What is the primary reason that insulators become polarized when placed in an external electric field?

When an external electric field is applied, the electrons in an insulator are attracted toward the direction of the field while the positively charged nuclei are pushed in the opposite direction because they are much heavier and less mobile. This slight shift of charge creates tiny dipoles throughout the material, which is called polarization. The induced dipoles then produce an internal field that partially opposes the external one, reducing the net field inside the insulator. For example, a glass rod in a uniform field will have its electrons displaced slightly to one side and its nuclei to the other, forming a chain of tiny dipoles that align with the field. This separation of charges is the primary reason insulators become polarized.