Why are metals opaque? Is it due to the free electrons in a metal or a material's intrinsic properties?

  • $\begingroup$ In a book , it is written it is beacuse of the free electrons in a metal which absorb the incident radiation . But insulators (wood) are also opaque . They are not conductors or have free electrons . $\endgroup$ – user170656 Jan 1 '18 at 11:15
  • $\begingroup$ you asked conducting metals. But, all metals are conductors $\endgroup$ – QuIcKmAtHs Jan 1 '18 at 11:18
  • $\begingroup$ Basically I'm asking when a material is opaque or transparent . Does it have to do anything with free electrons ? $\endgroup$ – user170656 Jan 1 '18 at 11:20
  • $\begingroup$ Related: physics.stackexchange.com/q/72368/2451 and links therein. $\endgroup$ – Qmechanic Jan 1 '18 at 11:45
  • $\begingroup$ Wood is actually transparent. Mostly fibers used to make paper, mostly cellulosa, the stuff that cellophane is made of. $\endgroup$ – Pieter Jan 1 '18 at 12:45

Metals have a high concentration $n$ of quasi-free electron constituting an electron plasma. This results in plasma frequencies $$\omega_p=\sqrt \frac {ne^2}{\epsilon m_e}$$ which are typically between visible and ultraviolet light. Below the plasma frequency an ideal plasma neglecting collisions has a negative permittivity, reflects transverse EM waves perfectly and has imaginary wave vectors, i.e., purely exponentially damped waves. Only above the plasma frequency EM wave propagation is possible. This can explain the opacity of metals in the frequency range of visible light and below.


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