Tempered Glass Rainbow Patterns (Stress Birefringence): Viewing Car Rear Windows Through Polarized Lenses

Automotive side and rear windows are made of thermally tempered safety glass. Heated to $650^\circ\text{C}$, the sheet passes beneath an array of high-pressure oscillating air nozzles ('air quench') that blast cold air onto both surfaces:

$$\text{Surface Chills Rapidly into Compression } (\sigma_c \approx -100\ \text{MPa}) \quad | \quad \text{Core Remains in High Tension } (\sigma_t \approx +50\ \text{MPa})$$

Because the air jets deliver uneven cooling corresponding to nozzle spacing, permanent 2D grid patterns of compressive and tensile mechanical stress are frozen into the molecular silica matrix.

Mechanical stress induces optical anisotropy via the Stress-Optic Law ($\Delta n = C \Delta \sigma$). When polarized blue sky light enters the glass, it splits into fast and slow orthogonal components, emerging with wavelength-dependent phase retardation:

$$\delta(\lambda) = \frac{2\pi}{\lambda} \cdot d \cdot C \cdot (\sigma_1 - \sigma_2) \implies \text{Destructive Interference at Specific Wavelengths}$$

Your polarized sunglasses serve as the optical 'analyzer.' At locations where $\delta = \pi$ for green light, green is extinguished, leaving purple/magenta fringes; where red is extinguished, cyan glows. The invisible thermal stress map of the glass becomes vividly visible.

Front windshields are manufactured from laminated annealed glass (two thin sheets of untempered float glass sandwiching a PVB interlayer). Because annealed glass cools slowly without high-pressure air quenching, it contains near-zero internal stress birefringence, leaving front windshields clean and artifact-free.