not pursue that here. The question is: how does the polarised dielectric modify the original external field inside it? Let us consider, for simplicity, a rectangular dielectric slab placed in a uniform external field E parallel to two of its faces. The field causes a uniform polarisation P of the dielectric.
Thus FIGURE . A dielectric develops a net dipole moment in an external electric field. (a) Non-polar molecules, (b) Polar molecules.