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Question about light

  1. Mar 13, 2009 #1
    I'm definitely not very experienced with physics, but I'm reading up trying to understand the basics. Basically, I need things really spelled out for me lol.

    I'm confused with light. I realize there's a lot we don't understand about it, and there might not be a good answer for my questions, but I'm hoping someone will be able to help me.

    1) I understand light is electromagnetic waves. But, what is actually "waving"? I'm very familiar with sound waves, which is the air molecules being pushed, causing changes in pressure. But with light, it goes through the ether and the vacuum of space where there isn't anything to "wave" and no pressure can be caused. So what is waving? I haven't read about Einstein's theory that light is particles yet, (but I will here in a few weeks) so that might be part of my problem. Before Einstein, though, at least for a little while the belief was that light was made of waves...so what did they believe was waving?

    2) I understand that electromagnetic waves are changing magnetic fields that cause electric fields which then cause magnetic fields, and so on. Are the waves an alternation of magnetic and electric? Like M, E, M, E, M, E, M, E. Or is it an "on/off" alternation of both at the same time? Like EM, off, EM, off, EM, off. The illustration I saw was like a sound wave (up and down) combined in a 3-D way with a horizontal version of the same kind of wave. So there was one wave going up and down, and another wave going left to right, both sharing the same median axis (the 0 line in a sound wave diagram). I'm really not sure what this illustration is meaning at all. Waves are changes in pressure, correct? So, it's more like a pulse with the wavy line just being a graph to plot the changes in pressure (the higher the line, the higher the pressure, the lower the line, the lower the pressure). So, what on earth does that illustration mean? I'm very confused about what these "waves" supposedly are.

    3) I'm also a bit confused on how the waves are caused. To create the changing waves there has to be a changing back and forth of electric and magnetic fields, correct? Only changing magnetic fields create electric fields, correct? So how does a light bulb filament create light? Because the current isn't changing. Or does a steady electric current create changing magnetic fields? If so, that would make sense.

    Thanks for any help you can give! :)
  2. jcsd
  3. Mar 14, 2009 #2


    Staff: Mentor

    The intensity and direction of the fields themselves are what is waving.

    That kind of picture is supposed to illustrate the amplitude and direction of the electric and magnetic fields for a plane wave.

    The light from a lightbulb happens simply because the filament is heated up to glowing hot. Any object that is a certain temperature will radiate a so called "blackbody" spectrum which is characteristic of the temperature. From a microscopic perspective you can think about it happening because molecules have charges, charges radiate when they accelerate, hot objects have molecules bouncing around a lot, and bouncing is acceleration.
  4. Mar 14, 2009 #3
    Thanks, although I think a lot of my original questions have gone unanswered. :-/

    Like, is the wave a pulsing in amplitude? What exactly is meant by amplitude of the field? Amplitude in a sound wave means the air pressure goes up and down. What is going up and down in the EM wave?

    And is it E, M, E, M, E, M... or is it EM, off, EM, off, EM, off...
  5. Mar 14, 2009 #4
    1. The media was assumed to be an ether. Classically, what is waving is the magnitude and direction of an electromagnetic field. This field is assumed to exist in accordance with Maxwell's equations. One question that some physicist like to toy with is whether the fields are real physical things or are consequences of something else we haven't discovered, yet. I'm surprised by how many students refrain from thinking about this kind of stuff.

