Quarter Wave Plate
quarter-wave-plate: Let us consider a quarter wave plate such that optics axis is making an angle of 
with the y axis and is placed in y-z plane. A linearly polarized plane wave is allowed to fall at normal incidence on this wave plate as shown in Fig 1.
fig..(1)
The direction of propagation of this wave is along x axis. The wave is polarized along y direction and its electric field is given by equation 1. The two orthogonal components; E|| , along the optics axis and 
, perpendicular to the optics axis are given by
(1)
(2)
Since the quarter wave plate introduces a pah diffrence of 
, or corresponding phase change of 
, therefore the two components after traveling inside the wave plate will come out as
(3)
Thus the two mutually orthogonal components are of equal magnitude and have a relative phase difference of . hence after traveling a quarter wave plate, for the orientation discussed above the plane polarized light is converted into a circularly polarized light.
Therefore quarter wave plate can be used to obtained a circularly polarized light from a plane polarized light.