Wave plates are optical devices made from birefringent materials that alter the polarization state of light passing through them. They work by introducing a phase shift between the two orthogonal polarization components of light, which can be used to convert linearly polarized light into circularly polarized light, or vice versa. There are two main types of wave plates: quarter-wave plates and half-wave plates. A quarter-wave plate introduces a phase shift of 90 degrees, while a half-wave plate introduces a phase shift of 180 degrees. These devices are commonly used in various applications, including lasers, optical instruments, and telecommunications.