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 |
