Electric field of an electromagnetic wave incident on a metal accelerates free electrons, and these accelerating electrons then emit radiation opposite to the electric field of the wave, thus reducing the resultant amplitude in the forward direction. Radiation emitted in the backward direction is reflected light. Now, this will only happen if the frequency of the EM wave is lower than the plasma frequency ωf of the metal, the metal being transparent to a wave of frequency greater than ωf. The reason given everywhere is that the index of refraction of the metal is imaginary for ω < ωf and real for ω > ωf. In my opinion, that is hardly a physical explanation. I understand what plasma oscillations are and I can derive the equation for ωf. I just don't understand physically why the phenomenon described in my first paragraph stops happening for ω > ωf. I have read that for high frequencies the electrons have trouble following the electric field, which makes sense, but I'd like to understand why the plasma frequency is the critical frequency. Thanks in advance.