
Um...

1) Your Professor if just plain wrong.  Although blue LEDs are one heck of
a lot less efficient than red ones.
2) "Typical visible light LEDs" don't really exist.  LEDs have a fairly
narrow region of transmittence.  (Since the Fermi energy around either side
of the band gap falls off exponentially, blah, blah...)


You might want to check the copyright date on that. :-)

Blue LEDs most certainly do exist, I've got a bunch at work.  Cree Research
makes them (probably along with other companies...  Cree is big in
materials science, though; they advertise in the stuff like Physics Today,
etc.).  You can buy them from JameCo (I think), DigiKey (definitely), and
plenty of other distributors.

Actually, these days you _can_ get a "typical visible light" LED -- it's
called a "rainbow LED", and I've forgotten who makes it (they advertise in
EDN and many of the other trade rags, though).  It combines a reg, green,
and blue LED all on one chip (and has four leads).  Nifty device, although
it's probably fairly expensive.