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

Qmechanic
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user170656
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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.

freecharly
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