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Author Topic: Why is glass transparent but graphite (carbon) is not?  (Read 735 times)

Offline memoryerase1

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... does the carbon not transmit light.
If they are both made from the same noncrystalline atomic structure.
Say for a minute the carbon CAN (I am not sure) have its atoms to be in a noncrystalline network just like the glass.
So the carbon is a amorphous solid made from noncrystalline, just like the four inch cubed glass, yet its electrons absorb light why.
Or maybe I should be asking why the carbons electrons are absorbing light, but in the four inch cubed glass the electrons are not absorbing light.
The carbon, and glass atoms are arranged in the same non crystalline network, why is carbon absorbing atoms.
Thank you for your help, anything helps even if it is a few words.
« Last Edit: 08/01/2016 08:02:56 by chris »


Offline chiralSPO

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Carbon definitely forms amorphous solids (that's essentially what makes up most charcoal). The  key to interactions with light is not just the position of the atoms, but how the electrons are shared between the atoms. In glassy carbon, there are many possible electron transitions over the whole range of visible light (which is why it looks black). However, in very highly ordered crystalline carbon (diamond) there are no transitions that can absorb visible light, so pure diamonds are colorless (and transparent if high enough quality!) Introducing small impurities into diamond changes the electronic structure, such that diamond that has a small amount of boron in it appears blue (like the Hope diamond).
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Offline evan_au

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why the carbons electrons are absorbing light, but in the four inch cubed glass the electrons are not absorbing light?
Adding to the comments by chiralSPO...
3D Amorphous Carbon in the form of charcoal, or 2D crystalline carbon in the form of graphene is a fairly conductive material. There is a "sea" of electrons which are shared among many atoms.

A consequence of Pauli's Exclusion Principle is that no 2 electrons can share the same energy level. In a material like this, there is not just a single energy level for outer electrons, but a "band" of energies which are shared among all the electrons. This means that no matter what energy a photon has, it will quickly hit an electron with just the right energy to absorb it. This results in a black material (although it takes many layers of graphene to become totally opaque).

However, silicon dioxide in glass is an insulator, each molecule tightly holding onto its own electrons. In this case, the electrons do not see the atoms of adjacent molecules so strongly, and the electron energies are more tightly confined. Silicon Dioxide does not absorb much in the visible range (but it does absorb in parts of the ultraviolet and infra-red spectrum).
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