Óptica Moderna Fundamentos e aplicações - Fotonica.ifsc.usp.br ...
Óptica Moderna Fundamentos e aplicações - Fotonica.ifsc.usp.br ...
Óptica Moderna Fundamentos e aplicações - Fotonica.ifsc.usp.br ...
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Difração 165<<strong>br</strong> />
S1<<strong>br</strong> />
n ) 1<<strong>br</strong> />
Volume de interesse<<strong>br</strong> />
Fig. 8.4 - Geometria utilizada para o cálculo da integral de superfície.<<strong>br</strong> />
Queremos encontrar o valor da função U no ponto de observação<<strong>br</strong> />
P e para isto tomaremos V(r,t) como sendo uma onda esférica da forma<<strong>br</strong> />
V( r, t)<<strong>br</strong> />
= V0exp{<<strong>br</strong> />
i(<<strong>br</strong> />
kr − ωt)<<strong>br</strong> />
} /r. O gradiente em coordenadas esféricas é<<strong>br</strong> />
dado por:<<strong>br</strong> />
1<<strong>br</strong> />
θ φˆ<<strong>br</strong> />
rsenθ φ<<strong>br</strong> />
ˆ<<strong>br</strong> />
∂ 1 ∂ ∂<<strong>br</strong> />
∇ = rˆ + +<<strong>br</strong> />
∂ r r ∂ θ ∂<<strong>br</strong> />
r<<strong>br</strong> />
(8.7)<<strong>br</strong> />
de forma que a integral de superfície em S2 fica:<<strong>br</strong> />
r r<<strong>br</strong> />
J V∇U<<strong>br</strong> />
- U∇V<<strong>br</strong> />
. nˆ dS<<strong>br</strong> />
⎡V<<strong>br</strong> />
S. C. Zilio <strong>Óptica</strong> <strong>Moderna</strong> – <strong>Fundamentos</strong> e Aplicações<<strong>br</strong> />
P<<strong>br</strong> />
ρ<<strong>br</strong> />
n ) 2<<strong>br</strong> />
( ) =<<strong>br</strong> />
= ∫∫ 2 2<<strong>br</strong> />
S2<<strong>br</strong> />
{ i ( kr − ωt)<<strong>br</strong> />
}<<strong>br</strong> />
r<<strong>br</strong> />
r ⎛ exp<<strong>br</strong> />
S2<<strong>br</strong> />
{ i ( kr - ωt)<<strong>br</strong> />
}<<strong>br</strong> />
⎞⎤<<strong>br</strong> />
. nˆ<<strong>br</strong> />
0<<strong>br</strong> />
∫∫ exp<<strong>br</strong> />
U - UV0<<strong>br</strong> />
S ⎢<<strong>br</strong> />
∇ ∇ ⎜<<strong>br</strong> />
⎟<<strong>br</strong> />
2 r<<strong>br</strong> />
r<<strong>br</strong> />
⎥<<strong>br</strong> />
⎣<<strong>br</strong> />
⎝<<strong>br</strong> />
⎠⎦<<strong>br</strong> />
2<<strong>br</strong> />
onde = ρ dΩ e nˆ = −rˆ<<strong>br</strong> />
, que substituidos na eq. (8.8) resulta em:<<strong>br</strong> />
dS2 2<<strong>br</strong> />
J = V e<<strong>br</strong> />
0<<strong>br</strong> />
−iωt<<strong>br</strong> />
∫∫<<strong>br</strong> />
S2<<strong>br</strong> />
⎡e<<strong>br</strong> />
⎢<<strong>br</strong> />
⎣ r<<strong>br</strong> />
ikr<<strong>br</strong> />
r<<strong>br</strong> />
∇U<<strong>br</strong> />
- Ue<<strong>br</strong> />
ikr<<strong>br</strong> />
⎛ 1 ik ⎞ ⎤<<strong>br</strong> />
⎜ − + rˆ<<strong>br</strong> />
2 ⎟ ⎥<<strong>br</strong> />
⎝ r r ⎠ ⎦<<strong>br</strong> />
r=<<strong>br</strong> />
ρ<<strong>br</strong> />
.<<strong>br</strong> />
2 ( − rˆ ) ρ dΩ<<strong>br</strong> />
2<<strong>br</strong> />
dS<<strong>br</strong> />
2<<strong>br</strong> />
(8.8)