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The direction of polarization of a transverse wave is defined to be the direction perpendicular to the direction of propagation of wave or the direction of oscillation of wave, right? But in the case of Electromagnetic waves, a class of transverse waves, there are two directions of oscillations that are perpendicular to the direction of propagation of the wave, the direction in which the electric field oscillates and the direction in which the magnetic field oscillates. But while referring to the direction of polarization of an Electromagnetic wave, only the direction of oscillation of Electric field is called the direction of polarization. But why isn't defined to be the direction of Magnetic field?

For this I thought like this: "For transverse waves like waves on a string, to determine the direction of polarization, just put a plane with slit in different directions and the direction perpendicular to the direction of slit in which the wave completely gets absorbed is its polarization direction."

Now, if I apply the same logic to the Electromagnetic waves by replacing the plane-with-slit with a material that can interact with the electric or magnetic components of the electromagnetic field, i.e., a polarizer, then I can define exactly what the polarization direction of EM wave is. First let us introduce a polarizer that can interact with electric field. Generally these polarizers contain some long chain linear molecules placed in the same direction. If an EM wave is allowed to fall on this polarizer, with the normal of plane of polarizer being in the direction of propagation of EM wave, in different directions each time, then the electric component of EM wave gets blocked if the direction of electric component and the axis of the long chain linear molecules are parallel. Hence the polarization of EM wave is along the direction perpendicular to the axis of such long linear molecules or simply the direction of Electric field.

Now if I conduct the exact experiment with such a polarizer that can interact with magnetic field then the polarization of the EM wave comes out to be the direction of oscillation of magnetic field. But when direction of polarization of EM wave is referred, only the direction of oscillation of Electric component of EM wave is considered. Why is that so? Don't the polarizers that can interact with magnetic fields exist? Or between the both directions of polarizations only the direction of Electric component of EM wave is considered conventionally because of simplicity? Or am I missing something?

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It's a semi-arbitrary choice. Not completely arbitrary since, in practice, we more commonly sense EM waves by their electric fields rather than their magnetic fields.

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  • $\begingroup$ @AnnaV please tell me if you agree that this comment (from memory, I might add) was irrelevant: "loop antennas measure the B field and are as common as linear antennas that measure the E field." To which I also would add now that the fact how loops are used to measure B underlies the view that the B field acts in a plane because for maximum reception one must turn the loop in that "plane of action" to be coincident with it. $\endgroup$
    – hyportnex
    Commented May 24, 2023 at 19:30
  • $\begingroup$ @hyportnex If we had magnetic charges, we could make linear magnetic antennas. It's the lack of magnetic charge that creates the distinction you wish to make. $\endgroup$
    – John Doty
    Commented May 24, 2023 at 19:49
  • $\begingroup$ I have no idea how the world would look like if it were not the kind we have, I can barely deal with the one we have let alone with something that we do not. $\endgroup$
    – hyportnex
    Commented May 25, 2023 at 0:26
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It is a natural choice because the $B$ field is not a vector in the usual sense, but rather it is a bi-vector representing a plane of action coincident with the polarization vector of the $E$ field. The bi-vector is not a vector, geometrically it represents a plane spanned by two vectors, and is an anti-symmetric tensor with 3 components that constitute the so-called axial-vector representation in which the plane of the bi-vector is perpendicular to this axial vector. Since both the $E$ vector and the anti-symmetric tensor of $B$ act in the same plane it is natural to assign that the polarization.

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  • $\begingroup$ I believe the choice came before this distinction was commonly recognized by physicists. $\endgroup$
    – John Doty
    Commented May 24, 2023 at 18:19
  • $\begingroup$ @JohnDoty almost certainly you are correct, and it just shows how brilliantly prescient these electrodynamicists can be! Strangely my previous comment to you under your answer here just disappeared without a trace; hmm, what could have happened with it? $\endgroup$
    – hyportnex
    Commented May 24, 2023 at 18:24
  • $\begingroup$ No idea, nor do I know where my response went. $\endgroup$
    – John Doty
    Commented May 24, 2023 at 18:33
  • $\begingroup$ Moderators may remove comments they decide are irrelevant to the subject without leaving a comment. $\endgroup$
    – anna v
    Commented May 24, 2023 at 19:07

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