Re: POLARIZATION

["Donald E. Simanek" <dsimanek@eagle.lhup.edu>]

The responses so far have several problems:

The model of a polarizer as an array of "slits". Where did *that* model
come from? Is it an extention of the infamous "rope through the
picket-fence" model some textbooks foist on students as an analogy to
polarization? 

The slit model and the picket fence model fail miserably when applied to
the case of a sandwich of three polarizers. The second's axis is at, say,
45 degrees to the first. The third is at 90 degrees to the first. The
picket fence and slit models would predict no light gets through. But it
does get through. Then remove the middle polarizer and then light
doesn't get through. Any model or analogy which can't deal with this case,
a case so easily demonstrated, isn't worth a moment's consideration.

Diffraction gratings (for light) in the laboratory are usually clear
transmission gratings, replicas of a metal grating. Therefore they
transmit light over their entire area, blocking none. They are essentially
phase gratings.

The microwave diffraction grating made of metal strips with spaces between
works because of oscillatory motion of electrons along the strips. The
electrons in these strips radiate.  The process is essentially classical,
the electron motion in the metal being over distances large compared to
the slit and wire widths. 

There's a model of light gratings which treats the process as a quantum
one of absorption/re-emission of photons in the solid material at the
edges of the slit. However, I see no reason why this process should be
coherent, preserving polarization direction, or biasing the direction of
the emitted photons, since only single atoms are involved in each event.
But someone here who knows more than I about such processes may come
forward to enligten us. The optics book by Ditchburn, or Born and Wolf,
should have something useful. Anyway, I wouldn't expect the grating to
polarize. 

	-- Donald

......................................................................
Dr. Donald E. Simanek                            Office: 717-893-2079
Professor of Physics                                FAX: 717-893-2048
Lock Haven University of Pennsylvania, Lock Haven, PA. 17745
dsimanek@eagle.lhup.edu                 http://www.lhup.edu/~dsimanek
......................................................................


On Fri, 22 May 1998, Charles Crummer wrote:

> > I am missing a fundamental point and would welcome clarification if
> > possible.
> >
> > A student asked "When does a polarizing filter stop polarizing if the
> > spacing is made greater and greater between the 'strands' in the
> > polarizing filter?"
> >
> > To this I would add; Does a diffracting grating polarize light? (It
> > certainly doesn't appear to as we tried in lab).
> >
> > I believe a clarification concerning polarization would be valuable if
> > anyone can help with this issue.
> >
> > Thanks,
> >
> >
> > David Abineri
>
> A polarizer transmits only light polarized in a certain plane.  It absorbs
> the rest. (We have several burned polarizers and no burned gratings.)
> Gratings do not absorb much.  They diffract light of any polarization.  I
> think the usual model of a polarizer as a lot of small slits is good if one
> keeps the absorption in mind.  Pasco's microwave apparatus can be used for
> both diffraction and polarization.  The metal in the grating absorbs.  Why
> don't polarizers diffract?
>
> 	Charlie