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Electromagnetic Waves' Magnetic Component

  1. Jun 17, 2012 #1
    This is just a conceptual question that I've been personally wondering about. So light has a magnetic field wave perpendicular to an electric field wave. I don't quite understand the magnetic field wave though.

    I thought that magnetic fields were not supposed to have a beginning or end. They continuously travel in a loop from north pole to south pole. How then does light's magnetic fields work? Where are the north and south poles and where are the loops? How is it that the diagrams just depict linear magnetic field vectors?

    Diagram example: http://www.google.com/imgres?um=1&hl...,r:8,s:0,i:164

    I have googled this to a great extent but have not been able to find an answer.

    Any help would be greatly appreciated. Thanks.
  2. jcsd
  3. Jun 18, 2012 #2
    this question struck a chord with me, because i basically wondered the same thing as an undergrad, then just ‘left it’ since no one else seemed to be ‘worried’.

    but now you set me thinking, i might have ‘got’ it. thanks.

    so, to start with, we know some things that have to be true,

    1. the photon experiences no time, so can't really be oscillating.(this is what confused me originally.)
    2. you can't ‘see’ the photon during a journey from emission to absorption, because any interaction would basically absorb it.

    so, what ‘must’ be the case, is that;

    any interaction with the light, is once only and destroys it (collapses its state) and the photon (its fields) are in the same state they were when they left the emitter, so oscillation of light radiation is not from an individual photon, but produced as an effect that comes from combining MANY as they arrive at a given point with changing values, and thereby give the usual observed oscillating fields.

    so an individual photons fields are fixed but displaced ‘snapshots’ of the emitter at the moment the photon was emitted, but are at a delay of the speed of light over distance for an observer, or, infact in 4-dimensions they ARE the same fields as those at the emitter!

    so, the original questions answer is that;

    the fields, at the photon, are just part of those at the original emitter, the magnetic field lines are discontinuous in the observer's frame, but they ARE continuous in the real 4-dimensional world taking in the photons fields and the emitters fields.

    so basically the concept of magnetic field lines/loops, as an intuitive way to visualise the underlying physics, proves to be not so helpful if you don't restrict to non-relativistic situations, which is pretty bad when you consider magnetism is basically a relativistic effect, so it seems to me that they will often not ‘work right’ and confuse rather than inform.
  4. Jun 18, 2012 #3
    The photon picture is not relevant here.

    Magnetic field lines are always closed as you say because we've ruled out magnetic monopoles. Any magnetic fields therefore have their origin in time-changing electric fields. (Steady currents are just a special case of time-changing electric fields where the electric increases are cancelled by the decreases but the rotations add so all that's left is the magnetic field.)

    When you launch a wave (your original question) the time-changing electric field near the oscillating charges gives rise to a magnetic field with closed lines. The initial electric field lines start and end on the charges. But the time-changing magnetic field also gives rise to a solenoidal electric field. This is an electric field whose field lines are also closed exactly like the magnetic field. If you watched an animation of this it would look like the electric field lines detach from the charges and move away hand-in-hand with the magnetic field.

    In free space (where there are no charges) both electric and magnetic fields are solenoidal (closed loops) and all field lines terminate on themselves (or at infinity for the special case of the infinite plane wave.)
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