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Electromagnetic wave constituents

  1. Jun 16, 2009 #1
    Ok, I'll try to make my question as clear as possible, but since the question arises from confusion, it may be difficult.

    In an electromagnetic wave, what exactly is waving?

    If the answer is field strength, then why do things like the size of the holes on the screen on a microwave oven, or the size of a radio antenna (how it should be proportional to wavelength) matter?

    If the answer isn't field strength, then is something physically moving up and down? If so, what?

    Back to the antenna idea. Let's say I have a dipole antenna that is vertically aligned and is one wavelength of whatever frequency I'm trying to transmit. This is often pictured as a cartoonish red or blue sinusoid emanating from the antenna with an amplitude of the height of the antenna. I have a feeling this is wrong, but let's assume it is accurate so someone can explain to me why it isn't. So for an antenna to emit this cartoonish sinusoidal wave, something in the metal is moving up and down sinusoidally that is spitting out blue pixels that travel parallel to each other away from the antenna at c.

    Now suppose I have a receiving antenna a km away that is exactly the same size and same height from the ground. It would seem these blue pixels hit the metal and cause the appropriate voltage oscillations in the wires. But what if I move the antenna up or down by 10, 20 or 200 percent of it's length? This doesn't pose a problem. Well, I might lose a little bit of strength, but if you think about the cartoon, I'm completely missing the blue pixels. But if I chop of, say, 10% of the antenna, I'm basically screwed. I'm assuming that moving it up or down doesn't hurt because the sinusoid is not alone, but has many copies, some going out radially (like ripples from a stone thrown in a pond), but also some not coming out directly horizontally but some sinusoids flying towards the sky and some towards the ground. But copies don't make sense either, because wouldn't they then interfere with each other somehow? So is it that it isn't a stream of pixels but more like a pressure that forces something that is already present to move up and down (like ripples on a pond)? If so, what is it moving up and down?

    And finally, if this is the case, why does changing the length of an antenna have such a great effect?
    And again, if a whole huge chunk of the ether (or whatever) is moving up and down, what do the size of the holes in the microwave screen have to do with anything?

  2. jcsd
  3. Jun 16, 2009 #2
    Cartoons are bad for you :)
    1) I you want to call it a medium it is the E/M field, but it has very different properties then what you would expect from a real medium like water (see Michelson-Morley experiment) So physicist prefer to say that there is no medium.

    2) A plane wave in x direction far away from the source has in the E-field roughly a formula like this (x,y,z are the coordinates where you measure the field strength):

    [tex]\left| E \right| (x,y,z) = \sin(x - t)[/tex]

    So you see it doesn't matter if you move up or down along z a bit or in the y direction, the E-Field is the same. But its strength oscillates with time. When time of one swing is exactly the time that the electrons need to get from one end of the antenna to the other, then we have a resonance. Just like two guitar strings that are tuned to the same frequency. When you pull one then the other one is excited by the first's airwaves and starts to swing.

    This is why the antenna length is important. Not because there is some wave that has some height.
  4. Jun 16, 2009 #3
    Thank you very much for your answer! I had a feeling the cartoons were horribly deceptive :)

    This clears things up a bit. Especially about the antenna length. I will look into the Michelson-Morlay experiment to further understand the medium (or lack thereof). I'm still left a little puzzled about the the mesh on the front of a microwave oven though (or the slits in the double slit experiment, etc.). If we picture the cartoon it would make sense that the wave can't fit through, but looking at the equation you gave me, it isn't obvious to me why hole size would have anything to do with wavelength.

    If it is not the case there there is some wave with some height in space, can you (or anyone) explain what it is about the waves that cause x-rays to be mostly stopped by metal but not so much by flesh, visible light to be mostly stopped by flesh, but hardly by the mesh on a microwave oven, and microwaves to be stopped by the mesh?

    Thanks again ;)
  5. Jun 16, 2009 #4
    The penetration of a metal sheet by an electromagnetic wave is a bit tricky to picture. I'll try to sketch a rough picture again. Inside metals there cannot be an E Field, because the electrons in the metal always try to flow in such a way to make the field 0. If a wave hits a barrier like this, then it is reflected. This happens at an open pipe end, too, where sound waves get reflected because there can be no pressure at the pipes exit (see Kundt's tube)

    So now the question arises, what happens if we punch holes in the metal sheet. Naively we could expect as much reflection as there is metal, but this doesn't happen, because of something called resolution. The formulas get a bit complicated, but the main idea is that waves stay stable by "interference" if two parts of a wave come in contact, then one part might be on its up swing (E field is positive) while the other one is on its down swing (e field is negative) and they cancel out, or if they go in the same direction they get stronger. This interference cannot pass a barrier if the hole size doesn't fit a down swing and an up swing at the same time (even if it looks as if this swing isn't realized see Huygens–Fresnel principle) So for a wave with a large wavelength the whole wall looks like a big sheet of metal.

    This happens with sound and water waves too and there should be videos on youtube.
  6. Jun 16, 2009 #5
    Excellent. Your answer is very clear. I'll check YouTube for the water/air videos.

    Thank you very much again!
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