"SQUEALING WALL"
Laser demonstration
On Tue, 27 Jun 2000, kyle forinash wrote: > Can anyone give me a reference to a demonstration I saw a few years back? > It was something along the lines of gluing a tiny mirror to a wall, > shining a laser off the mirror, collecting the laser light with a > sensor of some kind and turning the beam into soundIt's called "Laser Audio Interferometer" I tried this and it's simple. The mirror reflects the laser beam back down the bore of the HeNe laser tube, forming a second optical cavity external to the laser. Or use a laser pointer instead; most will work. A small photocell collects some of the spill-light from the beam, and the photocell output is sent to an audio amp and loudspeaker. If you move the wall (or the laser) a tiny bit, then as the "cavity" changes length, it sweeps across hundreds of resonant lengths located every 1/2 wavelength of the light. The light intensity sequentially grows dimmer and brighter, generating a sinewave signal. The frequency is proportional to the velocity of the wall. Connect the photocell to the microphone input of an audio system, and when the wall is pushed, you hear moans and squeals. It's like raking your fingernail along the teeth of a comb, but with the teeth spaced half the wavelength of light: every 300 nanometers!
Rather than using a fancy mirror with X-Y positioner, I just put the
laser on a cart, aimed its spot on the wall, then slapped a chip from a
broken mirror up on the wall with double-stick foam tape. I then could
move the cart around until the reflected spot fell upon the laser's exit
aperture. My photocell was a tiny (3mm) square silicon photodetector from
my junkbox, but I suppose that any silicon photovoltaic cell would work.
I taped the photocell to the front of the laser so it was very near the
aperature and facing outwards. The mirror chip wasn't perfectly clean, so
lots of scattered light surrounded the return beam, and the photocell
received a strong signal.
______________________
= | |
= | |
= - - - > - - - - - - - < - - - - -| HeNe Laser |
= || | |
= |______________________|
/\
MIRROR Small Photocell
Simpler Version
If you have a large photocell with a glass-smooth surface, you might consider mounting it on the wall instead of the mirror. Use the photocell as a crude mirror, and bounce the reflected beam back into the laser's aperature. I found it more convenient to use a mirror, just because the laser, the light sensor, and the audio amp could be placed on the same lab-cart.
I found that a concrete wall didn't respond. It was too stiff. Wood
or
sheetrock worked well. I couldn't even approach the wheeled cart with the
laser, since the flexing of the wood floor was enough to generate all
kinds of squeals. If you have only a concrete wall, then put the mirror
on a separate table, so that delicately touching the table will create all
kinds of squealing from the system.
I used a HeNe laser, and I don't know if all laser pointers will work in
this application. You need laser with a long coherence length. The
demonstration worked well even with the mirror separated by several
meters from the laser. If you only have laser pointers available,
perhaps the demo will work if the mirror is positioned within a few cm of
the laser.
Here's a mystery. I reasoned that the photocell might actually be bathed
in a MOVING PATTERN of light, and not just a changing intensity. I
mounted a white card on the front of the laser, with the beam shining
through a small hole. Sure enough, when I bounced the beam from the
distant mirror back into the laser, I saw a bullseye-shaped interference
pattern in the scattered light coming back from the dirty mirror. Tiny
motions of the laser's cart made this pattern expand and contract. I've
always wondered about the origins of this pattern, and how the spacing
between nodes might be calculated.
PS, if your photocell is too large, it might "wash out" the signal by
receiving several fringes of that bullseye pattern. Experiment by masking
down your photocell until it's only a couple of mm wide. In hindsight I
see that I also could have put a narroband red filter on my photocell in
order to reject the 120Hz noise from flourescent lights (the AC noise
forced me to demonstrate the effect in a darkened room.)
