[General] Coherent light

[Andrew Meulenberg]

Dear Hodge,

Like you, a small group of us* are re-examining the nature of light from
the experimental view-point by using lasers. However, whereas you are
studying diffraction patterns, we specifically try to avoid them and look
at the interference patterns without that complication. To do so, we use
the 'zero-slit' experimental setup. It uses beam splitters, rather than
slits, to obtain interfering beams. Nevertheless, we are doing very similar
work and pondering the same questions as you.

I considered sending this off-line from the NoL group because I have not
had time yet to read your papers, or even most of your emails. I will try
to catch up at some point. Like you, I found that a camera was not adequate
and spent the nearly $5k for a beam analyzer.necessary for the job. Almost
all of my other equipment came from E-bay. I have just donated much of it
to a local college (Randolph-Macon in Ashland VA). However, since they have
no present use for it and I will not be able to use it for a couple of
years, you might be able put it to use if you need it.

In one of your emails, you said:

"... The experiment I conducted was limited to equipment available to a
retired researcher. Note the difference in the left side predictions of the
model. Some show a low intensity diffraction pattern, others no or little
pattern on the left side. My camera could not detect these much lower
levels. That is why I’d like to see the experiment done with better
detection equipment - to verify or reject the model by looking at the left
side of the pattern.

What the experiment has to show is coherence of the impinging light from
the first mask. This was done. What the laser is or is not becomes
irrelevant once coherence is shown to be impinging on the second mask. The
source could be anything - from sun, from an incandescent bulb, etc. (light
from an incandescent bulb is not coherent but becomes coherent when passed
through a slit.) Likewise, the dimensions of the experiment produce the
diffraction pattern period."

We have observed the same effect on an interfered (but non-diffracted)
beam, which we know to be coherent. Your comment about coherence beyond the
slit was very interesting and forced me to look up some things [e.g.,
https://en.wikipedia.org/wiki/Coherence_%28physics%29#Spatial_coherence].
Fig 8 in the ref is particularly interesting. We know that light from the
sun has a coherence length that pertains here. Does an incandescent bulb
also have one that is adequate to produce the effect? I think that a test
would be to tilt the double slit so that the incident light front will
enter the two slits beyond the coherence length. If the coherence length is
too short, I think that a straight-edge or single slit will display
diffraction, but a tilted double slit would give the sum of two diffraction
patterns rather than true interference between the two diffracted beams. I
wish that I had time to actually compare laser, sun, and incandescent-bulb
light in this manner.

Andrew
__________________________--


*  Interestingly, one of them, Bob Hudgins, was called Hudge for many
years.
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