Date: Tue, 11 Aug 1998 14:23:55 -0700
From: Steve Beeson <beeson@asu.edu>
Subject: Rainbow maker
To: "'Pinhole Listserv'" <pinhole@exploratorium.edu>
At the Institute this summer, I made a portable rainbow-maker as one of my
Snacks. If you haven't seen this done, here's the recipe (quick n' dirty):
a piece of posterboard
spray adhesive (I used 3M Photo Mount)
retro-reflective glass beads (see below)
Spray the posterboard with the adhesive (cover uniformly) and
pour the glass beads over the surface (again, being as uniform as
possible -- not easy).
Spray the surface again to help keep the beads on the posterboard
Hold a flashlight (Mini-Mags work great) in front of you and shine
it
on the bead-covered board.
and/or
Take the rainbow-maker outside and view in the sunlight (w/ sun
behind you....)
The place to get these retro-reflective beads are 1) sand-blasting
equipment stores (so prevalent these days...), and 2) Departments of
Highways (where you may be able to get them free if you say you are
a
teacher). I found that the correct sizes are either #6 or #10 (I
haven't gone
to pick them up yet, but that's what the man at the sand-blasting
store
told me) and the beads look and feel like very smooth sand. They
run for
about $23 per 50 lb. bag (which should last a lifetime).
My questions, to anyone who has tried this: why does the rainbow
that is
seen when using the flashlight seem to actually _float_ around the
head
of the flashlight? If you haven't seen this, you have to see it to
believe it.
Truly an incredible thing. My pet hypothesis: it is the binocular
effect of
your eyes. If you close one eye, the rainbow appears to lie on the
paper,
but if you open both, it appears to float, much like a 3-D image in
a
ViewMaster.
My second question, does anyone know the index of refraction of
these
beads that makes them retro-reflective? And is there any references
where I can find more info on them (I've done a small bit of
research on
the Web, but have found nothing)?
If anyone has any ideas about this, please share. We are planning
on using this module this Fall, and I want to be more on top of it
than I
am....
Thanks, and sorry this was so long,
Steve B.