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Question in reflection and transmission at oblique incidence.

  1. May 24, 2013 #1
    I am reading Griffiths p387. It is my understanding that
    [tex]\tilde E(\vec r,t)=\hat r E_0 e^{j(\omega t-kr)}[/tex]
    Where ##\vec r =\hat x x+\hat y y+ \hat z z## is the positional vector from the origin to the observation point ( x,y,z).
    [tex]\Rightarrow\;\tilde E(\vec r,t)=\hat r E_0 e^{-jkr}\;=\;\left(\frac {\hat x x+\hat y y+ \hat z z}{\sqrt{x^2+y^2+z^2}}\right) E_0e^{j[-(\hat x k_x+\hat y k_y+\hat z k_z)\cdot(\hat x x+\hat y y+\hat z z)]}\;=\; \left(\frac {\hat x x+\hat y y+ \hat z z}{\sqrt{x^2+y^2+z^2}}\right) E_0e^{-j[( x k_x+ y k_y+z k_z)]} [/tex]


    In Griffiths, he let the incident TEM wave travels in ##\vec k_I## direction. So he let
    [tex]\vec E_I(\vec r,t)= \vec E_{0I} e^{j(\omega t - \vec k_I\cdot \vec r)},\;\vec E_R(\vec r,t)= \vec E_{0R} e^{j(\omega t - \vec k_R\cdot \vec r)},\;\vec E_T(\vec r,t)= \vec E_{0T} e^{j(\omega t - \vec k_T\cdot \vec r)},\;[/tex]
    To expand one out:
    [tex]\vec E_I(\vec r,t)= \vec E_{0I} e^{j(\omega t - \vec k_I\cdot \vec r)}=\hat x E_{0I}e^{j[\omega t - (xk_{Ix}+yk_{Iy}+zk_{Iz})]}[/tex]

    I have a problem with this, as you can see from the scanned page, the direction of the incident, reflected and transmitted wave is in direction of ##\vec k_I,\;\vec k_R\;\hbox { and } \;\vec k_T##. But he gave all three as ##\vec E(\vec r,t)##. This mean all three are in ##\vec r## direction. That is not right.
    Later, he actually equated
    [tex]xk_{Ix}=xk_{Rx}\;\Rightarrow\;k_{Ix}=k_{Rx}[/tex]
    This mean he used the same ##\vec r## in all three, that is questionable. I am not saying the final result is wrong, just the representation is questionable.
    Please help.

    Thanks
     

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  2. jcsd
  3. May 25, 2013 #2
    I think I got some of the explanation from Cheng's book that ##e^{\vec k\cdot \vec r}## is just to give ##e^{-jk_xx}e^{-jk_yy}e^{-jk_zz}=e^{-j(k_xx+k_yy+k_zz)}##.....which is like ##e^{-jkz}## in z direction.

    But I still have a question. It is obvious that in the book, the incident wave is traveling in ##\hat k_I## direction, reflected wave in ##\hat k_R## direction and transmitted wave in ##\hat k_T## direction.

    Why this book and other books all call ##\vec E(\vec r, t)## and not ##\vec E(\vec k_I, t)##? The incident wave IS traveling in ##\vec k_I## direction as indicated. Or this is just a general way of saying the direction of propagation has xyz components, not just z?
     
    Last edited: May 25, 2013
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