Why do car headlights turn yellow?

Because the glass cover of the headlight on the general car model is injection molded of high-grade polycarbonate, the common resin spectacle lens is this material. Polycarbonate has high hardness, high strength, high toughness, UV resistance, and good light transmission. . However, in long-term use, the lampshade will turn yellow due to the surrounding environment, high temperature or long-term ultraviolet light irradiation.