Electric fields do not penetrate conductors in static equilibrium, but they can exist inside conductors when charges move, such as in a circuit. When an external electric field is applied to a metal plate, it attracts free electrons, which move until the forces balance, creating an opposing electric field that cancels the original field inside the conductor. This charge separation does not eliminate the electric field on the outside of the plate. Electromagnetic (EM) waves cannot pass through metal because the free electrons in the metal respond to the wave, either absorbing it or reflecting it due to their collective oscillation. Understanding these interactions clarifies the behavior of electric fields and EM waves with metals.