Electromagnetic wave cut-off
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A Plasma, being a (partially) ionized medium, may inhibit the transmission of an electromagnetic field due to the skin effect.
The force of an electric field on a charged particle in the plasma accelerates it. An alternating field makes a charged particle oscillate. Surrounding charged particles exert a force on the oscillating particle. The interaction leads to a resonance phenomenon, called plasma resonance, see electron cyclotron resonance. In case of electrons oscillating in front of an ion background, which is quasi motionless at high electric field frequencies due to the larger mass of ions, the electron plasma frequency depends on the charge density of the surroundings and the frequency of electron collisions.
If an oscillating charged particle can follow an electric field in phase, the field is reflected. This field-frequency-dependent phenomenon is know as wave cut-off. Only waves with frequencies exceeding the cut-off frequency, which is equal to the plasma frequency, can traverse the plasma. A plasma with an electrondensity exceeding the critical density is called over-dense.