(∅)=0; hence α is a complex measure, and it is finite because λ2(R)=4ab by [F2]. If E∩S=∅ then R∩ȷ−1(E)=∅, so α is supported in S. For every countable Borel partition {Em} of R3, the defining values sum to ∣A0∣∑mλ2(R∩ȷ−1(Em))=∣A0∣λ2(R)=4ab∣A0∣ by countable additivity and [F2]; taking the supremum over partitions gives ∥α∥TV=4ab∣A0∣<+∞.

2.1step 1.1F2F3algebra

[Pairing identity] For a complex simple Borel function s, its defining sum gives ∫s dα=A0∫Rs∘ȷ dλ2, since each level set pulls back to R∩ȷ−1(E), by [F3]. For bounded Borel f, choose M>0 bounding both real coordinates. For each positive integer N, partition [−M,M] at tj:=−M+2Mj/N, 0≤j≤N, into half-open intervals [tj,tj+1) except for the last interval, which includes M. On the Borel preimage of the cell with coordinate indices j,k, set sN:=tj+itk. This gives a finite-range Borel simple function with ∥f−sN∥∞≤22M/N. By [F3], the error in the left integral is at most 22M∥α∥TV/N, and the error in the right integral is at most 8ab∣A0∣2M/N, using λ2(R)=4ab. Passing to the limit in the simple-function identities proves ∫f dα=A0∫Rf∘ȷ dλ2.

3.1step 2.1F4F5algebra

[Fraunhofer amplitude] Fix n∈S2 and put f(y):=e−ikn⋅y, which is continuous on the compact support; by step 2.1, ∫e−ikn⋅y dα(y)=A0∫Re−ik(n1y1+n2y2) dλ2(y1,y2) because y=ȷ(y1,y2) and n⋅y=n1y1+n2y2 by [F5]. The integrand is continuous on the compact rectangle R, hence Riemann integrable, so the Lebesgue integral equals the Riemann double integral by [F4], and the Riemann double integral factors as the product of the one-dimensional integrals ∫−aae−ikn1y1dy1 and ∫−bbe−ikn2y2dy2 by [F4]. For n1≠0 the map y1↦e−ikn1y1/(−ikn1) is an antiderivative with derivative e−ikn1y1 by the chain, algebra and trigonometric derivative rules of [F5], so the second fundamental theorem gives ∫−aae−ikn1y1dy1=(e−ikn1a−eikn1a)/(−ikn1)=2sin⁡(ka n1)/(kn1)=2asinc⁡(ka n1); for n1=0 the integral is 2a=2asinc⁡0 by the separate value of sinc. The second factor is analogous, giving the displayed identity 4abA0sinc⁡(ka n1)sinc⁡(kb n2) for every direction n.

4.1step 3.1F1F2algebra

[Fraunhofer intensity] For r≥2a′ the support bound ∣y∣≤a′ of [F2] puts the model in the regime of the Fraunhofer estimate of [F1] with aperture radius a′, so u(rn)=eikr(4πr)−1⋅4abA0sinc⁡(ka n1)sinc⁡(kb n2)+R(rn) with ∣R(rn)∣≤∥α∥TV(πr)−1[a′/r+k(a′)2/r] uniformly in direction by step 3.1. Taking moduli, ∣uF(rn)∣=ab∣A0∣πr ∣ ⁣sinc⁡(ka n1)∣ ∣ ⁣sinc⁡(kb n2)∣, so IF(rn)=12∣uF(rn)∣2=a2b2∣A0∣22π2r2sinc⁡2(ka n1)sinc⁡2(kb n2), and the intensity-error transfer of [F1] bounds ∣I(rn)−IF(rn)∣ by the Fraunhofer field error.

5.1step 4.1F6algebra

[Zeros of the pattern] By definition of sinc and the zero set of sine in [F6], sinc⁡ξ=0 exactly when ξ∈πZ∖{0}; since k,a,b>0 and the two factors multiply, IF(rn)=0 exactly when ka n1∈πZ∖{0} or kb n2∈πZ∖{0}.

5.2step 4.1F2F6algebra

[Global maxima] First, ∣sin⁡x∣≤∣x∣ for every real x, with equality exactly at x=0: for x>0 choose δ:=min⁡{x/3,π/6}>0; on [δ,2δ]⊆[0,π/2] the cosine is positive and strictly decreasing, so ∣cos⁡t∣=cos⁡t≤cos⁡δ<1 there, while ∣cos⁡t∣≤1 everywhere; the second fundamental theorem, the integral triangle bound and monotonicity of [F6] give ∣sin⁡x∣=∣∫0xcos⁡t dt∣≤∫0x∣cos⁡t∣ dt≤(x−δ)+δcos⁡δ=x−δ(1−cos⁡δ)<x, and for x<0 oddness of sine in [F6] gives ∣sin⁡x∣=∣sin⁡(−x)∣<∣x∣, while x=0 gives equality. Hence ∣ ⁣sinc⁡ξ∣≤1 with equality exactly at ξ=0; since k,a,b>0, the modulus of the product in step 4.1 equals the prefactor exactly when ka n1=0 and kb n2=0, that is n1=n2=0, which on S2 means n=(0,0,±1); these two poles are therefore exactly the global maxima of the pattern on S2, and the model's half-space x3>0 forces n3>0, leaving only the positive pole.

6.1step 3.1step 4.1step 5.2F1F3F4given∎

[Scope and choice accounting] The computation is exact inside the adopted scalar model: by [F1] it inherits the Fraunhofer and Fresnel error bounds rather than re-estimating them, it makes no measured-intensity, apparatus, sampling or calibration claim, and no polarization or screen-interaction content is introduced; only model comparisons of I between observation points are read from it, which is clause (c). The only choice principle used is the Countable Choice carried by the parent theorem and by the Riemann-Lebesgue comparison and Fubini interfaces of [F4]; the remaining steps are explicit finite computations, so no further choice is made.

Remarks

  • The pattern's mathematical maxima on the whole sphere sit at the two poles, but the model field is only defined for x3>0; the observable central maximum is the single direction n=(0,0,1), and the lower pole belongs to the extension of the formula, not to the model's observation region.
  • The zeros ka n1∈πZ∖{0} or kb n2∈πZ∖{0} are exact zeros of the ideal Fraunhofer factor; a finite measurement could not verify exact darkness, and no such empirical claim is made here.

Sources