Interaction of electric fields with conductors

An electric conductor has very low resistance. It's conductivity is extremely high. Since current density is product of conductivity and electric field inside a conductor, electric fields tend to zero to avoid the condition of infinite current density. Also, for such conductors, the current flows in a very thin layer on its surface. Since fields cannot exist inside the conductor, all of the incident energy is reflected in the case of a perfect conductor. This reflection can be seen as the following steps

1. Incident fields induce currents over a thin area on its surface.
2. The currents re-radiate fields into space.

Thus a perfect conductor behaves as a perfect reflector of electromagnetic energy.