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Total internal reflection: decaying transmitted field, vanishing normal flux and the reflection phase

Physical scope

Deterministic example of total internal reflection in the lossless interface model for declared indices n_1=3/2, n_2=1 at sin(theta_i)=4/5: the unit-modulus reflection factor with its phase, the evanescent transmitted field with decay factor e^{-beta x_3}, the vanishing normal and positive tangential averaged fluxes. Excluded: measured indices, absorption, information-speed or signal-propagation claims, and every empirical statement.

Example

Adopt the setting of Oblique-incidence TE and TM Fresnel amplitudes, the flux budget, Brewster's angle and total internal reflection with equal permeabilities μ1=μ2=:μ>0 and declared indices n1=3/2, n2=1 (declared model parameters, not measurements), choose the tangential wavevector κ=(6ω/(5c))e1, so κ^=e1 and y^=n×κ^=e2, and take the transverse-electric polarisation ej=Ejy^. Let the incidence angle be given by sin⁡θi=4/5, which exceeds the critical angle θc with sin⁡θc=n2/n1=2/3, so ∣κ∣=K1sin⁡θi=(6/5)(ω/c)>(ω/c)=K2. Then with β=∣κ∣2−K22=(11/5)(ω/c) and k13=(ω/c)(3/2)(3/5)=(9/10)(ω/c): (i) [Reflection] Y1s=9/(10μc), Y2s=i11/(5μc) and rs=9/10−i11/59/10+i11/5,∣rs∣=1,ts=9/59/10+i11/5,∣ts∣2=324125; the reflection factor is the phase rs=e−2iφ with φ=arctan⁡ ⁣(211/9)≈0.63514703 (about 36.39124∘), i.e. the reflected wave acquires the phase −2φ≈−1.27029406 (about −72.78249∘); (ii) [Transmitted field] the transmitted field is Et(x,t)=e−βx3Re⁡(Ety^ ei(∣κ∣x1−ωt)) with β=(11/5)(ω/c): it decays by the factor e−11/5≈0.51514 when x3 increases by c/ω, its period-averaged normal flux vanishes identically, ⟨St⋅n⟩≡0, while its tangential averaged flux is positive, ⟨St⋅κ^⟩=e−2βx3 (6/5)(ω/c)∣Et∣22μω=e−2βx33∣Et∣25μc; (iii) [Flux budget] ∣rs∣2+(Re⁡Y2s/Y1s)∣ts∣2=1+0=1, so the whole incident normal averaged flux is reflected, while the evanescent field carries a tangential averaged flux only; (iv) [Scope] the example is an exact deterministic computation of the lossless interface model for declared parameters; it reports no measurement, and the presence of a tangential evanescent flux is not a claim about information speed or signal propagation.

Verification

technique · direct substitution in the oblique-incidence and medium plane-wave theorems with exact rational and quadratic-irrational arithmetic, and an explicit phase computation via Euler's form

Given: The lossless two-media setting of Oblique-incidence TE and TM Fresnel amplitudes, the flux budget, Brewster's angle and total internal reflection with μ1=μ2=μ>0, n1=3/2, n2=1, the tangential wavevector κ=(6ω/(5c))e1, the transverse-electric data ej=Ejy^ with y^=e2, and the incidence data sin⁡θi=4/5.

[A1] The oblique-incidence theorem Oblique-incidence TE and TM Fresnel amplitudes, the flux budget, Brewster's angle and total internal reflection: clause (a) and (d) give Y1s=k13/(μ1ω), Y2s=kt3/(μ2ω), the amplitude formulas, and in the evanescent branch ∣rs∣=1, ∣ts∣2=4(Y1s)2/((Y1s)2+∣Y2s∣2)>0 together with the flux budget; clause (d) also records that the transmitted field decays and its normal averaged flux vanishes.

[A2] The medium plane-wave structure Plane-wave fields in a homogeneous Ohmic medium: amplitude conditions, dispersion, averaged flux and the evanescent branch: the transmitted evanescent field of clause (e) with the substitution n′=n, κ0=κ: the exact factorization, the vanishing normal component ⟨S⋅n′⟩≡0 and the positive tangential averaged flux ⟨S⋅κ^0⟩=∣κ0∣∣e∣2e−2βn′⋅x/(2μω) for the transverse-electric data e=E0(n′×κ^0).

[F1] Medium parameters (Homogeneous isotropic medium parameters: phase speed, impedance, refractive index, complex propagation constant and skin depth): Kj=ω(εjμj)1/2=ωnj/c and εj=nj2/(c2μ) for the common permeability. The selected evanescent wavevectors in Oblique-incidence TE and TM Fresnel amplitudes, the flux budget, Brewster's angle and total internal reflection give k132=K12−∣κ∣2, β2=∣κ∣2−K22 and kt3=iβ.

[F2] Complex arithmetic and modulus (Conjugation is an involutive real-field automorphism, zz‾=∣z∣2, and modulus is definite, multiplicative, and subadditive): z+w‾=z‾+w‾, zw‾=z‾ w‾, ∣zw∣=∣z∣∣w∣, ∣z∣2=zz‾ and ∣z∣≥0 with ∣z∣=0 only for z=0; in particular a complex number of modulus one is nonzero and the sum of a complex number and its conjugate is twice its real part.

