At low frequencies, or long wavelengths, transmission is limited by the ionosphere, which reflects waves with frequencies less than its characteristic plasma frequency.
22.
Radiation of this frequency is below the plasma frequency of the interstellar medium, thus this medium is opaque to it, and it cannot be observed from Earth.
23.
Mathematically, the dispersion relation leads to a band with negative group velocity everywhere, and a bandwidth that is independent of the plasma frequency, within the stated conditions.
24.
If the transmitted frequency is higher than the plasma frequency of the ionosphere, then the electrons cannot respond fast enough, and they are not able to re-radiate the signal.
25.
Pendry et al . demonstrated that the plasma frequency can be made to occur in the lower microwave frequencies for metals with a material made of metal rods that replaces the terahertz frequencies.
26.
This coupling only occurs when the frequency of the light is equal to or less than the plasma frequency and is greatest at the plasma frequency that is therefore called the resonant frequency.
27.
This coupling only occurs when the frequency of the light is equal to or less than the plasma frequency and is greatest at the plasma frequency that is therefore called the resonant frequency.
28.
At very high frequencies ( near and above the plasma frequency, usually ultraviolet ), the permittivity of a metal is not so large, and the metal stops being useful for concentrating fields.
29.
In particular, at extremely large frequencies ( above the plasma frequency ), metals become transparent to photons ( such as X-rays ), and dielectrics show a frequency-dependent cutoff as well.
30.
At high frequencies, on the order of the plasma frequency, the compression of the electron fluid is an adiabatic process and \ gamma \ left ( \ omega \ right ) is equal to three.
