What is the difference between a transparent material and an opaque one at the atomic level? I know that something is transparent to light because the energy of the photons that correspond to visible light does not match the transition energy of the atoms that make up that body. What happens when something is opaque? And why the angle of incidence is always equal to the angle of reflection? I know about the Fermat's principle and all the mathematical arguments, but I was looking for a "physical picture" of the phenomenon.
[Physics] the difference between a transparent material and an opaque one at the atomic level
opticsvisible-light
Best Answer
There’s often nothing easily intuitive about the difference between transparent and opaque materials on an atomic level.
Take, for instance, carbon, which can form as diamond (transparent) and graphite (opaque). What’s the atomic difference between the two? Well the atoms are the same, but in each case they have a different crystal arrangement. So something about the arrangement of the atoms is responsible for the difference in transparency.
Unfortunately, the difference in optical properties must be understood through complex calculations of the crystal lattice, including the species of the atoms, their relative orientations, the strength of their couplings, possible inclusions/dopants. In general, nothing can be easily understood without cranking through the calculations. And of course, once you’ve done a calculation, you might find that other physics (like excitons, phonons, or wavelength-scale interference in a slab of material) applies for the colors of interest.
But when the calculations are complete, you’re generally looking for an electronic band gap, which means that electrons cannot absorb photons of a range of energies because they have no available states at those energies. When that condition is fulfilled, the material has a good chance of being transparent. If not, it will certainly not be transparent (although it might be a little transparent if thin enough).
That said, there are a few intuitive rules which can sometimes be applicable. For example, stronger binding between atoms, and smaller atoms, usually result in wider electronic band gaps for insulators (because the atoms are closer together), meaning that it will be transparent over a wider color range. That’s why, for example, diamond is transparent while silicon (also group IV and of the same crystal structure) is not (for visible light; they are both transparent in the infrared).