What is that makes some objects transparent while some are opaque objects.? While considering the atomic theory of matter every thing is made of atoms. Glass as well as wood.

I was recently studying the photon absorption and emission theory which somewhat explained different refractive indeces. At that time I thought about this.

For instance, take example of Carbon allotropes ; diamond and graphite - both made with carbon atoms If bonding is considered : there are many solids analogous to diamond

Then what factors decide opaqueness, transparency and translucency of objects?


You know already that light interacts with the charged particles in the atoms. The main question to ask to determine the optical character of a particular material is: What states are available for electrons to occupy? This question can be answered by the band structure or energy levels of the material/molecule, which depend on the nature and geometry of the bonding between atoms as determined by quantum mechanics.

If there are no states available for the electrons, there can be no absorption. Diamond is an insulator because its crystalline geometry determines that it has a large band gap, and visible light simply does not have enough energy to promote electrons to the available upper states. So the electrons stay where they are, and light is primarily transmitted.

In contrast, graphite has a different crystalline geometry. The result is that it has no band gap whatsoever. So no matter what color of light you choose, there will be states available for the electrons to be excited. So the electrons will absorb the light power, going into the excited states, and the light will be attenuated.

A metal has lots of states at low energies that electrons can move into. Once again, this is determined by band structure. Since there are lots of low energy states, the electrons can basically just slosh around with the driving fields. This makes metals good conductors.

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