We are interested in the reflection coefficient for light with its electric field parallel to the plane of interface because we are dealing with p-polarized light. The corresponding angle of incidence is called the critical angle, q c . This angle marks the point at which the incident light no longer transmits light into the second medium. At this point the incident ray of light will be totally reflected into the high index medium. Dielectrics are materials that contain no free electrons.
Free electrons are able to traverse materials rather readily as in conductors.
In dielectrics the electrons are bound to their nucleus and are not able to travel freely. If the electric field is applied to a dielectric the positively charged nuclei will not move with the electric field, but remain tightly bound by interatomic forces. The negatively charged electrons, while not able to break free of the nucleus, seek to travel against the electric field. What follows is a phenomenon called polarization. The negative and positive charges within the dielectric are rendered neutral because there is a positive charge for every negative.