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Lustre of metals

  1. Aug 11, 2017 #1
    What gives metals their Lustre?
    One of my books(living science chemistry by arun syamal) says that the electrons achieve a higher energy state by absorbing energy and come back to their ground state by emitting it of which light is a part but my teacher tells that it is due to the crystalline structure which reflects light. But then why doesn't diffuse reflection occur? Why specular reflection?
     
  2. jcsd
  3. Aug 11, 2017 #2
    There are a couple of ways to describe this and real metals are always more complicated than the ideal description. However a simple description can give a pretty good idea.

    In a metal the valence electrons are free to move around relative to the net positively charged atomic cores (nuclei plus bound electrons). So you have a sea of electrons floating in a sea of positive charge. Under an external electric field the whole chunk of metal can polarize. The sea of electrons can shift a little in the direction against the field (toward some distant positive charge) and leave the positive lattice leaning in the direction with the field. If the field is localized, the polarization response can be localized, but in all cases, since the ideal charge in an ideal metal is perfectly free to move, it will polarize to perfectly neutralize the applied field i.e. until the net field on the free charge is zero.

    In an oscillating field the bulk charge will oscillate to cancel the applied field. If the applied field is a traveling EM wave going into the metal, the oscillating dipole will cancel the wave going into the metal. However the oscillating dipole itself produces a traveling EM wave, and that wave is equal to and for a flat smooth surface travels in the direction of a speculator reflection.

    So that is why an ideal metal reflects. Specular vs scattering is just a question of smoothness. If the surface is flat on a size scale comparable to the wavelength of light, the reflection will be specular
     
  4. Aug 13, 2017 #3
    I get it!
    But which one is the real reason ( post 1) ?
    Or is it both ?
     
  5. Aug 13, 2017 #4
    Ok, let me try to be more succinct. Reflection from a metal is due to the polarization of the sea of valence electrons and positive atom cores, not due to absorption and remission by the bound electrons of a single atom.

    The difference between the ideal description and a real metal is that the valence electrons are not 100% free to take on just any motion. The allowed energies are determined by the lattice (see density of states).
     
  6. Aug 13, 2017 #5

    Drakkith

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    Staff: Mentor

    I don't know if the lattice structure can be said to be a cause of the reflection of light from the metal, but I know that the absorption and re-emission of light by electronic transitions cannot explain it.
     
  7. Aug 14, 2017 #6
    My bad!!
    Actually, I did connect the two but I forgot all about it when I replied .
    Well, thank you!
     
  8. Aug 14, 2017 #7
    Yes, I get it too.
    Thanks!
     
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