What it shows:
Normally isotropic substances can become birefringent when under stress. This property can be used in stress analysis.
How it works:
To use birefringence in stress analysis, the sample is placed between two crossed Polaroids. The first Polaroid produces a linearly polarized light source for the sample. This source has components split into ordinary and extraordinary rays; the differing velocities of these rays in the sample creates a phase difference which is color dependent. The second Polaroid takes components of the ordinary and extraordinary rays; if the components are in phase, constructive interference occurs, and the color is seen. If out of phase, the color is not seen. As the level of birefringence is proportional to the stress, differently stressed areas will appear different colors.
1. A V-shaped piece of thin Plexiglass that can be squeezed to show stress at the elbow.
2. A block of urethane, 10×7×2cm, cast on mercury, that can be squeezed and twisted.
3. Plexiglass block whose holes have been plugged with tightly fitting metal pins. The stress caused the plastic by the insertion of the pins is a model for riveting metal plates in aircraft or ships.
4. C-clamp tightened on a Plexiglass block shows the compressive stress pattern.
The thickness of the material will also dictate which color emerges in phase. Cellophane is drawn in the manufacture of sticky tape, so is birefringent. Because the stress is uniform along its length, it is a single color. We have a sheet of glass with several strips of the tape crisscrossing; here a double layer of tape is a different color because a new color is in phase. Rotate the strips 90° to obtain the complementary colors. Old cellophane bags do the same thing.
Setting it up:
We've found the best way to display is to use two Polaroids, one circular (diameter 22cm) that sits on an OHP and the other square (28×20cm) which is on 12cm legs so that it straddles the other. There is space between them so that the sample can be placed and played with. All the above samples are available.
Really beautiful colors abound (better when viewed directly so invite your audience up at the end). Clear audio cassette boxes are another good source of stressed plastic.