From: pireddag@hotmail.com   
      
   Am 06.03.2018 um 18:40 schrieb rdelaney2001@gmail.com:   
      
   >   
   > The given explanation: if electrons were conducting,   
   > it would be reflective, like a metal.  Instead, it's opaque.   
   >   
   > I don't get this.  Reflection involve photons, which aren't   
   > choosy about positive vs. negative charges.  Can anyone clear   
   > this up?   
   >   
      
   The so-called plasma frequency, that is the frequency below which a   
   collection of free charges reflects e.m. radiation, decreases with the   
   mass of the charge carriers. More precisely, it decreases with the   
   square root of the mass.   
      
   If the plasma frequency for a collection of free electrons is in the   
   near UV, the plasma freq. for a collection of free protons with the same   
   number density is at about 20 µm, that is, in the far infrared.   
      
   For more information on this, you could learn about the Drude model for   
   the dielectric constant of metals.   
      
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