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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166<<strong>br</strong> />
−iωt<<strong>br</strong> />
= V0e<<strong>br</strong> />
∫∫<<strong>br</strong> />
S2<<strong>br</strong> />
ik<<strong>br</strong> />
⎡e<<strong>br</strong> />
⎢<<strong>br</strong> />
⎣ ρ<<strong>br</strong> />
ρ<<strong>br</strong> />
r<<strong>br</strong> />
ikρ⎛<<strong>br</strong> />
1 ik ⎞⎤<<strong>br</strong> />
2<<strong>br</strong> />
( − ∇U.<<strong>br</strong> />
rˆ ) + Ue ⎜−<<strong>br</strong> />
+ ⎟ ρ dΩ<<strong>br</strong> />
r=<<strong>br</strong> />
ρ<<strong>br</strong> />
Tomando o limite ρ → 0 obtemos<<strong>br</strong> />
ρ<<strong>br</strong> />
⎥<<strong>br</strong> />
⎠⎦<<strong>br</strong> />
Difração<<strong>br</strong> />
S. C. Zilio <strong>Óptica</strong> <strong>Moderna</strong> – <strong>Fundamentos</strong> e Aplicações<<strong>br</strong> />
⎝<<strong>br</strong> />
ρ<<strong>br</strong> />
2<<strong>br</strong> />
(8.9)<<strong>br</strong> />
= −V<<strong>br</strong> />
exp{<<strong>br</strong> />
− iωt}<<strong>br</strong> />
U(<<strong>br</strong> />
P)∫<<strong>br</strong> />
dΩ<<strong>br</strong> />
=<<strong>br</strong> />
J 0<<strong>br</strong> />
− 4πV0 exp{<<strong>br</strong> />
− iωt}<<strong>br</strong> />
U(P)<<strong>br</strong> />
∫∫ ∫∫ ∫∫ =<<strong>br</strong> />
=<<strong>br</strong> />
⎡<<strong>br</strong> />
4πV<<strong>br</strong> />
exp<<strong>br</strong> />
exp<<strong>br</strong> />
0<<strong>br</strong> />
{ i ( kr − ωt)<<strong>br</strong> />
}<<strong>br</strong> />
. Logo, como = + 0 temos:<<strong>br</strong> />
A S1 S2<<strong>br</strong> />
{ iωt}<<strong>br</strong> />
U(<<strong>br</strong> />
P)<<strong>br</strong> />
= ( V∇U<<strong>br</strong> />
- U∇V)<<strong>br</strong> />
. nˆ dS =<<strong>br</strong> />
− ∫∫S1 r<<strong>br</strong> />
r<<strong>br</strong> />
r<<strong>br</strong> />
{ i ( kr − ωt)<<strong>br</strong> />
} ⎜−<<strong>br</strong> />
+ ⎟ rˆ . nˆ 1dS1<<strong>br</strong> />
∫∫S ⎢V0<<strong>br</strong> />
∇U<<strong>br</strong> />
− UV0<<strong>br</strong> />
exp<<strong>br</strong> />
2<<strong>br</strong> />
1 r<<strong>br</strong> />
r r<<strong>br</strong> />
⎥<<strong>br</strong> />
⎣<<strong>br</strong> />
⎝ ⎠ ⎦ S1<<strong>br</strong> />
que nos leva à equação básica da teoria da difração:<<strong>br</strong> />
4πU<<strong>br</strong> />
⎡e<<strong>br</strong> />
r<<strong>br</strong> />
ikr<<strong>br</strong> />
⎛ ⎞<<strong>br</strong> />
∫∫S ⎢<<strong>br</strong> />
2 ⎥<<strong>br</strong> />
1 ⎣ r ⎝ r r ⎠ ⎦ S1<<strong>br</strong> />
⎛<<strong>br</strong> />
1<<strong>br</strong> />
1<<strong>br</strong> />
1<<strong>br</strong> />
ik ⎞<<strong>br</strong> />
ikr<<strong>br</strong> />
( P)<<strong>br</strong> />
= ∇U<<strong>br</strong> />
− U ⎜ − + ⎟ rˆ e . nˆ 1dS1<<strong>br</strong> />
1<<strong>br</strong> />
ik<<strong>br</strong> />
⎤<<strong>br</strong> />
⎤<<strong>br</strong> />
(8.10)<<strong>br</strong> />
(8.11)<<strong>br</strong> />
Esta expressão é chamada de teorema integral de Kirchhoff. Ela<<strong>br</strong> />
relaciona o valor da função no ponto de observação P com valores desta<<strong>br</strong> />
função e sua derivada so<strong>br</strong>e a superfície S1 que envolve o ponto P. Como<<strong>br</strong> />
tomamos ρ → 0, a Fig. 8.4 se modifica da maneira mostrada na Fig. 8.5.<<strong>br</strong> />
Particularizando a eq. (8.11) para o caso em que U é também uma onda<<strong>br</strong> />
esférica da forma:<<strong>br</strong> />
U 0<<strong>br</strong> />
U( r1,<<strong>br</strong> />
t)<<strong>br</strong> />
= exp{<<strong>br</strong> />
i ( kr1<<strong>br</strong> />
− ωt)}<<strong>br</strong> />
(8.12)<<strong>br</strong> />
r<<strong>br</strong> />
1<<strong>br</strong> />
o teorema integral de Kirchhoff pode ser escrito de forma mais explícita<<strong>br</strong> />
como:<<strong>br</strong> />
4<<strong>br</strong> />
( P)<<strong>br</strong> />
−iωt<<strong>br</strong> />
πU = ikU 0e<<strong>br</strong> />
∫∫S<<strong>br</strong> />
1<<strong>br</strong> />
exp<<strong>br</strong> />
{ ik(<<strong>br</strong> />
r1<<strong>br</strong> />
+ r2<<strong>br</strong> />
) }<<strong>br</strong> />
[ cosθ1<<strong>br</strong> />
− cosθ<<strong>br</strong> />
2 ] dS1<<strong>br</strong> />
r r<<strong>br</strong> />
1 2<<strong>br</strong> />
ikr2<<strong>br</strong> />
ikr1<<strong>br</strong> />
−iωt<<strong>br</strong> />
⎡e<<strong>br</strong> />
e ⎤<<strong>br</strong> />
− U 0e<<strong>br</strong> />
∫∫ ⎢ cos θ1<<strong>br</strong> />
− cos θ 2 ⎥ dS1<<strong>br</strong> />
(8.13)<<strong>br</strong> />
S 2<<strong>br</strong> />
2<<strong>br</strong> />
1 ⎣r2<<strong>br</strong> />
r1<<strong>br</strong> />
r1r2<<strong>br</strong> />
⎦