    2. A lot of info can be had from Maxwell's equations, including relative phases for E and B for various physical systems. One example would be when light propagates through a conductor. In this case, E and B may reach their maximums at different times. BTW, something to keep in mind if you start digging through pictures of EM waves is that E and B often differ in magnitude by a factor of c! B is tiny.

    http://www.ccrs.nrcan.gc.ca/glossary/images/3104.gif [Broken]
    Last edited by a moderator: May 4, 2017
  6. Mar 14, 2009 #5
    Great stuff, Derek, thank you! I'm a bit confused about the changing in magnitude. I assume that means how large the field is? So the field gets larger and smaller, but how would that create a wave? I would assume the field just gets infinitely larger at the speed of light, which isn't a wave at all. I know I'm doing a terrible job of phrasing the question in a way that makes sense, but I don't know how to ask it. lol
  7. Mar 14, 2009 #6


    Staff: Mentor

    Honestly, that is because you are asking too many questions. The internet is a limited format. You will be much better off thinking about your question a bit and really distilling it down into one or two key questions. The shotgun approach is not terribly effective.

    The amplitude and direction of the field at a point in space are the magnitude and direction of the force that would be exerted on a test charge at that point. The higher the amplitude the stronger the push that the fields will give.

    Look carefully at the picture in your book. Which way is it?
  8. Mar 14, 2009 #7
    Well, we can take the plane wave, for example. Typically, a plane wave is a sequence of infinite parallel planes that are propagating in the direction of the plane normal. For an EM plane wave, each infinite plane has an E and B with a certain magnitude and direction. If you were to sit down and watch these plane waves as they passed you by, you would notice a periodic character to how the magnitude and direction of the E and B field vectors changed. That is the wave people are talking about. It is like sitting at the beach and watching the water amplitude of the waves crashing on the shore, except we are looking at the magnitude and direction of fields. I think another thing that might be confusing is the locality of vectors, i.e. how we draw them on paper. So, for example, say you had two electrons, each at the z = 2 m coordinate, separated by a length of 300m and an incident infinite EM plane wave traveling in the +z direction. Assume E and B are varying as in phase sinusoids. Ignoring the electron-electron interactions, the motion of these particles under the influence of the plane wave is the same.

    Just in case my language is confusing, the waving is a waving of the magnitude and direction of the field. You could say that the amplitude is going up and down, for a particular wave that behaved like that. Alternatively, you can describe the wave as having an absolute value amplitude (magnitude) and say that it is pointing in the direction of up or down, depending on whether the amplitude is positive or negative. Just remember that a wave can be somewhat arbitrary. It doesn't have to just go up and then down and then up, etc...
    Last edited: Mar 14, 2009
  9. Mar 14, 2009 #8
    Oh okay, I think I understand. So it's the strength of the field that changes; the amount of force. That makes sense.

    The last paragraph makes it sound to me like the field is alternating between positive and negative charge, North and South (for the magnetic), is this what the wave is? Or am I totally misunderstanding you? :)

    Thanks for putting up with me!
  10. Mar 14, 2009 #9


    Staff: Mentor

    The magnetic does alternate between North and South. For the electric there is no actual charge in free space, but the force on a stationary charge alternates direction.
    Last edited: Mar 14, 2009
  11. Mar 14, 2009 #10
    EM fields are produced by charge, moving charge and changing fields. Alternating charge wil produce fields that change direction, sure. EM waves propagate due to a recursion of changing fields. E has changed, so B changes, but that makes E change, but that makes B change, etc...
    Last edited: Mar 14, 2009
  12. Mar 15, 2009 #11
    Just to clarify, when you say "moving charge" you are talking about current? So, current in one direction causes a magnetic field to change, which causes an electric field to change, etc?

    When you say an electric field "changes" what is changing? From positive to negative?
  13. Mar 15, 2009 #12

    \nabla\cdot\vec{E} &= \frac{\rho}{\epsilon_0} \\
    \nabla\cdot\vec{B} &= 0 \\
    \nabla\times\vec{E} &= -\frac{\partial\vec{B}}{\partial t} \\
    \nabla\times\vec{B} &= \mu_0\vec{J} + \frac{1}{c^2}\frac{\partial\vec{E}}{\partial t}

    I mean a change with respect to time, as you can see in the last two equations. When I say moving charge, I mean a charged particle, or particles, in motion. That could mean current, or current density, or a vibrating charge, etc... Charge in motion.
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