Hello Tim,
A recent article in Science News (see URL below) addressed the issue of
"sagging" glass, it gave the period in which glass would show any
appreciable flow to be on the order of 10^32 years, the Universe is
10^10 years (approx.). Unless Mac has some REALLY old glass I think the
effect of any differential thickness is a manufacturing issue and not a
glass physics issue.
http://www.sciencenews.org/sn_arc98/5_30_98/fob3.htm
Best Regards,
Luke Coletti
Tim Yu wrote:
>
> At 10:15 PM 11/27/98 EST, Tom McHugh wrote:
> >So long as the glass plates are parallel, it doesn't matter
> >what the refractive index of the substance is between the
> >glass plates so long as it is a homogeneous substance.
>
> One factor I haven't seen written about...For the incident and transmitted
> rays to be parallel, we also need to have the pane(s) of glass of uniform
> thickness throughout. Glass is a **very** slow fluid; over time, any
> non-horizontal sheet of glass will get thicker and thicker at the bottom.
> [Haven't mic'd any old glass, but that's what I've been told...]. For rays
> that strike such a non-uniformly thick pane (with, say, air on either side)
> from above the horizontal line, the transmitted ray will bend ever so
> slightly up toward the horizontal line.
>
> Tying up small details,
> Tim Yu
