From: "Bob May" 
To: "atmlist" 
Reply-To: "Bob May" 


Arthur, if you cross a pair of polarizers, the light going through the
polarizers gets real dim and usually turns a purplish color.  If you can't
do that, you don't have polarizers. The source of the light is irrelevant
as this is a function of the polarizers themselves.  I'll note taht
polarized light will need only one of the polarizers to get this condition
to some degree, depending upon the level of the polarization of the light
source. What happens with a piece of glass between is that it modifies the
polarization by rotation to some degree or another and that rotation is
what you are going to see as different parts of the glass will rotate the
light to differing degrees if it is stressed and that makes a difference in
the brightness of that area of the glass.  Thus you get to see where the
stresses are in the glass by a very simple method. Many descriptions of
tests on the list lately, including mine, are specific versions which try
to use the reflection at a large angle to produce the polarized light so
that the tester can use simple, very available sunglasses to complete the
necessary optics to do the complete test. Bob May
http://nav.to/bobmay
bobmay{at}nethere.com
NEW! http://bobmay.astronomy.net

--- BBBS/NT v4.01 Flag-4