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Light waves

  1. Sep 15, 2006 #1
    sound waves travel through air; waves on the sea travel through water; what do light waves travel through?
     
  2. jcsd
  3. Sep 15, 2006 #2

    jtbell

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    Light waves are waves of electric and magnetic fields, which don't require a material medium. Electric and magnetic fields can exist in a vacuum, so light waves can, too.
     
  4. Sep 15, 2006 #3
    I've never understood that. How then light moves through vacuum where there are no EM fields?
     
  5. Sep 15, 2006 #4

    jtbell

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    A vacuum can indeed have electric and magnetic fields in it. If EM fields could not exist in a vacuum, then two charges would not exert electric and magnetic forces on each other when placed in a vacuum.

    Place a single electric charge in a vacuum. It has a stationary electric field around it, given by Coulomb's Law. Now shake the charge back and forth. The electric field at each point in the vacuum starts to change; it oscillates. By Maxwell's Equations, this changing electric field gives rise to a magnetic field which is also oscillating. The oscillating magnetic fields in turn give rise to more electric fields. The pattern of mutually oscillating electric and magnetic fields spreads outward from the oscillating charge... a spherical electromagnetic wave. Shake the charge with a high enough frequency and you get light!

    Of course, you can have a vacuum with no EM fields in it, but then you can't have any charges in or around it, to produce EM fields. This vacuum doesn't have any light in it, simply because there are no light sources!
     
  6. Sep 16, 2006 #5
    Does it mean that when I for instance turn on the light in my room, I generate the oscilating source of electric field which fills the universe?
    Do you mean physycaly shake the charge?
     
  7. Sep 16, 2006 #6

    jtbell

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    Well, it won't fill the entire universe because it gets blocked by walls etc., but the light bulb is a source of oscillating electric and magnetic fields.


    Yes, that's what happens in a radio-broadcasting antenna: electrons move back and forth in a wire. This produces the oscillating electric and magnetic fields which form the radio wave. It's the same sort of thing as a light wave, except that it has a lower frequency and a longer wavelength.
     
  8. Sep 16, 2006 #7
    Question--can we say that the "medium" the light waves move through = "the vacuum", as an analogy to the OP question about air and water ?
     
  9. Sep 16, 2006 #8
    Why not call it aether and be done with it?:zzz:
     
  10. Sep 16, 2006 #9

    DaveC426913

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    No.
    10 char
     
  11. Sep 16, 2006 #10
    @jtbell:
    Thank you for your replies, but I don't understand it to the extent that I even don't know what to ask.
    For instance, I understand well the basic mechanical oscilation and wave. The atoms, or moleculs oscilate around their equilibrium location. That means that oscilation is a change in the position of particle which periodicaly moves forth and back. Measuring the path and time of one such oscilation we determin frequency and wavelenght.
    Now, is it posible to analogously explain the oscilation of EM field? What periodicaly changes in such oscilation?
     
  12. Sep 16, 2006 #11

    jtbell

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    The magnitude and direction of the electric and magetic fields oscillates, at each point in the wave. We can detect this by putting an electric charge at the point in question. The charge experiences an oscillating electric force. Or we can pass a current-carrying wire through that point, and it experiences an oscillating magnetic force.

    This is how a radio antenna works, in fact. The oscillating electric field in the radio wave causes the electrons in the antenna to oscillate back and forth, producing a small oscillating current or voltage that we amplify in suitable circuits.

    If you're looking for a purely mechanical "explanation" of electromagnetic waves, there is none that has any real acceptance among physicists. If you have problems with the "reality" of electric and magnetic fields, you're going to have problems with classical electromagnetism in general, not just with electromagnetic waves.

    I don't think moving to a quantum-mechanical picture would help much with these conceptual problems, either. The real and virtual photons of quantum electrodynamics are quite different from the particles of classical mechanics.
     
  13. Sep 16, 2006 #12
    If I may interject, the conceptual problem I see right away is the confusion between fields, waves, and the oscillation of particles.

    xAxis - a wave in the context you are considering is not actually a particle following a wave-like path. However, crossing over into quantum discussions won't quite help right now, as jtbell points out.

    Light propagates as the intersection of two waves, which are orthogonal to eachother, and exert a force on eachother. If you picture a 2D sine wave in the vertical plane, and a 2D cosine wave in the horizontal plane, you will get a good idea of how light travels in the single-wave model (as opposed to quantum wave packets). The electric field and magnetic field are oscillating, as you noted, and as one rises, the other falls.

    In doing so, they exchange energy in what might crudely be referred to as a perpetual motion device, which is really only true for the sake of argument, and in a fairly short span of cycles. There is a view which takes EM propagation to be harmonic motion, like connected pendulums.

    When considering a wave, discussions usually are limited to plane or linear effects. Exceptions include pressure and shock waves. A field, however, is a generally taken to be a volume with some directional or motion preference, and is visualized as a vector field. As such, a field doesn't move as a wave does. Or put another way, a wave is a small element of a field which has 'escaped' to travel outward in a straight line.

