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Understanding EM wave propagation

  1. Sep 8, 2009 #1
    I am generally an "artsy" person, though I am interested in science, too. I am curious about EM waves, particularly how they get started and what determines their amplitude and direction.

    I've read that an oscillating charge can generate a magnetic field, though I'm a bit confused about what is actually oscillating/fluctuating. For example, if I "shake" an electron, does the amplitude of the resulting EM wave depend upon how far it is displaced from its initial position via the shaking? Do the fluctuations of the E-field vector represent changes in charge? What exactly is periodically increasing and decreasing as per the crests and troughs of the waveform? Is it the charge at a given location?

    Also, when an EM wave starts, is it like a single ray or more like an infinite number of rays emanating from a sphere?

    Finally, why does EM field strength diminish with distance from its source, whereas amplitude and frequency of a beam of light do not vary with distance from the source?
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  3. Sep 8, 2009 #2

    Andy Resnick

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    You are using two different models, and the clash between the two is causing some confusion.

    First- Yes, an oscillating charge can generate an EM wave. However, the distance of oscillation does not really correlate with wave amplitude. The *frequency* of oscillation does. The EM wave is a way to describe how the energy propogates, by using a continuous field. By convention, the electric field is used to ascribe some parameters of the EM radiation (polarization, mainly). The field exists independently of any medium.

    The emission of EM radiation can be fairly complicated in its details, but to a good approximation light is emitted in a 'dipole' pattern. The light spreads as it propagates in accordance with diffraction theory.

    Now to the ray model- here, light is considered as composed of bundles of rays, however each ray carries no energy. How the bundle propagates (spreads) does not belong to diffraction theory.

    However, there must be concordance between the two models. For your final question, if you consider a bundle of rays originating from a point and speading uniformly in space, the density of rays that cross an imaginary sequence of spheres must follow the 1/r^2 function in order for energy (the total number of rays) to be conserved. Now, for light that does not spread uniformly in space (because there is a lens present, for example), diffraction theory describes the wave, while geometrical optics can usually approximate the behavior of the ray bundle.

    Does that help?
  4. Sep 8, 2009 #3
    Andy, thank you so much for taking the time to address my newbie inquiries!

    You answered most of my questions, though I still have some lingering areas that I could use clarification with. If I look at a sine-wave description of EM radiation, if the x axis is time, the y axis represents what, exactly?

    Also, the idea of an oscillating charge--is that some subatomic thing, or could I shake something like a maraca and, depending upon the frequency of my shaking, create EM waves of differing amplitudes?

    Given the answer to the last question you posted, I think I have a new appreciation for how lasers must work. They prevent the spreading of the light, thus maintaining the field strength over distances. (?)

    Thanks again!
  5. Sep 8, 2009 #4


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    Actually you would need all three axes. The x-axis would be time I guess and the other two axes would represent the amplitude of the electric and magnetic fields. Oscillating charges are not a subatomic thing, we generally calculate them as a macroscopic behavior. This is how any antenna usually works, oscillating the electrons in the antenna to generate electromagnetic radiation.
  6. Sep 9, 2009 #5


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    It's what's called the electric field. Loosely speaking, it is the force exerted on a unit of charge by the EM wave. So if there were, say, an electron in the path of the EM radiation, that electron would experience a sinusoidally varying force.

    And, as Born2bwire points out, there is a sinusoidally varying magnetic field present as well. That will modify the force that a charge experiences somewhat, but in most situations it is a negligible modification.
  7. Sep 9, 2009 #6
    To understand EM wave propagation, we first have to understand the nature of the atom. The orbit of the electron around the atom can exist in two states; rested or excited.
    It is the "falling" of an electron form a higher excited orbit to a lower rested orbit that causes this release in excess energy as electromagnetic waves.
    Vice versa, absorption of EM leads to a jump form rested to excited state.

    As a post above has mentioned, this energy is dipole in nature. Hence in one phase, it could be electric energy whereas at another phase it could be magnetic energy only. This is why an oscillatory charge can influence an EM field. This oscillatory pattern (sin wave) plus an initial momentum of the energy form the atom (obeying mc2) gives the motion of the EM energy at a maximum speed of c.

    It is thought that each single EM ray emanates from a single electron.

    The research onto how this happens is not very conclusive. Think of it as the Heisenberg uncertainty principle which states that certain pairs of physical properties, like position and momentum, cannot both be known to arbitrary precision. That is, the more precisely one property is known, the less precisely the other can be known.

    Hope i've answered some of your question.
  8. Sep 9, 2009 #7

    Andy Resnick

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    As others have mentioned, the y (and z) axes are field amplitudes. Usually all three axes are used to represent spatial coordinates (and then you can make sequential graphs into a movie to represent time).

    The oscillating charges must be *free* charges- I suppose if your maraca was full of metal ball bearings rather than seeds, you could indeed make an EM wave... interesting idea, actually... does anyone know if metal nanoparticles are conductive?

    The EM field within a laser cavity is a 'confined mode'. This is created fairly simply using two mirrors, one at each endface of the laser. The exit mirror is slightly less reflective than the other so some light can leak out. However, since the mode within a laser cavity is confined, the light exiting the cavity does not propagate freely in all possible directions. If you like, momentum must be conserved, so the light cannot all of a sudden 'change direction'.
  9. Sep 9, 2009 #8
    Let me make sure I've got this right: the increases and decreases along the y axis and z axis represent the changing strength of electric fields and magnetic fields, respectively. (I understand that the field doesn't "move" spatially other than in the direction of propagation, accounting for gravity.) I get the concept of field strength fluctuating, but what would it mean to have "negative" strength? Does that mean there is a change in charge or polarity? When the value at the y axis or z axis equals zero, does that mean there is no charge or polarity?
  10. Sep 9, 2009 #9


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    The electric and magnetic fields are vector quantities. A vector is composed of a magnitude and a direction. So a negative electric field would mean that the vector is now pointing in the opposite direction as it was for the positive electric field. As for charge, that is all independent of the electromagnetic wave. Charges and currents are sources that will produce the electric and magnetic fields and waves, but the sinusoidal oscillations of the fields occurs independent of whether or not there are sources in the region.
  11. Sep 9, 2009 #10
    "So a negative electric field would mean that the vector is now pointing in the opposite direction as it was for the positive electric field."

    I'm trying to visualize this in spatial terms. Let's say we are holding a light-gun with a special scope that allows us to look down its barrel, as it were, and to see instantaneously the fluctuations of the electric and magnetic fields it emits. When I pull the trigger, what will I see? Will I see a field of electrical force going up (in the direction of my forehead) and then going down (in the direction of my feet) and then up again, etc. as it gets further away from me? If I am aiming to displace an electron, do I need to calculate the phase of the wave so that when it reaches the electron when it's y-value is nonzero?

    Also, if the oscillations of the fields of an EM wave have no charge, does that mean they exert force upon positively charged, negatively charged, and neutrally-charged objects equally? If not, why not?
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