# Plasma frequency

## Main Question or Discussion Point

Why are electromagnetic waves reflected when they comme in contact with a plasma ( with a frequency lower then the plasma frequency, and im trying to find precisely why the wave cannot pass ). I was also wondering if conductors (metals) are considered to have a plasma frequency
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Born2bwire
Gold Member
A plasma is a sea of ions and electrons that is generally neutral in charge. Generally, we assume that the ions have much larger mass than the negative charges, being that the ions are the nucleus and electrons of an ionized atom and the negative charges the stripped electrons. When an electromagnetic wave hits a conductor, the electric and magnetic fields induce currents in the conductor. These currents produce their own electromagnetic waves that cancel the incident wave. In a perfect conductor, there is no resistance to these currents and so they perfectly cancel the incident wave in the conductor, causing the incident wave to reflect.

When an electromagnetic wave travels through a plasma, the electric field also induces currents due to the Lorentz force, just like with a conductor. We do not consider the ions to move though, because the frequency of the fields are too high. The heavy ions have too much inertia to move along with the high frequency fields. However, the light electrons do move with the fields. The electrons thus induce the same currents as we would find in a conductor giving rise to the cancelling and reflected fields.

However, there are secondary effects on the electrons in the plasma from the magnetic field of the electromagnetic wave. This gives rise to what is called the ponderamotive force. If I recall correctly, the ponderamotive force is what allows the wave to eventually propagate. The ponderamotive force is like a dispersion force, it is a force that acts on the electrons towards the volume of weakest electric field. So without the ponderamotive force, the electrons will be able to produce the wave cancelling currents willy-nilly. However, the ponderamotive force will create a drift in the electrons, upsetting the desired currents and thus allow higher frequency waves to propagate.

Yes, some metals, I don't know if all, do behave as plasmas. The plasma frequency is very high though, for example, silver has a plasma frequency in the terahertz. As I recall most of the plasma modes are surface modes though.

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ty for the information