If the electrons are cold, it is possible to show that the charge density oscillates at the plasma frequency \omega_{pe} = \sqrt{\frac{4 \pi n_e e^{2}}{m}} (cgs units) = \sqrt{\frac{n_e e^{2}}{m\varepsilon_0}} (SI units) \left[s^{-1}\right], where n_e is the density of electrons, e is the electric charge, m is the mass of the electron, and ε0 is the permittivity of free space. Note that the above formula is derived under the approximation that the ion mass is infinite. This is generally a good approximation, as the electrons are so much lighter than ions. (One must modify this expression in the case of electron-positron plasmas, often encountered in astrophysics). Since the frequency is independent of the wavelength, these oscillations have an infinite phase velocity and zero group velocity.

— Wikipedia on Plasma oscillation

2009.11.23 Monday ACHK