30. Crossed Polarizers

1. Crumpled cellophane, or patterns of cellophane tape on glass, or prepared slides (grapes, stained glass church window, American Flag) act as half-wave plates to produce pretty colored effects when projected between crossed polarizers. As the analyzer is rotated, the complementary colors appear.
2. You can pour various colors by pouring Karo syrup into a beaker between crossed polarizers. The syrup has cork-screw-like molecules that rotate the plane of polarization.
3. A plastic form placed between crossed polarizers and then stressed will show the stress pattern on the overhead screen.
4. In a spectacular demonstration ice crystals are grown between cross polarizers. This demonstration is usually performed by the assistant and advance notice is needed to obtain dry ice.

A polarizer and analyzer above and below the bulb end of the cryophorus are crossed to extinguish light transmission and dry ice plus alcohol to increase thermal contact are introduced into the cup end. Rapid evaporation of the water inside the bulb end causes a raft of ice to form. The first time the ice freezes, in about three minutes, many small crystals form at once and jump into view on the overhead screen. The dry ice is now removed, the projector focused carefully, and the cryophorus warmed by hand until the ice melts and the last crystal is just disappearing from the overhead screen. There will still be a few ice crystals left at this point as they melt thinner than the half-wave plate thickness. Dry ice is immediately reintroduced, and in thirty seconds or so beautiful large ice crystals will be seen to grow from the remaining seed crystals, accompanied by Ooh's and Aah's from the class. The demonstration can be repeated several times.