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Re: NPR's optics lesson



By now Earth&Sky has received some flack on this. Go to
http://www.earthsky.com/ and look at the 7/11/98 script and comments from
listeners.

Has anyone received the original message(below)? I have yet to see it
(and several others) posted???

Bob Sciamanda
Physics, Edinboro Univ of PA (ret)
trebor@velocity.net
http://www.velocity.net/~trebor

-----Original Message-----
From: Bob Sciamanda <trebor@velocity.net>
To: PHYS-L <phys-l@atlantis.uwf.edu>
Date: Saturday, July 11, 1998 10:13 PM
Subject: NPR's optics lesson


The following was broadcast on NPR about 7/10/98. It is also on the web
at http://www.earthsky.com/Shows/Latest/.

What think ye?

"JB: This is Earth and Sky, with a listener's question.
DB: John Clarke who hears us in Berkshire, England asks, "What's so
special about the structure of glass that it permits the transmission of
light, whereas wood and other materials are opaque?"

JB: John, to understand why glass is clear, consider why other
substances
aren't. Wood and many other objects are opaque because they're made of
atoms and molecules that are organized in such a way that they either
absorb or reflect light. Consider quartz -- a cousin to glass. Both
glass
and quartz are formed from silicon dioxide. When you shine a light on
most quartz, the light is reflected and scattered by the crystalline
structure and absorbed by impurities in the quartz. Some light passes
through, some is reflected, some is scattered and some is absorbed. The
net result is that the quartz looks cloudy.

DB: Glass, on the other hand, doesn't have a crystalline structure
because it's not a real solid. Glass is what's known as a supercooled
liquid, and, as such, the motion of the silicon dioxide molecules from
which the glass formed is extremely slow. It's so slow that
light-absorbing and scattering imperfections can't form. So it's the way
glass forms -- and its internal structure -- that makes glass clear.

JB: John, thanks for your question. With thanks to the National Science
Foundation, we're Block and Byrd for Earth and Sky. "
------------------------------------------------------------------------
-
-------
"More information" is offered at the web site:

" . . .Quartz does not appear transparent because it has impurities and
crystal
faces that scatter light. The light is scattered in all direction, some
passes through and some is reflected backwards and some is scattered in
all other directions. This makes the quartz translucent.

Glass, on the other hand, is not really a solid. Glass is a supercooled
liquid. Because it is not a solid, it does not have crystals to scatter
light. Pure glass does not have impurities that absorb light. Because
there are no crystals or impurities light passes through the glass. This
makes glass transparent.

Other substances such as wood, absorb most of the light that falls on
it.
If the wood is highly polished into a smooth surface it will actually
reflect some of the light. This makes wood opaque. . ."

Bob Sciamanda
Physics, Edinboro Univ of PA (ret)
trebor@velocity.net
http://www.velocity.net/~trebor