From: gawne@stsci.edu
Subject: Re: Why some materials you can see through?
Organization: Space Telescope Science Institute
Distribution: na
Date: Tue, 15 Dec 1992 15:04:52 GMT

Earlier YM Chen asked:

>>> I am wondering why some materials are transparency to our eyes?
>>> Can anyone give me some hint?
>>> Thank you.

and I replied:

>>Well, a photon can travel thru anything provided is doesn't get absorbed
>>or scattered.  Obviously transparent materials don't absorb or scatter
>>(some percentage of) the photons incident on them.  
>>
>>So the photon is not getting absorbed.  Why?  What else do you know about
>>solids like glass, such as their electrical conductivity, that might help
>>here?  Compare this to a metal which is opaque and has a very different
>>electrical conductivity.
>>
>>Finally, why isn't the glass in windows transparent to UV or IR radiation?
>>Clue, for UV photon energies the glass responds about like the metals do for
>>visible photons.
>>
>>Enough hints?  If you need more, read "Dance of the Solids", a wonderful
>>poem by John Updike that was in the 1967 September issue of Scientific
>>American.
>>
>>-Bill Gawne,  Space Telescope Science Institute
 
Then Raul Baragiola took issue with my posting:

> This does not really answer "why?" and may be misleading. Opaqueness is
> not necessarily related to electrical conductivity (take Teflon, as an
> example) and transparency is not necessarily a property of insulators
> (take salted water, for instance). The existence of a band-gap explains
> why photons with energy smaller than the gap are not absorbed (except
> for defects or impurities and for surface effects). But saying this does
> not really answer the question, since we have not said why some materials
> have a band gap and some don't. The answer here is not so straightforward:
> the gap is caused by destructive interference of electron waves scattered
> from the atomic centers, and so depends in a complex way on the electron
> density, distance between atoms, and type of atoms (for a compound). In
> carbon, for instance, different ways of arranging the atoms result in an
> opaque material (graphite) or a transparent one (diamond).
> 
> - Raul Baragiola
>   University of Virginia

I'd point out that the original questioner said, "I am wondering why ..."
and "Can anyone give any hints."  My intent was to do just that -- give
some hints without explicitly answering why.  I've no doubt that many of
us reading this can display our prowess with Quantum Mechanics, but that
wasn't what the poster required.

As for Raul's concerns, I'll concede everything he says is quite true.
But, I wasn't trying to "...really answer the question."  I left that up to
the questioner, with some "hints" from me.

I've found that when teaching you can a) Give the student the simple answer
outright, with little retention of knowledge; b) Give the complete and
exactly correct answer -- at which point the student usually smiles politely
and goes away confused; or c) Give the student sufficient information that
with a bit of skull sweat it mixes with previously obtained information to
provide a sense of discovery.  I was trying to do the last.  I do hope that
my hints weren't misleading.

-Bill Gawne,  Space Telescope Science Institute