The half wave plate can be used to rotate the polarization state of a plane polarized light as shown in Figure 1.

Suppose a plane-polarized wave is normally incident on a wave plate, and the plane of polarization is at an angle q with respect to the fast axis, as shown. After passing through the plate, the original plane wave has been rotated through an angle 2q. 

A half wave plate is very handy in rotating the plane of polarization from a polarized laser to any other desired plane (especially if the laser is too large to rotate).  Most large ion lasers are vertically polarized.  To obtain horizontal polarization, simply place a half wave plate in the beam with its fast (or slow) axis 45° to the vertical.  The l/2 plates can also change left circularly polarized light into right circularly polarized light or vice versa.  The thickness of half waveplate is such that the phase difference is 1/2 wavelength (l/2, Zero order) or certain multiple of 1/2-wavelength [(2n+1)l/2, multiple order].

Ordering information for standard products

Part No, - wavelength - Retardation
For Example, WPS110 - 1550 - l/2

Ordering Information for None Standard Products