Why You're Seeing Rainbows in Car Windows
If you put on your sunglasses and notice that your car windows have suddenly become covered in wavering colors, you’re not going crazy. Nor can it be chalked up to a bad window tinting job. And it doesn’t mean that your lenses are defective.
Many rear and side car windows are tempered — designed to shatter without causing sharp pieces. During the tempering process, the glass is heated and then rapidly cooled to room temperature. The surface of the glass cools much faster than the center of the glass and contracts, causing compressive stresses, while the center of the glass expands because of its temperature, producing tensile stresses.
What you’re seeing when you notice a checkerboard or rainbow pattern in a car window is an effect called stressed birefringence. Stress on optically clear materials often produces birefringence, which basically means that the material changes the polarization of the light.
Those little dots or lines you see are sections of the glass that partially polarize light on a horizontal axis. Since your polarized lenses have a vertical axis, that light is blocked, which is why it appears as dark dots or lines.
