polarization of light and power factor

The phase shift between E and B is 90 degrees in a propagating wave. If it weren't, the Poynting vector would point somewhere else, indicating decaying power or a "turn" or something like that. None of which makes any sense for a propagating wave (doesn't satisfy the wave equations) under ordinary conditions.

Polarization has to do with the apparent axis of the fields. The fields are always 90 degrees to each other, but relative to, say, the emission point, they can rotate to the left or to the right. This quantity is spin, which arises from quantum mechanics in 3 dimensions.

The linear combination of left and right produces the equivalent orthogonal polarizations of vertical and horizontal. Any nonorthogonal combination (from either basis) is "eliptical".

QM seems to suggest that circular is physically fundamental, but this has no practical significance for antennas or anything.

Tim

-- Deep Friar: a very philosophical monk. Website:

Wiki has a rather cute animation of it online:

Actually it does - weakly at least in radio astronomy. Powerful natural radio sources frequently have a significant linearly polarised component from synchrotron radiation and so the Stokes parameter I-V (using circular polarised feeds) is a much better representation of total brightness than I-Q (using linear polarised feeds). The latter almost always having to be corrected by a second set of observations of U+iV. Circular polarised feeds were harder to make though in the early days and ISTR the VLA was the first to exploit it fully.

I don't think any natural astronomical sources with strong circular polarisation have ever been observed, but I could be out of date here. There is a tiny amount of it from dust grain scattering in the galaxy.

Regards, Martin Brown

I mean in terms of circular vs. rectangular.

Polarization is indeed handy... in E&M lab, we were determining the radiation pattern of a horn antenna. After the measurements, I suggested turning the whole setup 90 degrees to observe polarization. Not a damn thing showed up on the HP reciever (at least -24dB, as you say).

Tim

Hi Tim, I used to think the same thing. Turns out in a traveling E-M wave in free space the E and B are in phase. Weird huh? How does it 'know' which way it is going?

George H.

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I'm not sure if you're implying that there's no practical significance in whether or not circular polarization is fundamental...?

Circular polarization itself *is* significant, of course. In the same way that a vertically-polarized antenna will tend to reject a horizontally-polarized signal, a clockwise-spiral antenna will reject an anti-clockwise signal... by 24dB in many cases.

Clifford Heath.

Hi,

Are you referring to the axis of the electric and magnetic fields being orthogonal or are you referring to a 90 degree phase shift between the electric and magnetic fields? I understand that the electric and magnetic fields are always orthogonal apparently, but is it also true that the electric and magnetic fields are always in phase? This picture from wikipedia seems to show the electric and magnetic fields (the two sine waves) orthogonal to each other, and on the travelling axis there is also a 90 degree phase shift between the electric and magnetic fields.

"

My main question is whether circular polarized light is caused by the electric and magnetic field rotating around the travelling axis of the light while maintaining their orthogonal alignment, or if circular polarized light is caused by a phase shift between the electric and magnetic fields, causing the phase relationship to change, similar to power factor.

cheers, Jamie

On 22/08/2010 9:20 PM, Tim Williams wrote: > The phase shift between E and B is 90 degrees in a propagating wave. If > it weren't, the Poynting vector would point somewhere else, indicating > decaying power or a "turn" or something like that. None of which makes > any sense for a propagating wave (doesn't satisfy the wave equations) > under ordinary conditions.

Hi,

Are you referring to the axis of the electric and magnetic fields being orthogonal or are you referring to a 90 degree phase shift between the electric and magnetic fields? I understand that the electric and magnetic fields are always orthogonal apparently, but is it also true that the electric and magnetic fields are always in phase? This picture from wikipedia seems to show the electric and magnetic fields (the two sine waves) orthogonal to each other, and on the travelling axis there is also a 90 degree phase shift between the electric and magnetic fields.

"

My main question is whether circular polarized light is caused by the electric and magnetic field rotating around the travelling axis of the light while maintaining their orthogonal alignment, or if circular polarized light is caused by a phase shift between the electric and magnetic fields, causing the phase relationship to change, similar to power factor.

cheers, Jamie

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I don't believe that the magnetic field is shown in that picture. It's just the rotating electric field.

Yes that's it.

No, there is no phase shift between E and B in free space (far field) propigation of light.

George H.

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-- Oops, that should be 90 degrees angle, not phase!

