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What Exactly is Light?

  1. Jul 3, 2014 #1
    I've heard that a changing electric field creates a magnetic field and a changing magnetic field creates an electric field, but I am still very confused about exactly how this propagation actually works.

    The visual representation of an electromagnetic wave I have seen are a sinusoidal electric and magnetic field waves travelling in the same direction, but a line connecting the electric waves peak and trough would lie perpendicular to the magnetic fields peak and trough. These lines would also lie perpedicular to the direction the wave is travelling. It looks like this:
    http://ffden2.phys.uaf.edu/webproj/212_spring_2014/Amanda_Mcpherson/Amanda_McPherson/em_electric_magnetic_propagating_waves.jpg [Broken]

    At this point I am already confused. To my knowledge, an electric field exists at all points in space with different intensities. It is a scalar quantity. How then, can this field be 'waving' in a sinusoidal pattern through space?
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Jul 3, 2014 #2


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  4. Jul 3, 2014 #3
    Yes, you are right, electric field is a vector. I spoke mistakenly.

    However, in the graph, the direction is taken into account by the wave being above or below the axis, I believe. What I meant to communicate was that the strength of the field is a scalar quantity.

    Perhaps the crux of the problem lies with the fact that I don't understand what the axes represent. The black axis seems to be the line of propagation. It represents time, and since the velocity is constant, space.
    The red and blue axes I am baffled by. My best guess is that it represents the strength of the field. But it must represent the strength of the field at some point in space (This is what I called a scalar quantity, I was thinking of it as a point in space with a certain strength). I would guess that this point is on the black axis.

    So we have a point on the black axis. We can know the strength of the electric and magnetic fields at this point by looking at the graph and seeing how high the wave is at this point. This would all make sense to me, except that it says that the E-field and M-field have to be perpendicular to each other. If the height of the wave only expresses the strength of the field, a scalar quantity, then it cannot be perpendicular to another scalar quantity.
  5. Jul 3, 2014 #4


    Staff: Mentor

    The field is a vector. It can be perpendicular to other vectors. I don't get why you seem to recognize that it is a vecor and then object to it being perpendicular to another vector. The fields simply are not scalars, they are vectors.

    I couldn't see the link in your OP, but usually the drawings illustrate the vector fields evaluated along a line in space at a single instant in time.
    Last edited: Jul 3, 2014
  6. Jul 3, 2014 #5
    I'll try another link: https://encrypted-tbn0.gstatic.com/...ODenuFlyiFPOf7sZGLBFvtNKaNfz2JqzKMVboSx3Iq076

    In this picture there are black, blue, and yellow axes. So I'll use that terminology.

    I was saying that the *strength* of the field is a scalar. I'm interpreting the amplitude of the wave as the strength of the field, where being on the other side of the axis is like making the number negative. If the amplitude is the strength of the field, and the strength is scalar, then the amplitude is a graphical representation of a scalar quantity, and scalars cannot be perpendicular.
  7. Jul 3, 2014 #6


    Staff: Mentor

    Let me ask you this: given that the fields are vectors and that those vectors are perpendicular, how would you draw it?
  8. Jul 3, 2014 #7
    Did you receive my personal message?
  9. Jul 3, 2014 #8
    I'm not sure how I would draw it because I am not certain of what I am drawing.

    Edit: That's not a very clear thing to say, sorry. I think I am beginning to understand now. Give me a moment to think about it.
    Last edited: Jul 3, 2014
  10. Jul 3, 2014 #9
    I think I understand now. Your question about how I would draw it really helped. Thanks!

    I have a bunch more questions, if you're still willing to help.

    When I look up why the sky is blue, I'm told the reason is blue light has a shorter wavelength, so it bounces into atoms in the sky more often. My new understanding of light is that it is essentially 1-dimensional. A shorter wavelength means that the distance between the peaks of the fields is less. So how would a shorter wavelength make it bounce off of atoms more often?

    When I lift my arm, aren't I creating a magnetic field? I'm moving a bunch of charges.

    In a material like a pencil, there are a bunch of atoms with electrons orbiting protons and neutrons. Don't they create a magnetic field, why can't I detect it.

    If light is a magnetic and electric field, shouldn't I be able to shine a flashlight on a magnet and get it to move or react or something?

    I still have questions about how light actually propagates, but I'll start a new thread for that.
  11. Jul 3, 2014 #10


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    Your arm and the pencil have equal amounts of positive and negative charges in close proximity. They generate magnetic fields which are equal in magnitude and opposite in direction, and therefore add up to zero, at least at macroscopic and even ordinary microscopic distances. If you get down to atomic-level distances, in among the atoms, there are probably small magnetic effects at that scale.
  12. Jul 3, 2014 #11
    Ok, so I understand about the situation when I lift my arm, but not about a stationary pencil. In the pencil, there are effectively a bunch of protons, which are not moving, and a bunch of electrons which are moving very rapidly. Since the electrons are the only ones moving, they create a magnetic field, and the protons magnetic field can't cancel out.
  13. Jul 3, 2014 #12


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    Which way are the electrons moving?
  14. Jul 3, 2014 #13
    They are moving in orbit around a proton.
  15. Jul 3, 2014 #14


    Staff: Mentor

    No worries, it takes time. For clarity, what you are trying to draw is two fields throughout all space and time, each of the form ##\mathbf{A}\cos(kx-\omega t)##, where the respective A vectors are mutually orthogonal. It is not easy to draw, so it can be confusing.
  16. Jul 3, 2014 #15


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    All of them? Moving? Around one proton? In the same direction? You know the Bohr model is wrong, right?
  17. Jul 3, 2014 #16
    Sorry I misspoke. If we take a hydrogen atom, we have a stationary proton in the center, and then its electron is moving about on the outside somehow. So the electron is moving and the proton is not. The electron would create a magnetic field and the proton would not. So there would be a net field. If this is happening in a bunch of atoms all packed together, couldn't that field be strengthened to some noticeable level?

    In the equation Acos(kx-wt) what do k, x, w, and t stand for?
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