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Q: Polarization of an electromagnetic wave ( Answered 5 out of 5 stars,   2 Comments )
Question  
Subject: Polarization of an electromagnetic wave
Category: Science > Physics
Asked by: jsczepek-ga
List Price: $20.00
Posted: 14 Apr 2004 15:30 PDT
Expires: 14 May 2004 15:30 PDT
Question ID: 330320
I would like to know some specific things about electromagnetic waves
and their polarization to completely understand the thing as a whole.
1. When and how is the spatial orientation of the electric vector of an  
   electromagnetic wave beeing determined ?
2. Does the spatial orientation of the electric vector change during one full 
   vibration of the wave or is it always constant ?
3. Does a polarization filter take away all but one polarization state of a wave
   or does it actually reradiate a light wave with a fixed polarization ?
4. Can you give me a simple idea of a circular polarized light wave ? 

Many thanks in advance !
Answer  
Subject: Re: Polarization of an electromagnetic wave
Answered By: eiffel-ga on 15 Apr 2004 12:03 PDT
Rated:5 out of 5 stars
 
Hi jsczepek,

I'm not sure how much you already understand about polarization and
electromagnetic waves. My guess is that you have been taught the
physics, but wish to add a "conceptual" understanding to your
"theoretical understanding".

I'm going to start by giving relatively straightforward answers to
your four questions. I will then invite you to request clarification
for any points for which you would like further elaboration, or an
explanation at a different level, and I will be happy to provide this.

===

1. When and how is the spatial orientation of the electric vector of
an electromagnetic wave being determined?

The orientation is determined at the time the electromagnetic wave is emitted.

For example, if we radiate a radio wave from a horizontal dipole
antenna, the electric vector will be in the horizontal plane and we
refer to the emitted radio wave as "horizontally polarized". This
means that if we look towards the antenna, the electric vector will be
horizontal (alternating left and right according to the frequency of
the radio wave), in line with the oscillation of the electric current
in the antenna.

Now if we take that same dipole antenna and orient it vertically (or
use an antenna made from a single vertical element above a ground
plane) the emitted radio wave will be "vertically polarized". This
means that if we look towards the antenna, the electric vector will be
in the vertical plane (alternating up and down).

===

2. Does the spatial orientation of the electric vector change during one full 
   vibration of the wave or is it always constant ?

For linearly polarized waves, the electric vector (at a given point,
as a function of time) changes in SIGN but not in direction. In other
words, the electric vector stays within one plane. If you were looking
directly towards the oncoming wave, you could imagine the "tip" of the
E-field vector wiggling back-and-forth repeatedly along a line. If
that line is vertical, we say the wave is vertically-polarized. If
that line is horizontal, we say the wave is horizontally-polarized. If
that line is at some other angle, we could either quote that angle as
the angle of polarization or, alternatively, treat the save as the sum
of a vertically-polarized component and a horizontally-polarized
component.

For circularly polarized waves (or, in the most general case,
elliptically-polarized waves), the electric vector points in different
directions during one full oscillation of the wave. If you were
looking directly at the oncoming wave, you could imagine the "tip" of
the E-field vector tracing a circle (or, in the most general case, an
ellipse).

===

3. Does a polarization filter take away all but one polarization state of a wave
   or does it actually reradiate a light wave with a fixed polarization ?

To quote from the "Physical Reality" site:

"A Polaroid filter can be used to polarize light. It works by letting
only photons polarized in a certain direction through, while absorbing
all the photons polarized perpendicularly to the filter."

"Physical Reality ? Quantum Physics"
http://library.thinkquest.org/C008537/quantum/cryptography/cryptography.html#polarization

(If this link does not return a page including a section titled
"Polarization" you may need to click "enter site" then click
"Polarization".)

You can easily see that we are dealing with absorption rather than
re-radiation by using two filters together (two Polaroid sunglasses
will do). As you rotate one of the filters to an angle of 90 degrees
from the other, the light transmission drops to practically zero
because one of them is absorbing the vertically-polarized light and
the other is absorbing the horizontally-polarized light. If they were
re-radiating, we would expect all of the incident light to emerge with
the polarization of the final filter ? and this is clearly not what
happens.

Strictly speaking, I should say that one filter is absorbing the
vertically-polarized COMPONENT of the light, and the other is
absorbing the horizontally-polarized COMPONENT. That's the because
ambient light is likely to be polarized in all directions. If a wave
that is polarized at an angle (being neither pure vertical nor pure
horizontal; we will call it diagonally-polarized) is passed through a
filter that passes vertically-polarized light, only the
vertically-polarized component will emerge ? and it will emerge with
an intensity that decreases as the angle of the diagonal polarization
gets further from vertical.

If we think in terms of photons rather than waves, we have to think of
it in rather different terms. The above site describes it thus:

"what happens to photons with diagonal polarizations? Half would be
let through the vertical Polaroid filter and would be changed into
vertically polarized photons. The other half would be blocked by the
filter."

