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Why can't I see magnetic fields?

  1. Aug 15, 2007 #1
    The question is a bit disingenuous. My real question is: How powerful would a magnetic field need to be to emit photons that fall in the visible spectrum - if that even makes sense. I know that magnetic force and electrical force are the same force: electromagnetism is transmitted by photons.

    P.S. I'm not worried about the damage such a field might cause to a person (for the purposes of the question, at least).
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  3. Aug 15, 2007 #2


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    I hate to say this, but you DO "see" magnetic fields. EM radiation has both E and B components, and in the "visible range", that is what you are detecting with your eyes.

  4. Aug 15, 2007 #3

    I understand that EM and M and E are the same thing at a particular level - but so are ice and steam, and they have very different properties on a human scale. A magnetic field does not behave the same way an electircal field does.

    A red laser pointer's beam is an EM field; can it attract iron? Could the kind of field - measured in Gauss, not eV and nm - ever be visible? Would it still be 'magnetic'?
  5. Aug 15, 2007 #4


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    Rembember that there is a huge difference between an EM field, which is composed of photons, and the kind of magnetism caused by dipoles aligning in a physical substance such as iron.
  6. Aug 15, 2007 #5
    Your eyes can only see EM fields in a narrow band of wavelengths (between 400 and 700 nm). Constant magnetic fields have wavelengths on the order of 3e8 meters (about half the distance from the earth to the moon). The energies of photons at that wavelength are far too small to cause any electronic transitions in the molecules of your retina.
  7. Aug 15, 2007 #6


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    Very nice post, Manchot. :smile:
  8. Aug 15, 2007 #7


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    How does a constant field have a wavelength? :bugeye:

    Does "constant" mean something different to you that it does to me? To me it means "not varying with time." If it doesn't vary with time, it's not a wave and threfore can't have a wavelength.

    Actually, as far as the receptors in your eyes are concerned, it's the frequency of the wave which is more relevant, because it determines the energies of the photons that interact with with them. Or in a classical view, a single receptor is at a single location, so it can sense only how the field at that location varies in time, not how the field varies with position.
  9. Aug 15, 2007 #8


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    Maybe it's because I never finished school, or maybe it's because I'm at work and am therefore somewhat inebriated, but the 'constant wave' reference made sense to me. The field surrounding a bar magnet, for instance, isn't propogating in the same way that a laser output does. It just kind of lies around waiting for something to happen. The photons involved still have an associated wavelength.
  10. Aug 15, 2007 #9
    Let me be clearer: I was referring to a field that is constant on a timescale of seconds, so that I could assign a finite numerical value to the wavelength (for the benefit of the OP). As for why I mentioned wavelength instead of frequency, it's because it's a lot easier to relate the difference between distances than it is to just say "the photon energies involved are a factor of 1e15 too small." You're right, though: to be more precise, I should have said, "the wavelength in free space."
    Last edited: Aug 15, 2007
  11. Aug 15, 2007 #10
    Partial Answer Achieved!

    Okay - so the photon wavelength in a magnetic field is 17 orders of magnitude too long for human sight. Fair.

    Follow up:
    1. Can the wavelength of a magnetic field be altered or set?
    2. If (1), then how?

    Follow follow up up:
    1. Is the strength of a magnetic field analgous to the intensity of a light source (e.g. number of photons, not wavelength of photons)?

    Thanks for your responses!
  12. Aug 15, 2007 #11


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    first 1.

    Magnetic fields are coupled to pretty much every electric field (depending on your frame of reference... but that's up with relativity) for now, just accept that every magnetic field is induced by an electric field. So if you have a wire that you're feeding voltage with a potentiometer (turn-dial), as you turn up the voltage, the magnetic field's strength increases.

    Magnetic fields also come from magnetized materials (like magnets)

    second 1.
    err... I don't think so... maybe you could rephrase the question?
  13. Aug 15, 2007 #12


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    It is pretty meaningless to speak of the wavelength of a magnetic field, we must talk of electo magnetic fields. Since a changing magnetic fields IMPLIES the existance of a changing electric field, they are inseperable.

    Given that, yes w commonly control the wavelength of E-M fields. Example, a radio station or a laser.

