The wavelength of an electromagnetic wave is described in terms of distance (e.g. 633 nm). I understand this physically as the distance over which the repeating peak to peak oscillations occur.

I am trying to understand what the Amplitude's distance is? E.g. what is the typical distance (in m) the oscillations travel from average to peak (or trough)? I see it generally refereed to as intensity, but in the diagrams e.g.


I am trying to understand if the amplitude has some physical 'height' as well? For a regular He-Ne laser is it in the order of nano meters? picometers?

I ask because I am considering light scattering from small particles of say e.g. 1 nm. I understand the particle can be though of a new source of light of the same wavelength and amplitude of the incident light.

What confuses me is what if the amplitude is 'taller' than the max height of the particle? Wouldn't the electrons be 'stuck' at the top of the particle until the electric field vectors come on their way back down... essentially making the scattered wave that looks something like this:

enter image description here

I hope this makes sense. Thanks


This isn't a wave of a rope. The thing that varies as the wave passes is not the position of something it is the strength of the electric and magnetic fields. Those are coupled so it is sufficient to give one and we usually use the electric field strength, so the units of amplitude for a EM wave are volts-per-meter (or joules-per-coulomb which is equivalent)..

  • $\begingroup$ Ahh, I see. So when the amplitude is depicted as being taller or shorter, it is not the physical electron displacement but the strength of the field? So does actual electron displacement come into it (since a wave can only propagate due to a moving charge)? I thought the height the electrons oscillate sort of are the amplitude, in the sense that a larger oscillation vs a smaller one impacts more 'force'... $\endgroup$ – Steve Hatcher Apr 3 '17 at 3:27
  • $\begingroup$ Electrons need not be involved at all. $\endgroup$ – electronpusher Apr 3 '17 at 5:37
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    $\begingroup$ @Steve The existence of an electromagnetic field does not depend on a charged particle being in that part of space. If take a parallel plate capacitor into space (so there is vacuum between the plates) and put a charge on it there will be an electric field between the plates. Likewise a solenoid with current running through it can has a magnetic filed in it even though it is `filled' with vacuum (i.e. nothing). A radio wave or light ray passing though space is nothing more or less than a fluctuation in the electric and magnetic fields. No physical object moves as that wave passes. $\endgroup$ – dmckee Apr 3 '17 at 23:00
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    $\begingroup$ A properly draw graph of the wave will have the transverse axes labeled with field strength. This one is better than the ones you exhibit about (which are quite generic and apply to any wave) w3.shorecrest.org/~Lisa_Peck/Physics/syllabus/soundlight/… $\endgroup$ – dmckee Apr 3 '17 at 23:05
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    $\begingroup$ @Steve In electrostatics the only source is charges, but in the full theory of electromagnetism changing electric fields are sources for magnetic fields and changing magnetic fields are sources for electric fields. A electromagnetic wave is a structure of changing electric and magnetic fields; the wave itself is the source of future wave. Once you have started a wave it is independent of the originating source. $\endgroup$ – dmckee Apr 3 '17 at 23:50

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