Birefringent materials are substances that have different refractive indices depending on the polarization and direction of light passing through them. This property causes light to split into two rays, each traveling at different speeds and angles. Common examples of birefringent materials include calcite, quartz, and certain types of plastics. These materials are often used in optical devices, such as polarizers and wave plates, to manipulate light for various applications. Birefringence is also important in fields like mineralogy and crystallography, where it helps identify and study the properties of different minerals and crystals.