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 polarizations of light. 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 optical communication, laser technology, and microscopy. By manipulating the polarization state of light, quarter-wave plates help improve the performance of optical systems and enhance image quality in scientific instruments.