Mixing my angles again...

Direction, of course, is the cross product of E and B, which is true at any point. If they were out of phase, the wave would have 0 power factor, in other words, it wouldn't carry any power, which is just silly!

Tim

elliptical

Reminds me:

Given linear, circular, and elliptical polarization.... why not ... hyperbolic polarization? ;)

Jon

I don't think it has an actual direction, rather it is constantly expanding.

cheers, Jamie

Thanks for the clarification(s)! So the electric and magnetic fields of light are crossing zero amplitude at the same time, and are displayed on orthogonal axis'. And for circular polarized light, both the electric and magnetic fields are rotating for some reason while maintaining their orthogonal alignment and phase synchronization.

So "circular" polarized light, is really two separate light waves, ie one with a horizontal and one with a vertical electric field (vertical and horizontal magnetic field respectively) that are out of phase, and the amount they are out of phase determines if they are elliptically or circular polarized and left or right polarized? That would seem to make sense as it is hard to understand why a wave would have any inherent rotating action while travelling. This image showing two light waves electric fields seems to support that idea:

"

cheers, Jamie

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Ahh OK, I use to have this mistaken idea that an EM wave was like an oscillator with the current and voltage out of phase.

In the near field there is a phase difference between the E and B fields. (I still find EM waves confusing at some point. What does it mean to say that they experience no time? Sorry for the anthropomorphizing.)

George H.

A yup.

Hmmm Well to say it is 'really' two linear waves is the same as saying that linear polarization is 'really' the combination of two circularly polarized waves.. added with the correct amplitude and phase. You can use either basis (is that the right physics term?) linear or circular to describe a photon, both are equally true.

George H.

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For a propagating wave, E and H are in phase in a lossless medium. Any phase difference is due to loss in the material.

For instance, take Faraday's equation,

curl E = -1/c dB/dt (in Gaussian units).

For a plane wave, _E_ = _E0_ exp(i _k_ dot _X_ - i omega t), so

curl _E_ = i _k_ cross _E_,

and

-1/c d_B_/dt = (1/c) i omega _B_

Both have a factor of i, so E and B are in phase.

Cheers

Phil Hobbs

This is a question I sometimes consider at night, when walking through my woods.

I'll suggest another question that arrived, many years ago, when I first asked it. "What does distance mean, if there is no time? And if light experiences no time and therefore no distance, what does this imply about the universe?"

Add to this, a question that arrives from the special theory of relativity, which flows entirely from two very simple expressions (four, after a step): x=c*t and x'=c*t'. These are two completely different frames of reference, entirely separate universes, if you will. The only thing they share is 'c'. Might particles reside in entirely isolated and completely empty separate universes, with perhaps the only thing shared between them being photons, which create simultaneously the concept of (x,t) for those particles?

What does distance mean?

Jon

A dipole has current at a maximum when voltage is at a minimum, and vice versa, and the position of these is off by lambda/4 (strong magnetic field towards the center, electric field towards the tips). Wouldn't seem to make sense. If anything, the magnetic field propagating should superimpose on the 1/4-wave-later electric field and propagate off into space (if not for the overall antenna being 1/2 wavelength, disallowing axial radiation).

So does the magnetic field part matter at all? I suppose in this type of antenna, it doesn't. It's like an electric field waveguide-to-coax antenna, expanded to free space. You can replace the magnetic part altogether with a loading coil (at the expense of physical size = gain), whose field usually points in completely the wrong direction anyway (an axial solenoid having an axial, rather than radial, field).

A loop antenna, OTOH, is all magnetic field. Little voltage appears on it.

Seems to suggest that an antenna is tailored to one or the other, and you let space sort it out as far as turning E into B into E. In the near field, the proximity of the antenna gives off E or B fields that apparently confuse the situation (electrostatic and induced E and B fields), giving rise to all sorts of voltages, currents and phases.

Also makes me wonder if one were to make an antenna which does both. Contrive a structure which launches E and M waves, in the correct phase, angle and proportion, so there is no near field. I wonder if it would look something like those metamaterials -- coaxial loops and sticks, with connections somewhere.

Tim

Yes, though not from physics, linear algebra actually. An intimidatingly simple subject, quite useful on occasion.

Tim

I just think of the impedance of free space as being 377ohms, so E and B pretty much have to be in phase. ...or space would be reactive. ;-)