# Understanding Plasma Frequency and Electromagnetic Wave Reflection

• Thierry12
In summary: you asked, metals with a plasma frequency lower then the plasma frequency will not allow the waves to propagate.

#### Thierry12

Why are electromagnetic waves reflected when they comme in contact with a plasma ( with a frequency lower then the plasma frequency, and I am trying to find precisely why the wave cannot pass ). I was also wondering if conductors (metals) are considered to have a plasma frequency
ty

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

## 1. What is the plasma frequency?

The plasma frequency is the frequency at which charged particles in a plasma oscillate collectively, also known as the natural frequency of a plasma. It is determined by the density and charge of the particles in the plasma.

## 2. How is the plasma frequency calculated?

The plasma frequency can be calculated using the formula ωp = √(ne2/ε0m), where n is the number density of particles, e is the charge of a particle, ε0 is the permittivity of free space, and m is the mass of a particle. It is measured in radians per second (rad/s).

## 3. What factors affect the plasma frequency?

The plasma frequency is affected by the density and charge of the particles in the plasma, as well as the external magnetic field and temperature. It also varies depending on the type of plasma.

## 4. What applications is the plasma frequency used for?

The plasma frequency is used in various applications such as plasma physics research, plasma processing in manufacturing, and in plasma display panels. It is also important in understanding astrophysical phenomena such as the behavior of charged particles in the sun's corona.

## 5. How is the plasma frequency related to plasma waves?

The plasma frequency is the resonant frequency of plasma waves, also known as Langmuir waves. These waves are created by the oscillation of charged particles in a plasma at the plasma frequency. The relationship between the plasma frequency and plasma waves is important in understanding plasma behavior and interactions.