A dense collection of equal number of electrons and positive ions is called neutral plasma. Certain solids containing
fixed positive ions surrounded by free electrons can be treated as neutral plasma. Let 'N' be the number density of
free electrons, each of mass 'm'. When the electrons are subjected to an electric field, they are displaced relatively
away from the heavy positive ions. If the electric field becomes zero, the electrons begin to oscillate about the
positive ions with a natural angular frequency '${\omega _p}' which is called the plasma frequency. To sustain the oscillations,
a time varying electric field needs to be applied that has an angular frequency \omega , where a part of the energy is
absorbed and a part of it is reflected. As \omega approaches {\omega _p} all the free electrons are set to resonance together and all
the energy is reflected. This is the explanation of high reflectivity of metals.
Taking the electronic charge as 'e' and the permittivity as '{\varepsilon _0}'. Use dimensional analysis to determine the
correct expression for {\omega _p}$.