This project was motivated by an interest in polarized light and the way that polarized light can reveal the internal structure of certain solid materials, such as crystals and polymer plastics. Normally, light is unpolarized, meaning that the electric field of the light is moving in every direction randomly. But simply passing such unpolarized light through a "Polaroid" sheet polarizer creates linear polarized light, in which the electric field oscillates in only one direction. Human eyes are not directly sensitive to the polarization of light, but if linear polarized light is passed through a second Polaroid sheet striking visual effects such as "extinction" (darkness) can be created. Even more distinctive effects can be produced by placing transparent birefringent materials between the two polarizer sheets. Birefringent materials have two indices of refraction, and can transform linear polarized light into elliptical or circular polarized light. Since these transformations depend on wavelength, striking colorful effects can result.

We started by examining varous birefringent plastic materials between crossed polarizers illuminated with white light. Cellophane sheets can produce intense colors, but the color is uniform across the material. On the other hand certain materials such as stamped plastic lids, plates or cups can create a distinctive pattern of color and darkness, the "Maltese cross." Maltese cross patterns occur when the material being examined is radially birefringent, meaning that the two different indices of refraction are in the radial and azimuthal directions, rather than up-down versus left-right. We have confirmed our understanding of radial birefrengence by illuminating our Maltese cross samples with circular polarized light, and seeing that the distinctive pattern was no longer present. Recently we have bee able to create "synthetic" Maltese cross patterns by arranging small wedges of linear birefringent materials in suitable patterns. An alternative approach might be to induce radial birefringence by stretching a plastic film radially.