So, back in Inorganic Chemistry, we had a lab about solid state physics... band gap etc. So, we dug out a box of LEDs and lit them up at room temperature and under LN2.
The stated premise was that BG is inverse with temperature, so the color changes "up" on cooling (red (GaAsP) towards yellow; blue (GaN) towards violet; etc.).
So, naturally, I grabbed a green LED (GaP). And it went red, not blue. Asked the instructor... he said "oh, well this one's
different, go pick a different color"...
![Roll Eyes ::)](https://www.eevblog.com/forum/Smileys/default/rolleyes.gif.pagespeed.ce.XcUK202Yqt.gif)
Chemistry is a lot like English: they like to have their rules and all (compare Lewis Octet Rule to "I before E except after C"), but... they have an awful lot of them. And a far more massive list of exceptions to those rules. So, they aren't really *rules*, are they?
![Wink ;)](https://www.eevblog.com/forum/Smileys/default/xwink.gif.pagespeed.ic.cldandycH0.png)
I suppose this is true of all fields. Certainly, textbook (HS to undergrad level) electronics has the same problem, where only the simplest, least useful approximations are taught as fact. Thevenin's theorem, maximum power transfer theorem: yes all well and good, but I don't suggest applying both at the same time to *my* audio amp. But go ahead, do try it with *your* amp.
![Wink ;)](https://www.eevblog.com/forum/Smileys/default/xwink.gif.pagespeed.ic.cldandycH0.png)
Tim