Re: POLARIZATION

[Ludwik Kowalski <KowalskiL@MAIL.MONTCLAIR.EDU>]

In a private message (see below)  XYZ wrote:

> The eye is a very non-linear instrument for comparing
> intensities.

I read the reply to students and asked what the "non linearity"
can mean in the context of our observations (total darkness
to the naked eye when polarizes are crosses versus acceptable
transparency when they are parallel). To my surprise most
of them could not answer. Here is the outline of an experiment
I improvised to clarify the concept.

The Pasco Light probe looks into the incandescent Pasco
Light Source. I just added the variac. What is the relation
between the light intensity and the applied voltage? Here
are my data in arbitrary units. I am happy for clarifying
the concept.

d.o.p. --->   100 V    90 V    80 V    70 V    60 V    50 V
Light   -->    2.32     1.59     0.95     0.45    0.27     0.10

XYZ wrote:

>  If you want to do the experiment accurately it would be
> best to use polarizes that provide a 100 % polarized beam,
> e.g. Nicol prisms. Polaroid polarizers don't come anywhere
> near that.

Very appropriate. The extinction ratio of two polarizers,
while the probe was looking to the lamp is also close to 6,
as in our Brewster angle experiment.
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I have one more question. The output of the Pasco Light
probe fluctuates under the constant illumination. Nothing
to do with 60 Hz which was debated last fall. About +/-10%
up and down without obvious regularity. Sometimes it takes
only 10 seconds to come back to the next maximum, and
sometimes ten or twenty times longer. And there are intervals
when the output remains nearly constant for up to one or two
minutes. I tried two probes with similar results. Did others
observed such drift? What causes it? I tried to get rid of the
drift by using a different light source and many different
intensities. No success. Please comment, if you can.

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Reply to my original message (see below) by XYZ:

> Dear Ludwik,

1.The eye is a very non-linear instrument for comparing
intensities.

2.  If you want to do the experiment accurately it would be
best to use polarizers that provide a 100 % polarized beam,
e.g. Nicol prisms. Polaroid polarizers don't come anywhere
near that.

3. "In general, highly polished metallic surfaces have a higher
reflectance than dielectrics. At normal incidence, for example,
silver and aluminum reflect over 90 percent of all visible light.
Experiments show that the reflectance depends not only on the
particular metal but on the preparation of the surface and on
the wavelength and direction of the incident light. If plane
polarized light is reflected from a metal at other than normal
incidence, the p and s components of the incident electric vector
are reflected with a phase difference and this gives rise to
elliptical polarization.  It is a general observation for all metals
that plane polarized light is not reflected as plane polarized
light except when it vibrates either in the plane of incidence
or perpendicular to it."   From:  Fundamentals of Optics
Jenkins & White, Fourth Edition, p. 534.

Best regards,

***************************************************
My original message:

> I would appreciate if somebody could comment on the following
> observations. Both are from today's lab.

> 1) Students in my Optics class were analyzing the beam of light
> reflected from plexiglas at the Brewster angle. Pasco Incandescent
> Light source, Pasco Light Probe and Pasco Polarizer were essential
> instruments. The reflected light intensity varied only by the factor
> of 6 when the polarizer was turned from the maximum orientation
> to the minimal orientation. Practically the same result was obtained
> at the reflection angle about 7 degrees away from Brewster. Why
> is the extinction ratio, Imax/Imin, so small? I expected it to be at
> least 20 (on the basis what the eye sees when two Pasco films
> are cross-oriented.)

> 2) The plexiglas was replaced by the aluminum foil (mirror) and
> the beam reflected at 50 degrees was again analyzed for linear
> polarization. I expected Imax/Imin ratio to be 1.0, the measured
> value turned out to be very close to 1.4. How can polarization
> from a metal be explained? Why is the extinction ratio so small?
> Should it be the same for all metals?
> Ludwik Kowalski
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