Re: Stupid question; telling plexi from polycarb

From: Mark Fergerson (nunya_at_biz.ness)
Date: 02/05/05 

Date: Sat, 05 Feb 2005 07:54:46 -0700

Franz Heymann wrote:
> "Mark Fergerson" <nunya@biz.ness> wrote in message
> news:hKQMd.9235$6u.1990@fed1read02...

>> I place a *** of transparent plastic between crossed polarizers
>> and _don't_ see colors; which am I looking at? I wear "high-index
>> polycarb"eyeglasses and they show colors, but neither of two sheets of what I
>>thought were different materials (one plexi, the other polycarb) do.

> In the late 1940's a friend of mine used perspex (plexiglas) models to
> investigate strains in civil engineering structures by studying the
> colours of which you speak. I think you have to put the material
> under stress to see colour bands..

   Both sheets were clamped in a bench vise for convenience (I don't
have three hands), but I suppose stresses parallel to line of sight
don't make bands? I'll carefully set a small bench anvil on one top
corner and see if that works.

   This is weird; one lens of my glasses shows gently curved bands, the
other sharp jagged ones reminiscent of those of crystals.

   Mark L. Fergerson