Wave plates are made from materials that exhibit birefringence. The velocities of extraordinary and ordinary rays through birefringent material vary inversely with their refractive indices. This difference in velocities gives rise to a phase difference when the two beams recombine. At any specific wavelength, the phase difference is governed by the thickness of the retarder (wave plate). Standard wave plates (including λ/2 and λ/4 wave plates) are based on air-spaced construction that allows their use for high-power applications. The damage threshold is more than 20 J/cm² for 10 ns pulses @ 1064 nm.