A quarter-wave plate is an optical device that alters the polarization of light. It is made from a birefringent material, which means it has different refractive indices for different polarization directions. When unpolarized light passes through a quarter-wave plate, it can convert linear polarization into circular polarization or vice versa, depending on the orientation of the plate. These plates are commonly used in various applications, including in laser systems and optical instruments. They play a crucial role in experiments involving polarized light and are essential components in devices like polarimeters and optical isolators.