Why can pure wide-band-gap ceramics transmit visible light?

Pure wide-band-gap ceramics are transparent to visible light because of their electronic structure. In these materials, there is a large gap of forbidden energy states between the highest filled electron level (the valence band) and the next unoccupied level (the conduction band).12

When this band gap is larger than the energy of photons in the visible spectrum, the material cannot absorb visible light. Because the electrons cannot be excited across such a wide gap by visible light energies, the light passes through the material rather than being absorbed, rendering the pure ceramic optically transparent.12

It is important to note that while the material itself is transparent, its physical form matters: powders or porous compacts of these same ceramics will appear white and opaque because their internal surfaces scatter light. Additionally, while wide-band-gap ceramics are excellent insulators, the presence of impurities or specific dopants can alter their optical properties, as seen in the creation of colored pigments or active devices like lasers and phosphors.12