From: Jeff Furman (jfurman@ocs.net)
Date: Thu Apr 22 2004 - 00:48:18 PDT
Date: Thu, 22 Apr 2004 00:48:18 -0700 (PDT) From: Jeff Furman <jfurman@ocs.net> Subject: Re: Pinhole Digest #1486 autompbile window polarization splotches - 04/22/04 Message-ID: <Pine.LNX.4.21.0404220022040.14951-100000@ocs.net>
Eric,
You are observing the change in optical rotation of polarized light
produced by clear materials under mechanical stress. This phenomenon is
used to view stress concentrations in plastic models of structures that
are put between crossed polarizers. It's called photoelasticity or
photostress on the web. Yhe Exploratorium may still have an exhibit
showing an arched structure, an also a femur model under variable
stress. You can probably try this by wearing polarized glasses, and
looking at (not just through) a flexible piece of clear plastic, with
another
polarizer lens behind the plastic, this lens rotated to look dark. When
the plastic is bent, the characteristic stress contours should be visible
in the plastic. The light from the sky is slightly
polarized, and some of the windows have been deliberately manufactured to
contain residual stress, so that they shatter into relatively dull
'cubes' instead of jagged shards. There may also be stresses on the
windows from just being mounted on the car. I hope this is what you are
looking for; and that it doesn't contain any urban legends.
Jeff.
> > Subject: automobile
window polarization splotches > From: "Eric Plett" <EPlett@serrahs.com>
> Date: Wed, 21 Apr 2004 14:44:33 -0700
>
> Hello All.
>
> For the past few years I've wondered why you see round splotches on windows while wearing polarized sunglasses. More expensive cars have smaller, more compact sets of splotches. I've offered extra credit to students who could find the answer - none succeeded.
> I just tried to find something on Google with no success. Can anyone explain this?
>
> Thanks in advance, Eric Plett.
>
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