From: SFPhysics@aol.com
Date: Thu Apr 22 2004 - 01:30:42 PDT
From: SFPhysics@aol.com Message-ID: <1d3.1f28f978.2db8dcb2@aol.com> Date: Thu, 22 Apr 2004 04:30:42 EDT Subject: Automobile Window Slotches?
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?
Greetings Eric:
The glass is made to not shatter into dangerous shards in case of an
accident. So a blow to the windshield will create small glass chunks rather than long
dangerous dagger like pieces. To do this the glass is heat treated in a
special way. This distributes the stress into small defined areas that polarized
light can detect. Each "splotch" as you call it is a stress area. The
process is called tempering and is one reason that windshield glass is so expensive.
Some cheap repair shops use imported glass that is not tempered and is
illegal in the U.S. Using polarized glasses to check for temper spots is always a
good idea whenever automotive glass is replaced.
Best wishes to all on the list,
Al Sefl
Whose splotches are just liver spots from advancing age...
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