Consider a dielectric placed in an external electric field E0 (say between the plates of a charged capacitor). Due to external field, there will be shifting of the centres of gravity of positive and negative charges. The positive charges are displaced in the direction of the applied field whereas the negative charges will get displaced in a direction opposite to the applied field E0 ( try to make the figure). Therefore, the positive charge is induced on one surface while negative charge on other.
This phenomenon of appearance of the charge on the surfaces of dielectric induced by the external electric field is called polarization or dielectric polarization.
The charge on the plates of the capacitor is called free charge and charge induced on the surfaces of the dielectric is called bound charge.
We can also say that when the atoms are placed in an electric field, they acquire an induced electric dipole moment in the direction of the field. This process is called the dielectric polarization and atoms are said to be polarized.The induced dipole moment p is proportional to applied electric field E,
p proportional to E
or p = αE
where α is proportionality constant known as molecular or atomic polarizability of dielectric.
Also, the induced surface charges appear in such a way that the electric field Ep set up by them opposes the external field E0. The resultant field will be
E = E0 – Ep (1)