Ah, the dual particle-wave nature of electromagnetic radiation sure
complicates things!

In addition to Polaroid filters, there are other ways to polarize
light that do not depend on absorption. For example, there are
crystals that have a different refractive index along different axes.
If we send some circularly-polarized light through such a crystal, we
can arrange the dimensions and orientation of the crystal such that
(by the time the light emerges from the filter) the horizontal
component will have been slowed by a quarter-wavelength compared to
the vertical component ? thereby bringing the vertical and horizontal
components back "in phase" and converting circular polarization to
linear polarization.

===

4. Can you give me a simple idea of a circular polarized light wave ? 

This shouldn't be too hard. Do you know those long coiled springs,
sometimes known by the trade name "Slinky"? Imagine that one end is
tied to a doorknob and that you have stretched the spring to the other
end of the room.

Now wiggle your end up-and-down. The transverse waves on the slinky
represent the electric component, and we are modelling a
vertically-polarized wave. Similarly, if you wiggle your end
left-and-right you are modelling a horizontally-polarized wave.

To model a circularly-polarized wave, you could just move your end in
a circular pattern and you would observe a circular wave travelling
down the slinky. But we can make this more interesting!

Imagine that you and a friend are both trying to move the end of the
slinky. You are moving it up-and-down, and your friend is moving it
left-and-right. If you are both moving "in step" such that you move
upwards as your friend moves left, the result will be that the slinky
is moving back-and-forth at a 45-degree angle ? half-way between
horizontal and vertical or "diagonally polarized".

But it gets more interesting still if you can time it so that the peak
of your friend's movement occurs at the time your own movement is
crossing the "zero" point (and therefore the peak of your own movement
will occur when your friend's movement is crossing zero). You would
now see the same circular motion that you created earlier by moving
your end in a circle. And if you and your friend are moving the end of
the spring with different amplitudes, then you will see the most
general case ? an elliptical wave.

The analogy with electromagnetic radiation is that we can produce a
slant-polarized wave by superimposing a vertically-polarized component
and a horizontally-polarized component that are "in phase" - and the
angle of polarization will depend on the relative magnitudes of the
vertical and horizontal components. If, however, we superimpose
vertical and horizontal components that are equal in amplitude but 90
degrees out of phase, we will achieve circular polarization, and if
the amplitudes are not equal we will achieve elliptical polarization.

===

I would like to conclude with a reference to a web document that
expounds on the final paragraph of your third question:

"Polarization and Polarization Control"
http://www.newfocus.com/Online_Catalog/Literature/apnote3.pdf

It's quite a specialized document, but it starts with a clear and
concise overview of polarization which you may find useful.

If that document is too technical for you, you may find the following
article both informative and enjoyable:

"Building the impossible kaleidoscope"
http://www.scitoys.com/scitoys/scitoys/light/polariscope.html


Finally, as I said at the beginning of this answer, feel free to
request clarification if this answer does not yet meet your needs.


Google Search Strategy:

"understanding polarization"
://www.google.com/search?q=%22understanding+polarization%22

polaroid filter "works by"
://www.google.com/search?q=polaroid+filter+%22works+by%22


Regards,
eiffel-ga
jsczepek-ga rated this answer:5 out of 5 stars
Very clear and readable answer that´s strictly oriented to what has
been asked. - That´s the way I like it !

Comments  
Subject: Re: Polarization of an electromagnetic wave
From: neilzero-ga on 15 Apr 2004 20:52 PDT
 
As far as I know all the information by eiffel is correct except I
think he has decribed special cases of cirular polarization. My guess
is the rotation rate can be any multiple, or even 28.371 times the
wave length. Atmospheric conditions can cause the polarization to
rotate to any angle between horizontal and vertical. ie my big dish
satellite receving antenna has contineously variable polarity and the
best reception is typically somewhere between horizontal and vertical.
 I dont know if the equivelent of a polaroid filter is available for
electomagnetic waves other than light frequencies. Polaroid filters
have high attenuation for waves at right angles to the minimum
attenuation, and intermediate (on a sliding scale) for waves at in
between angles.   Neil
Subject: Re: Polarization of an electromagnetic wave
From: eiffel-ga on 16 Apr 2004 02:18 PDT
 
Thanks for your comments, neilzero. I must admit I cannot see how
there could be multiple rotations per wavelength for a circularly
polarized wave. That would imply an electric component whose magnitude
in any specific orientation varies at other than the frequency of the
wave - and if we filtered the linear polarization corresponding to
that orientation it seems we would get a signal with a different
frequency from the original.

The "wikipedia" article on polarization is very readable, and is quite
specific about this:

"For circular polarization, it is also useful to consider how the
direction of the electric vector varies along the direction of
propagation at a single instant of time. While in the plane the vector
rotates in a circle (as time advances), along the propagation axis (at
one instant) the tip of the electric vector describes a helix. The
pitch of the helix is one wavelength"

Polarization - Wikipedia
http://en.wikipedia.org/wiki/Polarization

Regards,
eiffel-ga

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