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Thread: Having trouble understanding the magnetic field in light

  1. #1 Having trouble understanding the magnetic field in light 
    Time Lord
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    All other magnetic fields I'm familiar with, like the field around an electrified wire, or the field around a permanent magnet are either defined around a plane, or exist in three dimensions.


    Around a wire, the field line at any point is basically a circle drawn around the outside of the cross section of the wire, where North/South obey the "right hand rule". (Apparently no particular point is north or south, as such.) In a permanent magnet, the field always stretches between North and South, in a way so that it curves around the outside the magnet, in kind of a circular or elliptical way.

    How does the magnetic field in light work? If the field is perpendicular to the direction the beam of light is traveling, does this mean that North and South are aligned perpendicular, or and the field faces toward/away from where the beam of light is traveling? Or does it mean that North and South are facing toward/away where the beam is traveling, and the field is stretched perpendicular? Or am I visualizing this all wrong?


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  3. #2  
    Moderator Moderator Dishmaster's Avatar
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    I think, this article should shed some light on this.

    http://en.wikipedia.org/wiki/Electromagnetic_radiation


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  4. #3 Re: Having trouble understanding the magnetic field in light 
    . DrRocket's Avatar
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    Quote Originally Posted by kojax
    All other magnetic fields I'm familiar with, like the field around an electrified wire, or the field around a permanent magnet are either defined around a plane, or exist in three dimensions.


    Around a wire, the field line at any point is basically a circle drawn around the outside of the cross section of the wire, where North/South obey the "right hand rule". (Apparently no particular point is north or south, as such.) In a permanent magnet, the field always stretches between North and South, in a way so that it curves around the outside the magnet, in kind of a circular or elliptical way.

    How does the magnetic field in light work? If the field is perpendicular to the direction the beam of light is traveling, does this mean that North and South are aligned perpendicular, or and the field faces toward/away from where the beam of light is traveling? Or does it mean that North and South are facing toward/away where the beam is traveling, and the field is stretched perpendicular? Or am I visualizing this all wrong?
    The magnetic field at any point will be perpendicular to the direction of propagation of the wave front, as will be the electric field.

    That defines an entire plane in whch the field vectors can lie (the plan perpendicular to the vector in the direction of propagation). The electric field also lies in this plane, and in a vacuum will also be perpendicular to the magnetic field. This holds at any point. What happens as the field propagates is dependent on the state of polarization, of any of the field. For instance, if the field is plane polarized the magnetic and electric fields maintain their directions. If the field is circularly polarized the fields rotate.

    http://en.wikipedia.org/wiki/Polarization_(waves)
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  5. #4  
    Time Lord
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    Ok. That makes sense.


    Now the main question I'm left with is the one I always run into when I'm trying to figure out magnetism. If a compass needle were placed near the path of the beam of light. (We'll suppose this compass is somehow fast enough to respond to the field in time.....) Which way would the needle point to?

    Would the needle put itself parallel to the beam? (Perpendicular to the magnetic field), or would it align itself perpendicular to the beam? (But parallel to the field)? Mostly I'm trying to understand what it means for the field to point in a given direction. That's always so confusing with magnetism. You can have situations where the needle is pointing perpendicular to the magnet, and yet the compass is still being pulled toward the magnet. I find I understand best if I just ask where the needle is pointing, and go from there.
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  6. #5  
    . DrRocket's Avatar
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    Quote Originally Posted by kojax
    Ok. That makes sense.


    Now the main question I'm left with is the one I always run into when I'm trying to figure out magnetism. If a compass needle were placed near the path of the beam of light. (We'll suppose this compass is somehow fast enough to respond to the field in time.....) Which way would the needle point to?

    Would the needle put itself parallel to the beam? (Perpendicular to the magnetic field), or would it align itself perpendicular to the beam? (But parallel to the field)? Mostly I'm trying to understand what it means for the field to point in a given direction. That's always so confusing with magnetism. You can have situations where the needle is pointing perpendicular to the magnet, and yet the compass is still being pulled toward the magnet. I find I understand best if I just ask where the needle is pointing, and go from there.
    The needle would oscillate wildly as the magnetic field at a fixed spatial location changes in time. The frequencies of electromagnetic waves that correspond to visible light are pretty high. (This of course assumes that the mass of the needle is so incredibly low that the needle actually can respond mechanically to the changing field -- of course it cannot or else one would have to use a magnetic compass in total darkness, which makes it rather difficult to see the needle.)
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  7. #6  
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    So, the question is, where would it be pointing at the magnetic field's maximum point?

    I'm just trying to understand what it means to say the magnetic field points somewhere. With electric fields, it is a lot simpler. If the electric field points up, then the negative potential is above and the positive potential is below, and an electron placed in the field would move down.

    With a magnetic field, what are we saying? Are we saying that the north pole is above, and the south pole is located below?
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  8. #7  
    . DrRocket's Avatar
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    Quote Originally Posted by kojax
    So, the question is, where would it be pointing at the magnetic field's maximum point?

    I'm just trying to understand what it means to say the magnetic field points somewhere. With electric fields, it is a lot simpler. If the electric field points up, then the negative potential is above and the positive potential is below, and an electron placed in the field would move down.

    With a magnetic field, what are we saying? Are we saying that the north pole is above, and the south pole is located below?
    The electric field and the magnetic field are both vector fields and in fact are part of the single entity called the electromagnetic field, each one component thereof.

    So, the magnetic field, at any given spatial point, has a direction just like any other vector field, including the electric field.

    Moreover, while the electromagnetic field is invariant, the electric and magnetic components are observer dependent, and in special relativity what one observer sees as the electric field may appear to be the magnetic field to another observer, or some combination of the two.
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