    2, Common technology.

    I'll leave the ffuu to others.
  14. Aug 15, 2007 #13

    Remember, I started with "why can't i see the field my fridge magnet emits" :rolleyes:, and I know that E and M are coupled.

    > Pythagorean said:
    > Magnetic fields are coupled to ... every electric field ... if you have
    > a wire that you're feeding voltage with a potentiometer (turn-dial), as you
    > turn up the voltage, the magnetic field's strength increases.

    I know that the intensity of a photon source is usually separate from the frequency of the photon source (I can make more photons of the same wavelength). This implies that increasing the gauss of a magnetic field is not coupled to the frequency of the photons that make the magnetic field.

    So: Do all photons in a magnetic field have the same 3x10^8m wavelength? Is this a characterisitic of photons that carry magnetic force?

    If I'm asking circular questions I suspect it's a failure of my imagination: am I getting lost because of the many orders of magnitude of difference in the energy of the photons from 'light' source to 'magnetic' source?
  15. Aug 15, 2007 #14


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    The reason you can't see any static magnetic field is that your eyes are only sensitive to waves (and furthermore only to a specific range of wavelengths).

    The answer is no deeper than that. Manchot is doing you a disservice by putting specific numerical values in your head; forget the numbers. Constant magnetic fields don't wiggle any electrons around, so your eyes cannot sense them.

    - Warren
  16. Aug 15, 2007 #15


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    Please reread this thread. Your question has been answered several times.
  17. Aug 15, 2007 #16

    Claude Bile

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    It doesn't make sense to talk about magnetism on its own when talking about photons. You should be referring the the fields and forces as being electromagnetic and not just magnetic.

    With this in mind, Do all photons in an electromagnetic field have the same wavelength? - No. Is this a characteristic of photons that carry electromagnetic force? - Photons don't mediate the electromagnetic force, virtual photons do.

  18. Aug 15, 2007 #17
    :surprised Um. Thanks. I'm ignorant, not idiotic; I am trying to comprehend, and have not found an answer I can make sense of. (BTW, that doesn't mean the answers are wrong!) I think there are different answers in this thread, and I don't have the knowledge to assess the veracity of any of them. I do appreciate these patient attempts to illuminate me, though. :smile:

    So far, I think you (collective) have said I can't see photons emitted by magnets because either:
    a) the fields don't move (I visualized a standing wave of moving photons), or
    b) the wavelength is way to long for our eyes.

    (a) implies that if I move through the magnetic field (fast enough) I could see it.
    (b) implies that some method should exist where I could tune a magnet to produce visible photons, or tune a laser to make a ferrous material move.

    If I'm asking non-questions, point me to a physics text that will help. I can read the equations, even if I can't do them anymore.
  19. Aug 15, 2007 #18


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    A constant field can be considered the limit as wavelength goes to infinity. A field with an infinite wavelength is a constant field. Your (a) and (b) here are identical statements.

    You couldn't just move (as in along a straight line). You'd have to wiggle back and forth at a rate of some trillions of cycles per second.

    Furthermore, permanent magnets do not have energy sources, and cannot be made to radiate away energy in the form of electromagnetic radiation on their own. If you shake one back and forth at some trillions of cycles per second, though, the energy you impart in shaking it will be (partially) converted into visible light. I wish you luck in accomplishing this mechanical feat.

    A magnet producing a constant magnetic field is the same as your "laser" tuned all the way down to a frequency of zero cycles per second.

    Most of your misunderstandings come from a lack of intuition about what happens at zero frequency, or infinite wavelength.

    - Warren
    Last edited: Aug 15, 2007
  20. Aug 15, 2007 #19


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    Ah, I was unaware of that specific definition for a 'constant wave'. Thanks for the clarification.
  21. Aug 16, 2007 #20

    I will do some reading on photons at zero frequency - a concept that I was never exposed to before, and one that never occured to me as being possible.

    Thank you for the answer, chroot.

    In the meantime, I'm going to load up on caffiene and stare at my fridge magnet. I'll aim for a few hundred trillion S/s (Shakes per second); if I see any pretty colours, I'll let you know. :wink:
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