Contrary to appearances, the images above are not those of a stained glass window. They show a simple pane of glass covered with a transparent adhesive tape of various thicknesses and orientations. The colors only appear when the lens is illuminated with linearly polarized white light and viewed through a linear polarizer, in this case a pair of polarized sunglasses.
The trick to creating such art is to combine birefringent materials with polarization filtering. The refractive index of some materials depends on the polarization of light with respect to specific axes of the material. Therefore, the polarization of light can change as the light passes through the material, a change that depends on the initial polarization and the wavelength. For a given angle of a linear polarizer – sunglasses – only light with a polarization component in that direction will appear, so the colors change with angle. For example, the circular patch in the center of the glass illustration above is green when the sunglasses are set to transmit light vertically (top image), but pink when the sunglasses have been rotated 90° (bottom picture).
Aaron Slepkov, a physics professor at Trent University in Canada, created the artwork at the beginning of the article and above, along with a series of other pieces. In a recent American Journal of Physics article, he detailed the science and sometimes counter-intuitive methods behind art that derives its coloring from birefringent materials. The technique was introduced in 1967 by artist Austine Wood Comarow, who named it Polage – a portmanteau of polarization and collage. An example of his work, a Poling wall paint preserved at the Boston Museum of Science, is shown below. Its hexagons (background) contain hidden patterns that are revealed when the room’s polarizers (foreground) are set at specific angles.
The kinds of everyday materials that lend themselves to Polage make it an accessible classroom demonstration or rainy day activity, even for those, like me, who aren’t artistically talented. Following Slepkov’s example, I used a white computer screen as a linearly polarized light source and a pair of polarized sunglasses as a filter. For my birefringent materials, I used folded plastic wrap (top left, bottom) and layers of tape (bottom left). The best results, however, came not from my own design, but from a ridged plastic cup (right). Local stresses are locked into the plastic during manufacturing and create variable birefringence and a beautiful rainbow pattern.