Yesterday I alluded to the fact that natural and artificial fibers tend to respond quite differently to light. Witness the mousepad. This isn't really a surprising fact, given that they have totally divergent compositions. Natural fibers come either from plants or animals; the former are made of cellulose, and the latter of α-chain proteins. In both cases there is generally abundant microstructure. These have a tendency to be highly absorptive of light, often to the point of opacity. Synthetics are made from various hydrocarbon-derived polymers, typically with uniform composition and smooth surfaces. Dyes absorb specific visible wavelengths to give them color, but they are otherwise relatively transparent, and will scatter light freely.
For want of a more usual source, I took a couple of fibers from the most convenient and immediately handy place I could think of: my head. Observe.
This is a strand of my real hair. Contrary to popular opinion, it is dark brown, not black. True black hair is somewhat uncommon among the undyed crowd. It is faintly possible to discern the ruler marks through the strand, indicating that it is slightly translucent. However, although very brightly lit, it returns little light.
This is a strand of my fake hair, which is actually a spun modacrylic thread. It appears much more luminous in this photograph because acrylics are nearly transparent and have a glasslike refractive index. When held against the ruler underlying patterns are clearly visible. Small visible striations run the length of this fiber, probably a result of the machine spinning process.