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Direction of polarization for monochromatic wave?

  1. Dec 9, 2015 #1
    1. The problem statement, all variables and given/known data
    Write down the (real) electric and magnetic fields for a monochromatic plane wave of amplitude Eo, frequency w, and phase angle zero traveling in the direction from the origin to the point (1,1,1) with polarization parallel to the xz plane.

    I understand how to write the equations, I just don't understand how to get the correct direction for the electric and magnetic fields.

    2. Relevant equations
    ## E(z,t) = E_o\cos(\hat k \cdot \hat r - \omega t) \hat n ##
    ## B(z,t) = \frac{E_o}{c}\cos(\hat k \cdot \hat r - \omega t) ( \hat k x \hat n) ##
    ## k = -\frac{\omega}{c} ##
    ## \hat n \cdot \hat k = 0 ##

    3. The attempt at a solution
    This is what I did:

    ## \hat n = \hat x + \hat z ##
    ## \hat k = \frac{\omega}{c} (\hat x + \hat y + \hat z) ##

    So I thought that was all I was supposed to do to find the direction, but the solutions manual says these are the actual directions of n and k:

    ## \hat n =\frac { \hat x - \hat z}{\sqrt{2}} ##
    ## \hat k = \frac{\omega}{c} \frac{(\hat x + \hat y + \hat z)}{\sqrt{3}} ##

    So where did those factors of sqrt(2) and sqrt(3) come from?
    I appreciate any help!
     
  2. jcsd
  3. Dec 9, 2015 #2

    blue_leaf77

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    First, the question asks you to find the unit vector, so the magnitude of the vector which is supposed to be the answer should be unity. Second, you only know that ##\hat{n}## only has components along ##\hat{x}## and ##\hat{z}## but you are not given the length of each component, these are what you should calculate subject to the condition that the length of ##\hat{n}## is unity and that this vector is perpendicular to ##\hat{k}##.
     
  4. Dec 9, 2015 #3
    I don't think I understand.
    So I would do
    ## n = \sqrt(a^2 + b^2)) = 1## Therefore: ##a^2= 1-b^2##
    ## k = \sqrt(c^2 + d^2 + e^2) = 1##
    And then use this somehow:
    ## \hat n \cdot \hat k = nkcos\theta = 0##
    ## nkcos\theta = \sqrt((1-b^2) + b^2)\sqrt(c^2 + d^2 + e^2)cos\theta##
    But since n = 1 and k =1, wouldn't that just leave me with nothing again?
     
  5. Dec 9, 2015 #4

    blue_leaf77

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    If ##\mathbf{k}## is denoted such that it has components ##c##, ##d##, and ##e## then they must be known already since the problem tells you that ##\mathbf{k}## goes from the origin to the point (1,1,1). What you don't know yet are just ##a## and ##b##, i.e. two unknowns. You have figured out one equation relating these unknowns, which is
    .
    The other equation you need is the orthogonality condition between ##\mathbf{k}## and ##\hat{n}##. To do this, it will be easier with component-by-component multiplication instead of the one like ##kn\cos \theta##.
     
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