Re: LASERS

[Ludwik Kowalski <KowalskiL@MAIL.MONTCLAIR.EDU>]

William Beaty wrote:

> ... Not necessarily.  We are told that transparent materials have an
> index of refraction because photons are absorbed and re-radiated
> after a short delay.  How can this occur unless there are appropriate
> energy levels available?  But when there ARE energy transition levels
> which are matched to the illumination frequency, the transparency is
> not like that of glass, and instead is very dependant on frequency.

To defend this point of view refer to another Heizenberg relation;
dE*dT>h_bar. If the time dT is really very very short then dE is
large and you have a "continuum" of energy levels, not the discrete
levels observed by optical spectroscopists. Nuclear spectroscopists
actually observe a continuum (no more levels when the width of each
level is much larger than the distance between levels). For light elements
this certainly happens above 100 MeV, for heavy elements this happens
above 10 MeV, or so. (Again quoting from what I remember. Wasn't
this discussed in "Nuclear Physics" by Blatt and Weisskopf?)

But one may then ask: "why are some phenomena controlled by
discrete levels while others, such as transparency of glass, depend
on continuos levels?" Should we invent a new quantum number to
"understand" this? I know that some people on the list are just smiling.
Why are you smiling? Try to address this dilemma, as you would in a
class. William's point is worth addressing.

Ludwik Kowalski