P3D polarization

[Peter Abrahams <telscope@xxxxxxxxxx>]

>I'm attempting to get an intuitive feel for "circular polarization".

That's a good one......if successful, after this project, you should try
for an intuitive feel for the unix kernel, or quantum mechanics!

As already discussed, circular polarization is produced by passing light
through a linear polarizer and a quarter wave plate.  Polaroid makes
quarter wave plates of plastic, and the 'natural' plates are usually of
mica, which is birefringent (double refracting).  Light waves oriented in
one direction (polarization) travel at a different speed through the mica
than waves polarized at 90 degrees, and the proper thickness of mica will
give transmitted light the property that when it exits the mica, light
polarized at one orientation is one quarter of a wavelength out of phase
with the light waves at 90 degrees.  
In a quarter wave plate, light is first passed through a linear filter that
is oriented halfway (45 degrees) between the two axes of the mica.  As the
light traverses the mica, half of it is delayed, to emerge 1/4 wave out of
phase from the other half (how does the mica 'choose' which half? MY
intuition tells me that humans don't understand light well enough to know,
but I am hardly an expert here).  The linear pol has acted as a filter,
removing 'random' orientations, and the result can be thought of as linear
polarized light that rotates its orientation, 360 degrees of rotation with
each wavelength that passes.  This is not really an explanation, partly
because I have an amateur's understanding, but again partly because the
exact nature of light isn't understood -- or so I intuit.

Or...If you consider light as an electric field, as the light traverses a
point, this field moves around in a circle 360 degrees with every
wavelength that passes.  At 'adjacent points' along the path of the light,
the field of the light is a 'little ahead' of the next point.

The mica usually cut to about 0.034mm to give a quarter wave retardation to
yellow light.  I am not sure how it works with other colors, but it does
work, probably less effectively.  

>http://www.rutnut.com/nwocta/newsletters/99jul/kelton.html
>Which mentions "cross polarization" techniques, which I 
>could interpret either as linear *or* circular polarizers.

This is a pair of linear polarizers.  I'd just get some chalk, paint,
grease, etc.; and some slate, granite, basalt, etc; and give it a
try......unless you have some original relics nearby.  I'm sure this
technique is as difficult as he claims, but worth a try.

>If you took *stereo* pictures, ... if you used different polarization
>over each lens on the *taking* camera, you might produce retinal
>rivalry or pseudo stereoscopic effects which would further enhance
>picking up the faint images.

Another possible method of detecting obscure objects is blink
comparing........shoot a 'stereo' with a camera slide bar, then instead of
viewing both at once, view one, then the other, looking for any differences
(here, probably in reflectivity.)  Or you could view in stereo & look for
rivalry.

>http://www.algonet.se/~bengtha/photo/filters_faq/filters.html#index
Well, I see this site explains what I just said at least as well, but
having wrote it, I'll send it anyway.

>Many thanks to several people who have responded off list - '

What's the point of that??   I'd like to read anything substantial that was
explained.
_______________________________________
Peter Abrahams   telscope@xxxxxxxxxx


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