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

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