Re: [Phys-l] [PTSOS] How to explain glass being transparent

[Bernard Cleyet <bernardcleyet@redshift.com>]

The primitive theory of transparency of materials:

1  Do they know anything about traveling waves?  May demos here and  
their experience w/ water.
2  Show sympathetic vibration w/ sound.  e.g. two sound board mounted  
tuning forks.
3  Show lack thereof by detuning the receiving tuning fork w/ the  
clamped mass often supplied w/ the set.
4  Show similar to (2) w/ coupled pendula and lack thereof w/ one not  
resonant (3)
5  Show absorption by having the bob of the receiving (not driven)  
pendulum's bob partially dipped in a fluid.  Note how the initially  
driven pendulum, when allowed to freely decay, decays much more  
rapidly when the receiving pendulum is damped by the fluid.
6  Finally, show that the freely decaying pendulum decays more slowly  
when the damped pendulum is non resonant, but still dipped in the  
fluid. (Its rod is longer or shorter than the driven pendulum.)

This can all be done using coupled spring oscillators also.

Now explain that the glass molecules are very tiny damped spring  
oscillators and the light doesn't excite the oscillators because  
their resonances are in the IR and the UV, and, therefore, there is  
little absorption.   If one has a spectroscope for the UV and, or,  
the IR, one may thereby show that glass isn't always transparent.

bc prays these are unusual sixth graders.

p.s. Being an elementary student > 60 years ago,  I have no idea  
whether this'll fly like lead of helium.  BTW, this is the classical  
Drude theory * of the turn of the century and discussed in all the  
optics texts I've read.  Some members of the PHYS-L (list) rail  
against such descriptions w/ out at least pointing out that this  
description is not true.  It requires QM.

p.p.s. IIRC, Hewitt, in his texts, explains this well w/ cartoons.

* <http://209.85.173.104/search?q=cache:Ews07tnBicMJ:www.pdi- 
berlin.de/drude.pdf+optical+absorption 
+drude&hl=en&ct=clnk&cd=2&gl=us&client=safari>




On 2008, Oct 06, , at 12:02, Gunjan Chakravarty wrote:

> Dear Physics Teachers,
>
> How do I explain why a glass (made out of glass) without any water  
> looks transparent (on Granite table) to elememtary school kids?
>
> Thanks in anticipation,
> Gunjan