Re: A list of textbook miscon: spatial coherence

[William Beaty <billb@eskimo.com>]

On Fri, 13 Feb 1998 SCIAMANDA@edinboro.edu wrote:

> Bill Beaty wrote:
> > . . .
> > To simplify my laser, I assume that all atoms emit very long wavetrains. I
> > assume that the wavetrains are so long that any phenomena associated with
> > finite wavetrain lengths and broadened line widths can be ignored.  If the
>
> Hi, Bill!
> In trying to simplify the laser for your K-12 audience, I think you are 
> grievously sinning against Einstein's 11th commandment (paraphrased):
> "Thou shalt make things as simple as possible, but no simpler!"
> You are also short-changing the laser and its inventors.  You have made 
> assumptions which obviate the need for a laser.
>
> The triumph of the laser was not so much to provide a mechanism for 
> enhancing spatial coherence as much as harnessing poulation inversion 
> and stimulated emission (both absent in your model!) so as to provide 
> a new mechanism for enhancing TEMPORAL coherence.

I recognize this.  It belongs in a discussion of temporal coherence.  I'm
not trying to explain lasers with my present model, I'm only trying to
explain spatial coherence.

Can you tell me how my (temporary) removal of temporal coherence affects
the "spatial coherence" concept?  This is what I'm trying to figure out. 
Fear of shortchanging the inventors is not a good reason to ban someone
from simplifying a complex device.

> In short, given the extreme temporal coherence which you assume to 
> already exist with ordinary sources, no fancy wierdness is needed to 
> achieve (or explain) spatial coherence.  If we had such temporally 
> coherent sources (in the visible, etc) there would have been no need for 
> Light Amplification by the Stimulated Emission of Radiation. 

Perhaps the problem is that I have misconceptions regarding the nature of
spatial coherence.  I imagine it this way: spatial coherence without
temporal coherence looks like broadband starlight.  Temporal coherence
without spatial coherence looks like a large, diffusely glowing, extended
object which emits perfectly monochromatic light without even any sudden
jumps in phase of the emitted waves.

Does that cut to the heart of the concepts, or am I confused?

If my examples are OK, then my laser misconception can be restated thus: 

   I understand how lasers create nearly-perfect monochromatic light, but
   how do lasers create point-source light?  Textbooks seem to claim that
   spatial coherence is caused by the mechanism which also creates
   temporal coherence.  Either they are extremely wrong, or I am extremely
   confused. 


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