Date: Tue Oct 7 10:44:43 2008

Author: Adam Beehler

Subject: Re: LED's in LN2

Post:

Hi Adam

I saw a good version of this demo at a physics teachers conference in
the Czech Republic.

They had an unsilvered dewar they lowered in the green LED and as it
cooled it turned yellow.

The lecturer solicited opinions about how it worked.

Then after many suggestions he smiled and said now let's apply your
hypotheses to this experiment.

He lowered a yellow LED (at room temperature) into the liquid
nitrogen and it turned green.

oops.

Paul D



On Oct 6, 2008, at 2:09 PM, Adam Beehler wrote:

> Tappers,
>
> I finally have an instructor that is interested in showing a
> demonstration I put together after listening to this group. I of
> course put it together awhile ago and now do not remember exactly
> how it works. Every time I give the instructor an explanation, he
> comes at me with something else. Well, I have no more ammo to dish
> out. Anyway, the demo is lighting an LED (let's say it is green).
> Then, without changing anything, I dip the LED into liquid nitrogen
> and the LED now glows yellow. Remove it from the liquid nitrogen
> and as it slowly warms back up it glows green again. One can
> increase the voltage across the LED while it is cold and not blow
> it. Is this sounding familiar to folks? I have temporarily posted
> a link to a video I took of it here:
>
> http://www.physics.utah.edu/~beehler/uofu/pnJxnLEDLN2.mpg
>
> So.....what I would like to know is how it really works. Does the
> temperature really change the band gap energy and thus the photons
> emitted? Or is there some type of diffusion layer that changes its
> size and thus effectiviely changes the band gap? Or is it just
> colder so it emits light of lower energy? Does anything physically
> change inside the semiconductor material? Thanks for any added
> insight!
>
> Adam Beehler
>


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