[F3] Trigonometry and phases: arctan⁡ is the principal inverse tangent with tan⁡(arctan⁡y)=y (The principal inverse tangent arctan⁡:R→(−π/2,π/2)); tan⁡x=sin⁡x/cos⁡x (Tangent, cotangent, secant, and cosecant on their exact natural domains); sin⁡2x+cos⁡2x=1 and 1+tan⁡2x=sec⁡2x (Pythagorean and parity identities for all six trigonometric functions on their natural domains); cosine is strictly decreasing on [0,π] with cos⁡(π/2)=0 (Signs, monotonicity intervals, and ranges of sine and cosine, The zero sets of sine and cosine and the least positive common period 2 pi); and for real y, exp⁡(iy)=cos⁡y+isin⁡y with ∣exp⁡(iy)∣=1 (exp⁡(x+iy)=ex(cos⁡y+isin⁡y), ∣exp⁡(x+iy)∣=ex, and eiπ+1=0).

1.1givenA1F1algebra

[The geometry and the evanescent data] With κ=(6ω/(5c))e1 one has κ^=e1, ∣κ∣=(6/5)(ω/c) and y^=n×κ^=e3×e1=e2; K1=(3/2)(ω/c), K2=ω/c by [F1], so ∣κ∣=K1sin⁡θi with sin⁡θi=4/5, and ∣κ∣>K2 because 6/5>1. Hence the evanescent branch applies: β=∣κ∣2−K22=(36/25−1)(ω/c)=11/25 (ω/c)=(11/5)(ω/c) and k13=K12−∣κ∣2=(9/4−36/25)(ω/c)=81/100 (ω/c)=(9/10)(ω/c); the critical angle satisfies sin⁡θc=n2/n1=2/3<4/5=sin⁡θi, both angles being in (0,π/2).

2.1step 1.1A1F2algebra

[Admittances and reflection data, clause (i)] By A1 and [F1], Y1s=k13/(μ1ω)=9/(10μc), Y2s=kt3/(μ2ω)=iβ/(μω)=i11/(5μc) and ts=2Y1s/(Y1s+Y2s)=(9/(5μc))/((9/10+i11/5)/(μc))=(9/5)/(9/10+i11/5); the numerator 9/10−i11/5 of rs=(Y1s−Y2s)/(Y1s+Y2s) is the complex conjugate of its denominator, so ∣rs∣=1 by [F2] (and A1). For the transmitted modulus, ∣ts∣2=4(Y1s)2/((Y1s)2+∣Y2s∣2) by A1; computing (Y1s)2=81/(100μ2c2) and ∣Y2s∣2=11/(25μ2c2)=44/(100μ2c2) gives ∣ts∣2=(324/100)/(125/100)=324/125.

2.2step 1.1A1A2algebra

[The transmitted field and its fluxes, clause (ii)] By ∣ts∣2-normalised data we take the transmitted electric amplitude Ety^ with Et=tsEi; the evanescent structure of [A2] applied with n′=n=e3, κ0=κ, β as in step 1.1 and e=Ety^=Et(n′×κ^0) gives exactly Et(x,t)=e−βx3Re⁡(Ety^ ei(∣κ∣x1−ωt)), the identity ⟨St⋅n⟩≡0, and the tangential averaged flux ⟨St⋅κ^⟩=e−2βx3∣κ∣∣Et∣2/(2μω); substituting ∣κ∣=(6/5)(ω/c) gives ⟨St⋅κ^⟩=e−2βx3(6/5)(ω/c)∣Et∣2/(2μω)=e−2βx33∣Et∣2/(5μc). The decay over a distance c/ω is the factor e−βc/ω=e−11/5, whose decimal display in clause (ii) is the stated rounding.

3.1step 2.1F2F3algebra

[The reflection phase, clause (i)] Put Y1:=9/10 and β′:=11/5 in the units 1/(μc) and α:=β′/Y1=211/9, so that rs=(1−iα)/(1+iα); let φ:=arctan⁡α∈(0,π/2) and w:=1+iα, ∣w∣=1+α2. By [F3], tan⁡φ=α and 1+tan⁡2φ=sec⁡2φ, so cos⁡2φ=1/(1+α2)=1/∣w∣2; since cos⁡φ>0 (cosine is strictly decreasing on [0,π] with cos⁡(π/2)=0 and φ∈(0,π/2)), cos⁡φ=1/∣w∣ and sin⁡φ=αcos⁡φ=α/∣w∣. Euler's form of [F3] then gives w=∣w∣(cos⁡φ+isin⁡φ)=∣w∣eiφ and w‾=∣w∣e−iφ by [F2], so rs=w‾/w=e−2iφ. The decimal values displayed in clause (i) are the stated roundings of φ=arctan⁡(211/9) and of the reflected phase −2φ.

4.1givenstep 2.1step 2.2A1F2∎

[Flux budget and scope, clauses (iii)-(iv)] By A1, ∣rs∣2+(Re⁡Y2s/Y1s)∣ts∣2=1; here Re⁡Y2s=0 by step 2.1, so the identity reads 1+0=1: the whole incident normal averaged flux is reflected, while the tangential averaged flux of step 2.2 is positive and transports no normal flux. All quantities are declared model parameters; the computation is deterministic classical interface algebra, reports no measurement, and neither the tangential evanescent flux nor the reflection phase is a claim about information speed, signal propagation or empirical data. No choice principle is used.

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