Re: Laser light

[William Beaty <billb@eskimo.com>]

On Thu, 12 Dec 1996, Brad Shue wrote:

> Bill and David,
> 	I think you both need to give a short glossary of your terms.  I
> hear what you both are saying and I partially agree with both of you, but
> I am still confused.  I hope I'm not jumping out of my league here and
> these may sound stupid but...

I know what league I'm in!   ;)  As a non-physicist, I'm trying to figure
out how to explain lasers to K-6 students and teachers.  I became involved
in all this when I started looking at K-6 textbooks and realized that my
childhood confusion about lasers was in part due to misleading
explanations and outright mistakes in the books. 

> A couple questions:
> 1)  Isn't the "speckle" we see when a laser strikes a surface, just say an
> index card, caused by the interference of the "in step", same phase,
> photons as they reflect off the surface?

Yes, speckle is an interference phenomenon, but interference involves the
wave nature of light, rather than the particle nature.  QM issues intrude
if we start discussing light particles in flight.  Photons in flight are
not localized, even single photons cause interference, wave reflection,
etc., so I think it would make more sense to speak of the reflection of
electromagnetic waves off the surface, rather than the bouncing of
in-phase photons.

> 2)  If the photons aren't in phase, then why does a 1/4 wave plate do what
> it does to a laser beam?

The photons, being part of a single train of plane waves, are "in phase," 
meaning that there is only one wave train, even if it contains numerous
photons.  But unless you are bringing up virtual particles or QM, I think
it makes more sense in this situation to talk about e/m fields and waves
rather than the phase of photons.  The photons coming from a dipole radio
antenna are also all in phase, but we don't describe the antenna's
emissions this way.  The photons in *any* single train of plane or
spherical waves are all going to be "in phase," since there is only one 3D
wave train involved. 

> 3)  Bill, are you saying that all the photons have the same frequency, but
> are not necessarily in phase with each other?

Nope, ideally they have the same frequency, and transverse to propagation
direction they have the same phase.  But in this situation photons and
phase can be a misleading concept which obscures and disconnects light
from other wave phenomena which might be easier for the student to grasp.

When I continuously wiggle my finger in a pond, I don't describe the
resulting ripple patterns as being composed of "in phase phonons".  Yet
this description is correct.  I could spend days trying to communicate QM
concepts, mechanical waves as phonons, and discussing the phase of the
quantized particle nature of water surface waves, but I think I would get
further by showing a film of a pond and showing how a single "bulls eye"
pattern of ripples is created.  Same with light.  Ignore the quanta when
the waves make for a far simpler explanation.

> 4)  What is the purpose of the brewster window and why don't small HeNe
> lasers have them?

Hmmm, I'm not entirely sure.  I thought that the purpose was to prevent
multiple modes by reflecting away all but one polarization, so that one
polarization dominates the resonance, and the laser cavity creates a
single sphere wave rather than multiple competing wave trains, rotating
polarization, etc. 

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