Simpler VersionIf 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 haven't tried this yet.) 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 concrete or cinderblock walls didn't respond. They were too
sheetrock worked well. Even with the concrete wall, I couldn't
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. That, or stand near the cart on a
wood floor, and
shift your body weight from one foot to the other!
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 short-coherence laser pointers
perhaps the demo will work if the mirror is positioned within a few cm of
Here's a mystery. Rather than the light simply having an AC component, I
reasoned that the PV cell might actually be bathed
in a MOVING PATTERN of light. 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 rapidly expand and contract.
always wondered about the origins of this pattern, and how the spacing
between nodes might be calculated.
PS, if your PV cell is too large, it might "wash out" the signal by
receiving several fringes of that bullseye pattern. Experiment by masking
down your PV cell until it's only a few mm wide. In hindsight I
see that I also could have put a narrowband red filter on my solar cell in
order to reject more of the 120Hz noise from flourescent lights (the AC
noise forced me to demonstrate the effect in a darkened room.)