When wearing polarized sunglasses, you may notice unexpected color patterns, grids, or “rainbows” when looking at certain surfaces, particularly car windows, plastic dashboards, or digital screens. These vibrant, swirling effects are not a defect in the lenses or the objects you are viewing, but rather a direct consequence of how the specialized lenses interact with light that has already passed through another material. The phenomenon occurs because the sunglasses are filtering light in a way that reveals hidden physical properties in these other transparent objects.
Understanding Light Polarization
Light travels as an electromagnetic wave, with its electric field vibrating in many different directions perpendicular to the wave’s path. Light that vibrates in all these orientations is known as unpolarized light, which is typical of natural sources like the sun. Polarization is the physical process of restricting these vibrations to a single plane. This restriction is achieved by using a polarizing filter, which acts like a microscopic grid or slit. The light that emerges after passing through this filter is considered polarized, meaning its electric field oscillates primarily along one axis, typically vertical.
The Function of Polarized Lenses
Polarized sunglasses are designed to manage reflected glare, which is a specific type of polarized light. When unpolarized sunlight strikes a flat, non-metallic surface like water, a wet road, or snow, the reflected light becomes predominantly polarized in the horizontal plane. This horizontally vibrating light is the intense glare that reduces visibility and causes eye strain. The lenses in polarized sunglasses contain a chemical filter, often a plastic film with aligned molecules, oriented vertically. This vertical alignment acts as a barrier, effectively blocking the incoming horizontal light waves that constitute the glare. By absorbing this horizontal component, the lenses allow only the vertical light waves to pass through, eliminating the harsh glare.
Birefringence and the Visible Color Patterns
The unexpected colorful patterns seen through polarized lenses are the result of a phenomenon called stress-induced birefringence, also known as photoelasticity. Birefringence describes a material’s optical property of having a refractive index that changes depending on the polarization of the light passing through it. While most transparent materials are normally isotropic, meaning they have uniform optical properties in all directions, manufacturing processes can introduce internal stresses that make them birefringent.
Materials like tempered glass, commonly used in car side and rear windows, are heated and rapidly cooled during production, causing uneven expansion and contraction. This process locks in areas of high internal stress within the glass, which temporarily alters its molecular structure. When light passes through this stressed glass, the different wavelengths of light are slowed down and refracted at different angles, effectively changing the light’s polarization angle.
When this light, whose polarization has been twisted and altered by the stressed material, then reaches the polarized sunglasses, the lenses act as a second filter. The lens attempts to block the light based on its original vertical orientation, but because the light’s polarization has been rotated by the external material, some wavelengths are allowed to pass while others are blocked. The amount of rotation and the resulting phase shift between the light waves are dependent on the color, or wavelength, of the light.
This complex interaction between the two filters—the stressed material and the sunglass lens—causes a phenomenon called interference. As the light waves recombine after passing through the lens, the phase differences create visible interference patterns, manifesting as the characteristic colorful fringes and swirls you see. These “rainbows” are essentially a visual map of the internal mechanical stress patterns within the glass or plastic. This is why the effect is also commonly seen on plastic dashboards, which have internal stresses from injection molding, and on liquid crystal display (LCD) screens, which rely on layers of polarizing filters and birefringent liquid crystals to function.