    So, a field implies that we can move things in an out of if, and that it can intersect with other fields. By way of example, some electronic equipment that is very sensitive to these fields must be shielded. One form of shielding is to set up a destructive field that must be adjusted in space to exactly counteract any existing fields. Electron microscopes are particularly sensitive, and it is common to build 'cages' of wires around them that are tuned to remove surrounding EM fields.

    That was a really long way to say that EM waves do not have an exact analog to mechanical waves. Similar equations can apply in limited circumstances, but the similarities in behaviour is anecdotal at best. EM waves are a model to describe observed effects, and it's important to understand the limitations of that model.
     
  14. Sep 16, 2006 #13

    rbj

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    light waves (as well as other electromagnetic waves) travel through this medium called "aether" that permeates all of space in the entire universe. an observer that is not moving through this aether would measure the speed of light to be the nominal value of 299792458 m/s. another observer moving toward a source of light that is stationary with respect to the aether will measure the speed of light to be faster than the nominal value and an observer moving away from the same source would measure the speed of light to be slower, just as one would with sound waves moving through air.

    a few years ago, a couple of researchers named Michaelson and Morley set up an apparatus to measure the relative speeds of a split beam of light at right angles to each other expecting to measure this different speeds for c for the two different directions (they can't both be moving along with the aether) and expected to see a significant fringe shift in interference patterns of the two recombined split beams as they turned the apparatus to a different orientation. they have not reported any measurable fringe shift even though the speed that the earth moves around the sun should be sufficient to notice a different speed (via the fringe shift). what this must mean (unless there is professional misconduct and they ain't reported a fringe shift they have detected) is that somehow the frame of reference of this aether is following our planet around the sun meaning no matter what season of year it is, we are never moving through this aether at a speed sufficient to sense this difference in the speed of light.
     
    Last edited: Sep 16, 2006
  15. Sep 17, 2006 #14
    Thak you for such detailed reply. I think I am getting somewwhere.
    Still I will try to clarify few more things.
    At which point?
    Where does this oscilations come from? Is it created by another device in the vicinity, or the space is full of various oscilating EM fields from various objects.

    Whenever an object is electrified It generates electric ( and magnetic field) which is a vector field? But only when it starts wigling, it produces EM wave, right?

    @ redlokki
    Your post was also very helpful but for some things Im not sure if I got them.
    I know that EM wave propagates as two sinusoidal waves perpendicular to each other.

    I'm surprised that this hasn't been determined yet.

    Is that a field in which to every point in it we map a vektor of the strenght of the field E? And they are radial vectors? If so it is like a ball around the source which should have it's radius right? So for instance if we shoot electron pass that field a few cm's, elektron won't generate EM wave, which it would ,if it passed through the field?

    I suppose that wave was created by a small element whic oscilated in that field (which was statical electromagnetic field??) So it, being a wave, carries the energy, and EM oscilations in a strait line (forget relativity for now) leave the initial field?

    Uh, too many questions but I'd like to understand ElectroMagnetic waves
     
    Last edited: Sep 17, 2006
  16. Sep 17, 2006 #15
    Space. Or spacetime if you prefer. This can be "electrified" and "magnetized", so while it's empty it isn't the same as nothingness.
     
  17. Sep 17, 2006 #16

    jtbell

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    At whatever point (location) we want to examine. The electric and magnetic fields exist everywhere in some volume surrounding the source.

    The oscillating force on the charge comes from the oscillating electric and magnetic fields that exist at the location where we placed the charge. The oscillating electric and magnetic fields ultimately come from the oscillating charges and currents that are the sources of the fields.


    Right! Stationary charges or currents produce stationary (static) fields.
     
  18. Sep 18, 2006 #17
    Ok, so from this confused set of answers to my question, is it yes or no (please explain)---can we say the medium that light waves move through = "the vacuum", which then = "the aether". That is, is the vacuum = the aether ?
     
  19. Sep 19, 2006 #18
    I think we can't. There is no such thing like an eather, (at least not in the form classical physicist would propose).
    it turns out that we can say that light travels through vacuum, but it is not the media propagating them
     
  20. Sep 19, 2006 #19

    jtbell

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    Only if you play games with words. Among physicists "aether" has a specific historical meaning, and redefining it so that it includes the modern understanding of the vacuum is confusing and/or misleading.

    If you relax the definition of "horse" enough, you can make the word cover a pig as well, but that doesn't make a horse = a pig.
     
  21. Sep 19, 2006 #20

    Claude Bile

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    To say that light travels through a medium implies some universal frame of reference. No such universal frame of reference exists, evidenced by the fact we observe the speed of light to be constant everywhere.

    To answer your question, vacuum =/= aether, since the aether was hypothetical and never proven to exist. A vacuum though is quite real.

    Claude.
     
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