When a free charged particle is placed in an electric field, it is moved by this field under the action of the coulomb force. This is for example what happens with free electrons in a conductor. Not all materials are conductors though, and in most, the charged particles (ions and electrons) are not completely free in their movement, forming clusters (of atoms or molecules) of total charge zero. However, under the action of an external electric field, the individual particles (electrons or ions) will be locally displaced from each other inside these clusters, the particles with opposite electric charges moving in opposite directions (but over limited distances, unlike free charges). We refer to the phenomenon of "charge separation".
Thus, after applying an electric field to an insulating material, the molecules or atoms of which it is made are deformed such that the centres of mass of the positive and negative charges in it no longer coincide, and this situation arises for certain molecules even in the absence of an external electric field (for example, water molecules). Such an atom or molecule possesses what is called an "electric dipole moment" (induced or spontaneous), a quantity that reflects both the distance between the centres of mass and the absolute value of the displaced charges. If it exists, this dipole moment is carried by each of the "neutral clusters" forming the material, and we define the polarisation vector "P" as the volume-average of the dipole moment. This polarisation P itself creates an electric field E which is superimposed within the material on any initially applied field. In the simplest situations of induced polarisation, the polarisation vector is proportional to the applied electric field "E", the proportionality factor being a second order tensor (a matrix) if the material is anisotropic. Furthermore, if the electric field is strong enough, the reaction of the local charges to the field E is more complex and the relation between P and E is not necessarily linear.
The property of a light wave that describes the behaviour of the electric and magnetic vectors during its propagation (a light wave being an electromagnetic wave). The polarisation of a light wave gives the directions that the electric field vector (or the magnetic field vector) follows over time or along a given light ray in the orthogonal plane to the wave vector. There are some special cases:
- linear polarisation for which the electric field vector "points" constantly in a given direction
- elliptical polarisation (of which circular polarisation is a special case) for which the electric field vector describes an ellipse.
Often natural light is not polarised, resulting in the "displacements" of the electric and magnetic field vectors being disorganised.