Re: PHYS-L Digest - 29 Feb 2004 to 1 Mar 2004 (#2004-63)

[Leigh Palmer <palmer@SFU.CA>]

On 1-Mar-04, at 1:00 AM, Automatic digest processor wrote:

(From Chuck Britton):

> I have a similar problem with those physics instructors who declare
> forcefully that the 'picket fence' model is a totally bogus, wrong,
> without merit, way of visualizing polarized light.

I'm one of those instructors. There is no reason to invoke skipping
ropes through picket fences when discussing polarized light.
Constructing a physically realistic model of absorption of one
component of unpolarized radiation by parallel conducting (and
therefore dissipating) wires is easy, and the student is not left with
the idea that the transmitted wave is polarized with its E-field
parallel to the "pickets". I demonstrate cake grid polarizers and
analyzers with microwaves in lecture, so the emergent misconception
should not arise. I will not that, if the picket fence argument is
introduced first, the cake grid experiment may fail, since it can as
easily be interpreted in terms of the parallel component being
transmitted! The student must accept his instructor's initial premise
that the incident microwave radiation has the polarization he alleges.
It is all a bit of a swindle, you see, so I say that one should swindle
on the side of the angels.

> Any student who is in the least danger of suffering injury by this
> model - NEEDS to be introduced to the important relation between
> amplitude and intensity.

I take it as given that every student needs to have this introduction.

> Most folks can go their whole lives without knowing the difference
> and suffer no severe consequences.

Yes, but we are elitists. We want our students to be the cognoscienti.

> Does not the 'picket fence' model do a pretty good quantitative job
> when applied to the amplitudes?

Yes, but it reinforces a misconception that is easily avoidable.

> At 9:49 AM -0600 2/27/04, RAUBER, JOEL wrote:
> > |
> > | In the second place, I can't think of a worse example than
> > "intensity
> > | in interference and diffraction phenomena" being used as a
> > | counterexample for which superposition does not work. Surely one
> > incurs
> > | the peril of making some of the duller students think that
> > | superposition is inappropriately applied to interference phenomena.
> > |
> > [***]
> > Leigh,
> >
> > That is certainly a serious danger; but one I think a teacher of
> > interference phenomena has to confront anyway.  One must get across
> > the fact
> > that intensities don't add but the amplitudes do add; so the source of
> > confusion is already there; unless you choose to ignore intensity
> > considerations altogether.  So I feel that one might as well confront
> > the
> > source of confusion directly.
> >
> > Joel R
> >
> > PS,
> >
> > Glad you are back to reading (and posting every now and then) phys-L !

Thanks. It's good to see both of you guys are still at it, too. I
should tell everyone that I am cutting back on active participation in
plys-l because I am taking an undergraduate GR course this semester,
filling an important lacuna in my early training. It is great to be a
student again. It gives me a wonderful new perspective. It made me
wonder how any student could keep up with five or six courses like
this, until I realized that, in truth, no student could possibly do so!
I guess I didn't when I was a student, either.

Physics education is a wonderful thing. To paraphrase Galileo: "And yet
it works."

Leigh