It sounds as if the kind of strain viewer you are asking about is the
optical kind. These consist of two polarizing filters, one of which can
be rotated relative to the other. In between the two filters is where
you put the object whose strain is to be measured. Only certain kinds
of objects will do -- they have to be clear enough to let the light
through, and the strain in the object has to affect the polarization in
some way.
When the two polarizing filters are arranged so that their
directions of polarization are perpendicular to each other, very little
(ideally, zero) light passes through both filters. One filter will let
through light waves which have their electric fields pointing, say,
vertically, while the other polarizing filter only lets through the
part of the waves where the electric fields point horizontally -- no
light wave can satisfy both. If the first filter is rotated a little
bit, then some light will get through the second filter, as a small
component of the electric fields will point in the horizontal
direction. Quantum mechanically, we say that a small fraction of the
photons pass through.
Some materials can rotate the plane of polarization of light that
passes through. Common plastics, such as cellophane, do this
particularly well. Liquid crystals also rotate the plane of
polarization, and change their behavior when an electric field is
applied. Liquid crystals, along with polarizing filters, are used in
LCD displays. Have a look someday at a liquid crystal display with
polarizing sunglasses -- it's fun!
The material whose strain is to be investigated is usually made
out of a clear plastic that rotates the plane of polarization of
polarized light going through it. The amount of this rotation depends
both on the strain in the plastic, and on the color of the light. By
adjusting the polarizing filters and the amount and strain of the
plastic, you can get a desired color to appear. You even see this
effect on sunny days when looking at the glass windows of cars with
polarizing sunglasses. Most car windows has a sheet of plastic inside
to keep the glass shards from going everywhere when the window breaks.
It is pressed there under high stress and has an interesting
deformation pattern (usually a grid of spots). You can often see these
spots in different colors with polarized glasses on a sunny day.
To get a colored background other than the gray that's usual for
these things, I'd suggest putting a thin layer of cellophane between
your polarizing filters. It may not be the best, since most
cellophane's thickness isn't controlled all that well, and there may be
stretching. Try different kinds -- food storage wrap, freezer bags, and
the clear plastic wrap found around many boxed items. Try rotating the
polarizing filters until you get the color you like. More than one
thickness of cellophane may be needed -- experiment around!
You can also "cheat" and get a magenta filter for your light
source -- you'll be sure to have magenta light if you keep the filter
(especially if it is made out of plastic) outside of the space between
the polarizing filters. If the filter is between the two polarizing
filters, then any strains in your colored filter may appear as colored
stripes in colors you may not be that interested in.
Good luck!
Tom
(published on 10/22/2007)
