CREATE A GIANT PRISM
1997 William BeatyYears ago I *made* a big prism from scratch, using polyester casting resin. This material has very good dispersion (makes good rainbows.) For an earlier science museum exhibit, our machinist had milled and polished a Plexiglas prism, but the dispersion wasn't great and the rainbow was narrow. We had to find something better. We ended up using holographic diffraction film such as Edmund Science $8 sheets. But while trying various things, I ended up making a nice large polyester prism which casts good rainbows.
For the prism mold, I took a couple of 4" x 16" plates of glass 1/4"
thick, cleaned with alcohol, put a very tiny bit of spray-on silicone
mold-release on them, taped them together on one edge to form a V-shaped
trough, and plugged the ends of the trough with big chunks of plasticine
clay. I arbitrarily picked 45deg as a prism angle, but I think a smaller
angle would work OK too. I filled it with water to check for leaks and
determine the volume of liquid plastic needed, then emptied and let dry.
I saw that the plasticine wasn't holding the trough together by itself
very well, so I added lots of duct tape strips across the top.
Next, I pre-coated the glass with highly-catalyzed plastic to give the
final product a hard finish. To do this I mixed up a half cup of clear
paperweight-making polyester resin, using plenty of hardener, poured it
in, tilted the mold around to coat the glass, then poured out the excess.
( Ernst hardware had one-gallon cans of the stuff.) I don't know if this
mold-coating step is required, but I think it guarantees a much harder
surface for your prism. Thick castings sometimes are sticky, since you
cannot use lots of hardener without overheating and warping or cracking
them. The more hardener in the plastic, the less sticky and jelly-like it
is, but the hotter it gets while hardening.
After the coating in the mold jelled, I mixed up the measured amount of
resin with a small amount of hardener (3 drops per ounce? something like
that,) and poured it in. Note: to attain a clear casting with no internal
"ripples," mix the hardner very well. I mean VERY well, and when done,
pour the mix into a second clean container and continue stirring. This
eliminates the boundary layer of unmixed plastic which clings to the sides
of the first container. Use uncoated paper containers (plastic ones will
melt, and waxpaper ones will scrape off flakes.) If the hardener isn't
mixed in totally uniformly, there will be swirls of index-variation in the
finished plastic block. Also I whacked the container hard onto the table
and waited a while for any bubbles to rise, then swept them to the sides
with a toothpick, then poured the plastic into the glass mold. I taped
the top edges of the trough together with stretched duct tape, since the
oily plasticine clay looked like was going to de-adhere from the glass and
dump the liquid plastic on my workbench.
After it was nearly hardened, I made a mix of black-dyed plastic with lots
of hardener, then poured it on top to form a thin 1/4" layer. This forms
one black face on the prism's "bottom." It eliminates that one grungy
clear top surface, and removes both the stickyness and internal optical
reflections from the prism.
The result worked great. Everything seen through the prism glowed with
spectra. The one black side improves the esthetics, if not the optics. I
sawed off and sanded/polished the ends where the clay had touched. The
end result was about four inches wide and a foot long.
I didn't have a lead-glass prism for comparison, and I never did try
actually measuring the dispersion. A slide-projector with a slit-
aperature slide made a nice rainbow in the dark.
SUGGESTIONSFill your mold with water and look through it. This gives you an idea of how the plastic prism will behave. (If you're doing a science project, maybe a water-prism would be lots easier to make than a cast-plastic prism.)
When a 1mw classroom laser was sent through my prism, I noticed that the
beam was visible inside the bulk of the plastic. This effect could
be harnessed. Instead of a prism, cast a small, cubical "light beam smoke
tank" made of solid plastic, with no milk and water to spoil over time.
The effect might be enhanced with a VERY small amount of white pigment.
I've noticed that variations in the amount of hardener in the bulk plastic
will create variations in refractive index. The plastic shrinks under too
much hardener, increasing the index. This effect could be used to build
an "artificial mirage," inside a plastic block, by pouring two layers
having differing amounts of hardener. If a plastic cube with polished
faces was cast in this way, then a laser should bounce off the interface
between the layers when directed at a glancing angle. Direct observation
should reveal a layer of "silver" when viewed at a low angle. Or, a
narrow layer of differing index could be cast within a block of plastic,
giving an "optical duct" similar to an optical fiber, and analogous to
the "acoustic ducting" which lets whales communicate with each other
across the world's oceans.
I wonder if it's possible to harness the above effect to build your own
crude GRIN lens (gradient index lens?) If a straight streak of
overly-catalysed LIQUID plastic could be put in a large block of partially
gelled plastic, the catalyst should diffuse outwards and produce a
cylinder with radial gradient index. Maybe this could be done just by
dripping in some hardener, or pouring a little highly-catalyzed chilled
liquid plastic into a volume of heated, less-catalyzed mix and hoping it
sinks and leaves a trail behind? Or dunk a long needle in some hardener,
then stab it into a block of partially gelled plastic? Inject it with a
syringe? Or somehow make a thin "wire" out of plastic (pump it into a
narrow hose?), then when it is hard, dip it in hardener and stick it into
a cup of lightly-catalyzed plastic. When it has completely hardened,
there might be a gradient-index rod which runs through the larger block.
If all of the above is too complicated, you can also get a "giant prism" effect by using Holographic Diffraction Gratings. These are cheap at Project Star website:
5" plastic spectroscope gratings & filters, 654-0025 $19.95