Electronic polarization:

• Electronic polarization can be calculated by considering the atoms of given substance. In absence of applied electric field an atom will be spherical as shown (in figure a).
• The atom consist of point nucleus of charge +Ze surrounded symmetrically by electron cloud of charge-Ze in a sphere of radiusr.
• If an electric fieldE is applied to an atom then force of |ZeE| acts on nucleus in the direction of applied electric field and on electron cloud in opposite direction, so that they shift with respect to each other by a distanced as shown in figure b. The electron cloud is assumed to remain spherical for simplicity.