How do EM waves propagate?

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Hi everyone, I m not a physicist and I don't really speak english... please forgive me if I write any "rubbish". I'm quite curious, and I was wondering how do electromagnetic waves travels. I mean, from a "point" source, they propagate in every direction (I've been told) so I tend to imagine them as "compression and decompression" waves in sound... nevertheless they are always represented in a sinusoidal form. I really cannot visualize that concept. Sometimes I imagine light as tiny photons shooted from a point (the electron) in every direction, some others as little sinusoidal worms waving out from that point. Sorry if it makes no sense, I'm quite confused.
 

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  • #2
PeroK
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A radiowave, like all electromagnetic waves, comrises oscillating electric and magnetic fields. That they propagate through space is supported by Maxwell's equations of classical electromagnetism.

1620325764999.png
 
  • #3
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Thank you, that's what I meant: everywhere are pictures like the one you attached. I'll be "provocative" to clarify my point: where in the pic is the electron? is it "fluctuating" in which direction? up and down? If that's the case, from that image one should deduct one single ray of light (visible or not depending on freq) should be casted. My point is, I understand that image pictures a concept, but its like sayng that sound (from my prev analogy) is represented by a graph of a soundwave (which from my point of view is totally misleading for a common person). With today powerful 3d graphics, is there not an animation or a picture that better represent how em waves are generated and propagates?

Thank you very much. Best regards
 
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PeroK
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In general a light source such as a simple light bulb produces a huge number of such waves of different frequencies in all directions. You need to imagine trillions of such waves all superimposed and in all directions.

The electrons that create the radiation stay in the filament!
 
  • #5
tech99
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Look at the picture sent by Perok. It shows two transverse waves, not longitudinal waves like sound. Look at the representation of the electric wave, which is in a vertical plane. It is a graph of the electric field strength plotted against distance.
Now visualise a microwave beam transmitted upward from a dish. It diverges, so imagine that at a height of ten miles it has a diameter of 1 mile. The energy is mostly contained in this 1 mile diameter. The electric field varies little in strength across the beam, and we see it alternating in direction at the frequency of operation. But we do not see a wiggling line, like a rope, as shown on the graph.
Regarding the electrons which caused the wave, they each contributed to the total field strength. We do not have "one electron, one photon, one wave".
 
  • #6
DaveE
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Sorry, I don't have time to give a good explanation. However, I do want to point out a couple of key concepts that you need to learn about to understand EM waves (radio, light, x-rays,...):

1) EM waves do not need any "stuff" to travel through. Sound waves travel by atoms bumping into each other. Water waves travel on the surface of a liquid. Radio waves are fundamentally different, they can travel through empty space.

2) Charged particles make an electric field (E-Field). You can think of a field as a description of the force another fictitious "test" charge would feel at each point in space. If a charged particle moves, that makes it's electric field change; your test particle would be pulled in a different direction. The magic of EM is that a changing E-field creates a magnetic field (H-Field), a moving charge will attract (or repel) a magnet. In turn any changing H-Field will create an E-Field. It is this regenerative process that creates a wave that can propagate, essentially forever. In fact, it's called "electricity and magnetism" because for any moving charge, you will always have both E-Fields and H-Fields, you can't make just one type.
 
  • #7
tech99
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I think it is worth mentioning that the two fields are in-phase.
 
  • #8
sophiecentaur
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where in the pic is the electron?
There is no "electron" in the picture.
Moreover, the photon ( which many people think of as a little bullet) doesn't exist in that sense, either. This is hard to accept or visualise but the photon can only be thought of as existing when it interacts with, say, an electron around an atom (at each end of the path). At that time, the photon represents a quantum of energy. The rest of the time, the photon could be anywhere and has no actual defined size at all. (Nothing to do with the wavelength or the source and detector blah blah). Hard eh?
You can't expect to get a grasp of Quantum Mechanics or the quantum aspect of EM without throwing the conventional ideas out of the window. But you can explain EM very well in terms of waves and the variations of Electric and Magnetic Fields. Maxwell's equations were developed many years before anyone introduced photons (QM) and they describe pretty much every large scale EM phenomenon.
 
  • #9
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Thank you very much everyone. I think maybe the most "clarifying" video I could find is from youtube by Eugene Khutoryansky (a bit naive, but nice, for anyone that could be interested). Still lots of questions in my mind, but I think they rise for lack of studying. Still I think there could be some better way to represent in a 3d animation. If you can suggest some further readings not books :( (I m not a great reader, maybe just lazy).

Again thank you very much.
 

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