Polarization of electromagnetic waves

[broegger]

I have read briefly about polarization of electromagnetic waves and from what I understand an electromagnetic wave is said to be polarized in some direction if the E-field is aligned along this direction..

what then is unpolarized light?? - the E-field has a definite direction at every point in space and so it is always aligned along some direction (does the E-field of unpolarized light point in different directions at different points or...?)

 

[turin]

In general, light is not a well defined, monocromatic, coherent electromagnetic wave. For one thing, white light is a band of frequencies, and so the waveform doesn't even appear "stable." For another, the sources of most light signals are reflections from grossly smeared regions of space (i.e. the walls of a room). At any point, it is almost impossible to predict what the fields will do, even if you know what the waveform looked like the moment before. That isn't to say that white light cannot be polarized, but to "justify" that there is no reason to think it strange for light in general to be unpolarized.

More simply put, light, in general, is a superposition of polarized waves that are not necessarily all polarized the same, and so, the aggregate will have a superposition of polarization.

 

[broegger]

but isn't a superposition of polarized waves itself polarized (with respect to some superposed direction)??... what exactly is polarized light as oposed to unpolarized light??

 

[broegger]

I have read some stuff on the internet and I have come to this conclusion:

"an electromagnetic wave is linearly polarized if the E-field is - at all times - aligned along some fixed axis.. an unpolarized wave is a wave in which the E-field is randomly oriented (that is, it has different orientations at different times and positions)"

 

[turin]

What is the source of the E-field? Another way to ask the question: what (physically) determines the direction of the E-field at some point in space at some time? (These are rhetorical questions, but I would be interested to hear your answers.)

Here's an idea. To make things simple, consider only two dipole antennae in the x-y plane with 0 mutual impedance (i.e. they are in each other's far fields). Let one dipole be alligned along the x direction and the other along the y direction. Let these antennae be equidistant from the z-axis and both lying on the x axis.

Consider a point on the z-axis in the far field of both antennae. The antenna alligned along the x direction will induce an x-directed polarization at this point. The antennae alligned along the y direction will induce a y-directed polarization at this point. Let's say that the antennae alligned along the x direction induces an electric field E_xsin(10t) at this point and that the signal alligned along the y direction induces an electric field E_ysin(11t) at this point.

What is the polarization of the wave at this point? What is the polarization of the wave in general? At this point, the wave (which is a superposition of two individually polarized signals) has a somewhat erratic polarization. If the signals from the two antennaes were allowed to be arbitrary, the polarization at any far field point could be made arbitrarily erratic (i.e. random). Combine this with the added effect of an arbitrary number of sources while at the same time the effect of these sources diminishing to arbitrarily small individual influence.

 

[Njorl]

I have read some stuff on the internet and I have come to this conclusion:

"an electromagnetic wave is linearly polarized if the E-field is - at all times - aligned along some fixed axis.. an unpolarized wave is a wave in which the E-field is randomly oriented (that is, it has different orientations at different times and positions)"

It is more correct to say that unpolarized EM waves have no predictable preferential direction of polarization at any time or place. Saying that they have different polarizations at different times and places is misleading.

Njorl