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EM Waves and Polarization

  1. Oct 26, 2011 #1
    Hi

    I don't understand the polarization of EM waves. I have same questions about that.

    1-) As you know, EM wave is consist of both electric and magnetic field/wave. And there is 90 degree between them. Is that possible to separate these two fields or waves? I mean is that rule: If there is a magnetic wave so there must be a electric wave and vice versa?


    2-) Related with my first question, what is polarization of Em wave? For vertical polarization, is magnetic field eliminated or for horizontal polarization, is electric field?

    3-) To be able to imagine polarization I am thinking of 3d movies. How can one em wave or light be vertical or horizontal? What makes them vertical or horizontal or none of them? Is human eye capable of notice the differences between vertical or horizontal polarized or non-polarized light?

    Thanks.
     
  2. jcsd
  3. Oct 26, 2011 #2

    cepheid

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    Welcome to PF
    Yeah, it's a rule. There must be both an electric field and a magnetic field, and the two of them must be perpendicular.

    We define the polarization of the EM wave as the direction in which its E-field points. In other words, differently polarized EM waves have their E-fields pointing in different directions. We ignore the direction of the B-field, which will always just be perpendicular to the E-field anyway.


    The EM wave travels in one direction, and the E-field and B-field vectors must both be perpendicular to this direction of travel. But if you think about it, what this means is that the E-field is confined to oscillate in the plane that is perpendicular to the direction of travel. But it can point in any direction in that plane. The polarization is a description of which direction the E-field is pointing in within this plane. I did a Google image search for polarization and came up with many helpful diagrams to visualize this.

    No, I don't think the human eye, just by itself, can detect a difference between different polarizations of light.
     
    Last edited: Oct 26, 2011
  4. Oct 26, 2011 #3

    Andy Resnick

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    some people can:

    http://en.wikipedia.org/wiki/Haidinger's_brush
     
  5. Oct 27, 2011 #4
    Thank you.

    I have been looking polarization and I mostly understand it. I have just 2 more questions about that.

    1- )
    Is there a way to separate electric wave and magnetic wave? I mean can we see a light that is just made of electric wave (or magnetic wave)? If possible, Does still remains as a light? Can our naked eyes notice the difference?

    2-) Can we say in nature all Em waves are circular polarized by default? Or, for example, sun produces EM waves with random polarization and we can't say specific thing about that?
     
  6. Oct 27, 2011 #5
    No, you cannot separate EM field. According to Maxwell, change of electric field gives rise to magnetic field, vise-versa. Basically, in EM waves, fields are constantly changing, thus you cannot have only one of them. I believe in more advanced physics, they are sorted as the same type of interaction, namely electromagnetic interaction. They are just different sides of one thing.
     
  7. Oct 27, 2011 #6
    Since in waves, fields are oscillating, while meeting certain obstacles, there would be induced current. For example, while meeting a metal grating the oscillating field would cause electrons in the grating to move thereby absorbing energy.

    And I bet you cannot tell the difference. I've heard that some 3D movies make use of polarization. If your eyes can tell the difference, I think what you see would become funny! Also some sunglasses also make use of polarization. There isn't really difference except that it is darker.
     
  8. Oct 27, 2011 #7

    Andy Resnick

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    In this context, it's important to note that 'polarization' is a statistical measure of light, related to the amount of coherence. Light can be randomly polarized, partially polarized, or fully polarized. Sunlight (or any blackbody radiation) is randomly polarized, but atmospheric scattering generates a net linear polarization in some directions (right angles to the sun). Certain clouds also generate polarization (noctilucent clouds), rainbows are partially polarized, and some insects (Cetonia) generate iridescence through polarization effects.
     
  9. Oct 27, 2011 #8
    I find this hard to believe..or understand.....Are they trying to tell us on the opposite side of a building from a source of EMF propagation, there is a magnetic field but not an electric field??
     
  10. Oct 27, 2011 #9
    But when electric field is blocked, how can there be electromagnetic induction? No flux change in electric field means no magnetic field induced.
     
  11. Oct 27, 2011 #10
    I don't think they mean electromagnetic waves but rather quasi-static fields. Their concern is with the fields produced by power lines and appliances.
    I am not sure what they mean by the electric field produced by a hair-dryer for example. Maybe the original EPA document may be more illuminating.
     
  12. Oct 28, 2011 #11
    As post # 2 implies, I think, nobody does!!...

    as soon as you see "principle", "rule", or the like you've got a pretty good indication that's about as far as science can take us....rather than back to first principles.

    What I still find mind numbing is how people like Einstein, Lorentz, and in this case Maxwell
    ever managed to describe this kind of thing quantatively....
     
  13. Nov 6, 2011 #12

    Claude Bile

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    1. It is not possible to separate E and B fields in an EM wave. A quick examination of Maxwells equations will demonstrate this to be so. If there is a time-varying B field, an E field must exist (and vice-versa).

    2. The polarization of an EM wave is typically defined by the direction of the E-field. Note that this is not necessarily constant. I suggest looking up "Poincare sphere" to get a complete idea.

    3. The polarization of a wave is dependent on lots of things. Essentially the direction the charge "wiggles" will determine the polarization, though it is possible to change the polarization by placing optics between the source and observer. While most human eyes cannot directly distinguish polarization (though many insects can), all one needs is a good pair of sunglasses to do so! For linear polarizations anyway. Devices such as quarter wave plates can help us distinguish other polarization states.

    Claude.
     
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