Re: moving wall laser demo

[William Beaty <billb@ESKIMO.COM>]

On Thu, 29 Jun 2000, kyle forinash wrote:

> I'm not quite sure where/how the photocell is oriented relative to
> the beam. Do you mean right next to the laser opening and facing
> towards the beam returning from the mirror?

Anywhere near the beam seems to work.  I taped mine to the front of the
laser near the aperature and facing out, so the output beam just barely
grazed the photocell's edge, and the scattered returning light from the
dirty mirror hit the photocell (while most of the returning light went
back into the laser. )

> Do you have a suggestion for a photocell choice?

I had a 1/8" silicon cell in my junk box.  Probably anything will work,
however maybe a large cell will gather more than one interference fringe,
and it might average out the AC signal to zero, because it sees a bunch of
moving fringes rather than a spot of light that changes brightness.

> How good a mirror is needed (front surface, high quality or will an
> ordinary mirror work)?

I used a chip from a broken rear-surface mirror, and connected it to the
microphone input of those amplified speakers from radio shack.

The signals produced sound very much like something on a shortwave radio.
All sorts of squealing carrier beat frequencies.  This might corrctly give
people the idea that light and radio are similar.


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