Light is an electromagnetic wave. It oscillates perpendicular (transverse) to the direction of propagation of the light beam. Such a transverse electromagnetic wave can be divided into unpolarized and polarized light.
The plane of polarisation of unpolarized light, also called natural light fluctuates arbitrarily around the direction of propagation, so that in average, no direction is preferred. A beam of light can be thought of as being composed of two orthogonal electrical vector field components.
Polarized light occurs when these two components differ in phase or amplitude. All field components of polarized light have a fixed phase difference to each other. Each state of polarization (SOP) can be split into the two principal orthogonal states.
Different SOP‘s are shown in the following picture.
There are three states of polarization. The linearly, elliptically and circularly polarized states.
The first drawing in Figure 4.1 displays linear polarized light. This linear state of polarization can generally be described as a superposition of two circular states of polarization. The superposition of both orthogonal states is oriented in only one direction of the transverse plane. The projection of the field vectors to the plane results in a line.
The second drawing shows an elliptical state of polarization. Both orthogonal waves have a fixed phase difference (Delta) between 0 and 90°. The projection results in an ellipse with a left or right direction of rotation.
A special case is the circular state of polarization where the phase difference (Delta) is 90°. Then the electric field vector rotates by 360° within one wavelength and the projection results in a circle.