Waveplates (retardation plates or phase shifters) are made from materials which exhibit birefringence. The velocities of the extraordinary and ordinary rays through the birefringent materials vary inversely with their refractive indices. The difference in velocities gives rise to a phase difference when the two beams recombine. In the case of an incident linearly polarized beam this is given by α=2πd (ne-no) / λ， where α is phase difference; d is thickness of waveplate; ne,no-refractive indices of extraordinary and ordinary rays respectively; λ is wavelength). At any specific wavelength the phase difference is governed by the thickness of the retarder.