A half-wave plate is an optical device that alters the polarization of light passing through it. It is made from a birefringent material, which means it has different refractive indices for different polarization directions. When unpolarized light enters the half-wave plate, it can change the polarization state, typically rotating it by 90 degrees if aligned correctly. These plates are commonly used in various applications, including laser systems and optical communication. By manipulating the polarization of light, half-wave plates can enhance the performance of devices like polarizers and optical isolators, making them essential components in modern optics.