Re: moving wall laser demo

[Herbert H Gottlieb <herbgottlieb@JUNO.COM>]

On Tue, 27 Jun 2000 17:56:50 -0400 kyle forinash <kforinas@IUS.EDU>
writes:
> 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 sound (or some kind of
> display). If I recall correctly the idea was to somehow have the
> sensor set-up sensitive enough that you could 'hear' the wall move
> if  you pushed on it with your hand. I don't recall exactly how the
beam
> detector or setup was sensitive enough to the wall's location to be
> able to detect its very slight motion.
>
> Any hints or a reference would be appreciated. Thanks in advance.
>
> kyle > -----------------------------------------------------
>    kyle forinash   812-941-2390
>    kforinas@ius.edu  Natural Science Division   Indiana University
Southeast
>    New Albany, IN    47150    http://Physics.ius.edu/
> -----------------------------------------------------

As I recall it,  a laser beam was transmitted through a beamsplitter
(small piece of plate glass) to a small mirror glued to a distant
wall. Reflections from the glass plate and the mirror were superimposed
back to a photodetector (solar cell) near the laser.

When the wall was pressed, the reflected laser beam distance
(from the wall to the photodetector) increased by a distance
equal to many wavelengths of  the laser light.  Thus, the reflected beams
from the transparent glass and those from the mirror would go in
and out of phase each time the wall distance was increased by
only a half wavelength of the laser light.  The alternate constructive
and destructive interference of the two reflected laser beams
produced changes in amplitude of the light at the photodetector .

When the electrical output of the photodetector is connected to
an audio amplifier and speaker, audio frequencies are produced
each time the wall is pressed or released.  A pressure on the wall,
applied slowly, produces a sound of low frequency.  If the
pressure is applied quickly, a high frequency sound is produced.

I suppose that the main reason that the demonstration works is that
most plaster covered walls can easily be distorted by distances
equal to several wavelengths of light by applying a relatively small
force.  However, I doubt if the demonstration would work with
thick stone, or reinforced concrete, walls.

Herb Gottlieb from New York City
.(Where talking to walls can be more rewarding than talking to
some of our inner city students)