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  • Water is (broadly) transparent to optical frequencies, but opaque in ultraviolet and mid-far infrared. (graph)

  • Human flesh is (broadly) opaque in the optical frequencies, but transparent in the X-Ray frequencies

Would it be possible for a material to have a uniformly high or loss transparency across the whole frequency spectrum?

  • If yes, then do we know of any materials that have those properties?
  • If no, why? What would it mean?
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  • $\begingroup$ Black bodies absorb all EM radiation incident on it, so that would make it opaque. $\endgroup$
    – Yubin Lee
    May 25, 2019 at 6:39
  • $\begingroup$ Sure, but that's just a definition of a conceptual idea, right? Does any black body material actually exist? $\endgroup$
    – Brondahl
    May 25, 2019 at 7:05

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The transparancy of a material is dependent on the arrangement of atoms within the material and, more crucially, the arrangement of electrons. If the energy gap between the ground state of an electron and the next energy level is greater than the incoming energy of the photon, the photon will not be absorbed and we say the material is transparent.

In terms of gamma photons, the energy gap would have to be on the order of 50,000 eV in order that the photon is not absorbed. I do not know of any materials with energy gaps as large as this. The opposite argument applies for opaque materials, with very low energy gaps.

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