Notes on Opacity

Silica xerogel from the exploding gel lab
(disclaimer: this is not the sample that was blown up)

Before school started, a friend asked me what makes a material transparent. And despite me being from Materials, the question got me stumped. To understand why, here's a summary of what stories people have come up with to explain it.

Prof Li, NUS Materials: A material is transparent if it is amorphous. For example, in glass, the molecules form a network structure, allowing light to pass through the holes.

Prof Moriarty, Nottingham Physics: No, no, that's rubbish! A material is transparent if it is a semiconductor, with a bandgap energy well higher than the range of visible light.

Prof He, NUS Materials: A polymeric material is opaque because of the electron density differences between crystalline and amorphous regions. (At this point I wished my Finland bosses were around so that they could feel my excitement)

Prof Kasap, Univ. of Saskatchewan: Attenuation of light in a dielectric medium is explained through the complex permittivity of the dielectric material.