Ask A Scientist Physics Archive

    
name         Simona
status       student
age          17

Question -   How does a light wave travel in terms of its magnetic and electric fields?
What happens to the magnetic and electric fields as the light is
polarized? Is one of them completely reduced?
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The electric and magnetic fields are perpendicular to one another and to
the direction of propagation of the light beam. When light is polarized the
electric field direction is constrained to a specified direction i.e. the E
axis. The magnetic field is constrained to the perpendicular direction, but
it does not disappear.

Vince Calder
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Imagine a plane-wave propagating from left to right, with its polarization such that its 
electric field is vertical.
The electric field oscillates up/down.  The magnetic field oscillates perpendicular to 
both E-field and propagation.
In this visualization, that would be to/from the viewer.

what is left to decide is:
   does the magnetic field oscillate in-phase or 90 degrees staggered from the electric 
   field?
   what is the +/- sign applied to the magnetic field, relative to the electric field?  
   (One place your "right-hand rule" helps out.)

The picture is not fully complete yet, but already we can see:
  - this wave has already been through a polarizer, and yet it has its normal balanced 
  quantities of E and M fields.
  - if we put it through a polarizer at  0 degree misalignment, the E-field, M-field, 
  polarization, and power intensity will be unchanged.
  - if we put it through a polarizer at 90 degree misalignment, it will absorb all the 
  E-field _and_ all the M-field.
  - if we put it through a polarizer turned 45 degrees "left", it will absorb all the 
  E-field that is parallel to 45 degrees right,
          and pass all the E-field that is parallel to 45 degrees left.  If the E-field 
		  is attenuated, it's M-field will be, too.
          so what comes out is E-field rotated 45 degrees to the left, and 1/Sqrt(2) as 
		  big, with a matching M-field.
          The M-field is be affected exactly the same, rotated same amount and direction, 
		  and weakened by the same amount.

You have a point, in that most plastic polarizers work by draining only the E-field, I 
think.
But it does not happen instantly or at exactly one point in space.
A little E-field energy is drained, then the M-field donates some energy to keep the 
balance.
The underlying electromagnetic medium, space itself, does that.
Then the wave travels a little farther through the polarizer, drains again, and balances 
again, until the attenuation is done.

Listening to all that, you may wonder, OK, what is UN-polarized light?
Well, it is a mess.  To be a mess, it must have: multiple wavelengths, multiple 
directions, or instability over time.
It is not one wave, but many different ones.
Then, some of the waves can be one way, while different waves are the other.
A perfect laser makes only one wave, so it can never really be un-polarized (or white, 
for that matter).
Circular-polarized light is made of equal parts of vertical and horizontal, though.  But 
it is a polarization in its own right.

another picture:

Question:  How many independent light-waves can exist, of exactly the same wavelength, 
going in exactly the same direction?
Answer: 4
Any wave can have either of   2 phases:     {0-degree phase   and 90-degrees phase },
also called {sine, cosine},
                    and either of  2 polarizations:   { vertical polarization, horizontal 
					polarization}
2x2=4 options, so there are 4 waves:
       1) 0-degree phase vertical wave,    2) 90-degree phase vertical, 0-degree phase 
	   horizontal wave, 90-degree phase horizontal wave
 Any of these waves can, in principle, be inserted or removed, strengthened or weakened, 
 without bothering the others.
What phase you call "0-degrees", and what direction you call "vertical" are arbitrary.
But once you choose, you must then keep that frame of reference while you figure out the 
strength of the other phases and polarizations.

Once you understand that picture, you can clearly understand circular polarization.
   "Right_hand_circular_polarized"    =    cosine vertical wave  +  sine horizontal wave
     Because the horizontal wave is delayed relative to the vertical wave, the E-field 
	 looks like a long cork-screw sliding through space, and has a circular motion at 
	 any 
	 one spot.
    "Left_hand_circular" is the same thing, but spiraling in the opposite sense, i.e. 
	clockwise instead of counter-clockwise.
    Left- and Right- are mutually independent, just like vertical and horizontal.
    You can use either system (vertical/horizontal, or left/right) you prefer, for 
	whatever problem you are currently thinking about.

wish I could draw pictures here...
but you can probably find lots of them by surfing the web.  search for "circular 
polarization" and "course".
Teachers often make the best stuff.   wikipedia is good, too.

hope that clarifies, Simona
Jim Swenson
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Simona Rahela M.,
A changing electric field produces a magnetic field.  A changing magnetic
field produces an electric field.  These fields are produced perpendicular
to each other.  The magnetic field is a little bit in front of the electric
field.  Then the electric field is a little bit in front of the magnetic
field.  This results in the fields traveling through space.

When light is not polarized, the electric field can be in any direction that
is perpendicular to the light ray.  The magnetic field will be perpendicular
to the electric field.  When you polarize the light vertically, for example,
Only the vertical component of the electric field remains.  Only the
horizontal component of the magnetic field remains.  The light continues
moving.

Dr. Ken Mellendorf
Physics Professor
Illinois Central College
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