Date: Thu, 21 Mar 2002 11:16:26 -0700

Author: Jerry DiMarco



The setup I described creates a dark spot in the middle of an
illuminated circle of light. My understanding is that it's the same
physics that predicts a bright spot in the shadow of an object. In the
former case destructive interference at the center creates a dark spot, in
the latter case constructive interference at the center creates a bright


At 3/21/2002 12:03 PM, you wrote:
>I thought and thought about this, and I can't picture it. Why in the
>world would a small circular aperature throw a dark spot on the wall?
>Are you puttin' us on here, or is there something I'm missing?
>-St. John
>Boston University
>On Thu, 21 Mar 2002, Jerry DiMarco wrote:
> > Another neat demo to do is the inverse of Poisson's spot, a dark spot
> > in the middle of a bright area. I can't find this one in the DCS although
> > 6C20.30 - Airy diffraction rings seems similar. The Pasco optics kit also
> > contains a variable aperture that works well for this demo. Just put it in
> > front of an expanded laser beam and gradually close the iris. You can
> > watch the diffraction pattern developing, then at minimum aperture a dark
> > spot appears in the center. It is especially nice and sharp with a
> > spatially filtered beam...
> >
> > Jerry