When a polarized light wave enters a wave plate, it splits into two orthogonal polarized waves, each traveling at different speeds due to the two distinct refractive indices of the birefringent material. These two waves accumulate a phase difference as they pass through the material, and when they recombine upon exiting, they produce a light wave with a changed polarization state.

A wave plate, also known as a retarder, is an optical device that alters the polarization state of a light beam travelling through it. It is typically constructed from a birefringent material, which has different refractive indices along different axes.

In summary, wave plates are essential tools in the manipulation of polarized light and are widely used in scientific, industrial, and telecommunications applications.