The thickness of the quarter waveplate is such that the phase difference is 1/4 wavelength (True-zero order) or some multiple of 1/4 wavelength (multiple order).

If the angle q (between the electric field vector of the incident linearly polarized beam and the retarder principal plane) of the quarter-waveplate is 45, the emergent beam is circularly polarized. When a quarter waveplate is double passed, i.e. by mirror reflection, it acts as a half waveplate and rotates the plane of polarization to a certain angle. Quarter waveplates are used in creating circular polarization from linear or linear polarization from circular, ellipsometry, optical pumping, suppressing unwanted reflection and optical isolation.