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Re: laser light [2]



On Mon, 27 Sep 1999, Stefan Jeglinski wrote:

Thanks Bill. I plan to follow up on the laser light thread...

Hi Stefan! Have you taken a look at these entries in my "K-6
misconceptions" page?

Laser light is NOT "in-phase" light
http://www.amasci.com/miscon/miscon4.html#phase


Lasers are coherent NOT because the atoms emit in-phase
http://www.amasci.com/miscon/miscon4.html#coh

In the past, physics professionals and laser people have objected to the
above, but in exchanging messages with them I've always been able to let
them "see the light" (sorry), and they say things like "well, yeah, you
are right, but that's not the way the physics is usually explained." My
point is that the usual explanations are incorrect.

This all goes back to QM and the equivalence of wave explanations and
particle explanations. Lasers are explained using particle concepts. If
we attempt to explain them using wave concepts, we succeed without much
trouble, but there is an interesting result. Once we understand lasers
from a "waves" viewpoint, it becomes clear that the traditional
explanation of lasers is incorrect because it attempts to combine waves
and particles into a single explanation, rather than respecting the dual
nature of light. The result is a "wiggling snake" concept of light, where
particles ARE waves. This is not a proper QM concept. Light is either
waves OR particles, and we can only explain things using the wave-concept
OR the particle-concept, not both combined.

If light is tiny moving dots, then lasers can be explained. If light is
broad, space-filling electromagnetic waves, then lasers can be explained.
However, if we imagine light to be little wiggly lines, like the graphs of
a sine wave, then we fall into error, and begin to believe that these
wiggly lines can be packed together like stacked egg-cartons, and we start
talking about foolish things like "in-phase light."


There is another misconception here which I've never added to my list. If
light is supposed to look like little wiggling lines, and if the up/down
motion of the lines indicates the transverse vibration... then how wide is
that up/down motion?

For example, if the sideways motion is small, then the light waves should
fit through a narrow slot, but if it is wide, the edges of the slot would
limit the transverse motion and "snip off" the peaks of the wave. I find
that K-12 educators are shocked to discover that there *IS* no up/down
motion, and that light behaves as *straight* lines or "rays", without
having a transverse motion. Those wiggling lines are actually the graph
of intensity of the e-field (or of the b-field). Educators have made a
mistake. They've mistaken the instantaneous value of the EM field for a
transverse physical motion. They've labled the transverse axis of the
graph with "meters" rather than with "volts" or "teslas."

This "wiggling snake-lines" misconception is probably the origin of the
"in-phase laser light" misconception. If we should fall into the belief
that light is little wiggling snakes, then it makes sense that the
"snakes" could pack together under certain circumstances: when they are
all in phase with each other. Thus the concept of "in-phase light" is
born, and we come to believe that the photons in laser light are like a
stack of interleaved sinusoidal "snake-lines."

If we dispense with the "snakes" mental model, then we remove a learning
barrier from our minds. Once the "snakes" have been banished, another
obvious mental model presents itself: Huygens' wavelets. If the
fluorescing atoms in a ruby rod should each emit a spherical train of
"wavelets", then it becomes obvious that, if the wavelets are phase-locked
with peaks which describe a global plane-wave, then the wavelets will meld
together and CREATE such a plane-wave. In other words, in the long run
the individual wavelets are lost as they superpose to form a plane wave.
"Four plus six is ten", and once we have "ten", we cannot know whether it
was composed of 4+6, or 5+5, or 16-6. In other words, laser light is
simply plane-wave light, and is not a bunch of in-phase wavelets, and it
is not a bunch of "snakes" who practice synchronized swimming as then
proceed forward through the vacuum. In other words, the in-phase emission
explains why the laser medium can amplify, and it explains why the laser
medium is transparent, but it does not explain why laser light is
spatially coherent.

Laser light is spatially coherent because the geometry of the resonator
acts to convert incoherent light into coherent light over time. In other
words, a laser resonator contains its own "spatial filter", and acts as if
it were passing the light repeatedly through a pinhole aperature. If we
can explain why a "spatial filter" converts spatially-incoherent light
into spatially-coherent light, then we can explain how lasers produce
"coherent light". If instead we think that laser light is a pack of
"in-phase" snakes, then we remain stuck at the K-6 grade level in our
concepts of lasers.

I've met laser professionals who are stuck in this way. They survive by
diving deep into the mathematical description of lasers. This effectively
limits the harmful effects of their misconception. It lets them survive
in their careers just fine. But it also prevents them from ever attaining
a visual, gut-level understanding of how lasers really work, and it nearly
shuts off their creativity.

Once the "wiggling snakes" misconception is vanquished, lasers become as
easy to understand as any other optical component. Lasers even become
fun! Those competing wave-modes in a laser cavity are bizarre, and under
some conditions lasers exhibit chaos and other "organic" behavior as if
they were living things. I found in my own case that the "wiggling
snakes" misconception precluded my ever approaching these interesting
aspects of laser behavior. The "chaos" aspect of laser behavior, when it
touched the "snakes" misconception, produced mental pain in my head and
forced me to flee from the whole conversation. What I didn't know was
that the "snakes" misconception was like an open wound on my conceptual
structures. When I felt "pain", I assumed that it came from lasers'
horiffic complexity, and that I had no prayer of understanding them. I
was wrong, and the source of my "pain" was the misconception-disease
living within my skull. Once I was able to "cure" myself, the pain went
away, and (at least at the non-mathematical conceptual level) I became a
laser expert. I became able to converse with the experts on subtle and
interesting laser topics. But these topics are anything but complicated.
"In the land of the blind physicists, the one-eyed man is regarded as an
insightful genius!" But any child could do the same, if she had somehow
avoided being given the "wiggling snakes" misconception in K-6 or K-12
schooling.

Whew. Maybe I should learn to type slower. Or not read email at 4AM when
the time-limits for writing responses do not apply! :)


((((((((((((((((((((( ( ( ( ( (O) ) ) ) ) )))))))))))))))))))))
William J. Beaty SCIENCE HOBBYIST website
billb@eskimo.com http://www.amasci.com
EE/programmer/sci-exhibits science projects, tesla, weird science
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