Somehow I had not noticed the effect Brad Shue pointed out, that myopes
and prebyopes see opposite direction motions of spots. I'll try that out
next time. Thanks, Brad. I did want to share a little bit of my
experiences with speckle. There is theoretiker at UC Santa Cruz, Bill
Burke, who has done some lovely experiments on speckle. I don't know if
he ever wrote them up.
I have just had a rude shock. I just found his web page and I learn that
Bill died of injuries in July. He was a brilliant fellow whom I only met
once. I highly recommend his book "Spacetime, Geometry, Cosmology". I
regret very much never having taken him up on his invitation to visit.
His page looks interesting: http://www.ucolick.org/~burke/. I'll peruse
it on the weekend. Anyway, I'll return to the topic at hand.
I once had a student who is quite blind. I don't know what the clinical
description of her particular type of blindness is, but she can sense
light and vague shapes. She was very excited when I showed her speckle
because, for the first time in her life, she saw sharp-edged images.
My favorite demonstration of speckle involves dumping as strong a beam
as I think safe into the center of a large block of paraffin. I cast a
cylinder in a large glass beaker using clear melted sealing paraffin.
On cooling the paraffin shrinks a lot, and a somewhat conical
depression appears at the top surface. After the cylinder has thoroughly
cooled I remove it from the beaker by heating the outside quickly and
gently. The cylinder is laid on its side and the laser beam enters the
depression horizontally. When the diffused light is viewed from the side
of the cylinder on gets the illusion that the entire volume of the
cylinder is suffused with speckles. The effect is startling when one
realizes that the cylinder is actually not transparent, but rather
translucent. The spatial dispersion of the speckles is an illusion. The
brain, in its attempt to make sense of two uncorrelated diffraction
patterns presented binocularly dichotically, somehow associates each
dot in one pattern with a dot in the other pattern and fills the
perceived "boundary" of the distribution of dots.
I always point out to my students that what they are seeing isn't a
real phenomenon in the outside world, that it is all in their heads!
To prove the point I have one student move her head while another
remains stationary. Then I ask each whether the pattern is moving or
stationary, a convincing demonstration of the subjectivity of the
pattern. Of course I always ask those wearing glasses to remove them
to see the effect, and to lend their glasses to a neighbor so those
without glasses can see the effect, too.
As a foil for this demo it is a good idea to have a glass of milk
about the same size to point the laser into. Of course the speckle
pattern disappears because, even though the emergent light remains
coherent, path differences vary over times short compared to the
time characteristic of persistence of vision because of convection
in the liquid.
N.B. Don't turn the glass of milk on it side to do the demo. I didn't
try it, but I don't expect it would work well. Pointing the laser in
from the back side with a mirror works fine.
Sorry about the crepe edge on this message. I hope the rest of you
are well and will have a pleasant holiday. I will, right after I get
this stack of exams off my desk.