REFRACTIVE INDEX
Fibers vary in shape but are almost always thicker at the center than near the edges. Thus, they act as crude lenses, either concentrating or dispersing the light that passes through them. If a fiber has a higher refractive index than the medium in which it is mounted, it acts as a converging lens, concentrating light within the fiber. If the fiber has a lower refractive index than the mounting medium, it acts as a “diverging” lens and the light rays diverge from the fiber, as illustrated in Figure 1 with a fragment of glass. In most fibers, the light rays only slightly converge or diverge and thus appear as a thin bright line, called the Becke line, after the Austrian mineralogist Fredrich Becke who first described the phenomenon, at the interface between the fiber and the mounting medium (Good and Rothenberg, 1998). When an analyst is observing the fiber, the working distance on the microscope is increased (the stage is moved down); if the fiber has a higher refractive index, the Becke line moves toward the fiber as the working distance is increased. If the mounting medium has a higher index, the Becke line moves toward the medium (away from the fiber) as the working distance is increased (see Figure 1). If fibers are mounted in a medium that has a refractive index of 1.52, such as Permount, then the fibers can be described as being greater than, equal to, or less than 1.52. The refractive indices of a fiber can be measured directly by placing the fiber in a series of liquids of specific refractive indices until the refractive indices of the fiber and liquid are the same. At this point, the fiber “disappears” because the fiber and the liquid are now isotropic, meaning that light is traveling at the same speed through both the fiber and the liquid.
FIGURE 1 Fiber acts as a crude lens, either focusing light into or away from it. This has to do with the refractive index of the fiber, that is, the ratio of the speed of light in the fiber material over the speed of light in a vacuum. Appearing as a line of bright light, the Becke line can be used to determine the relative refractive index of a material, in this case, a fiber. When the distance between the microscope lens and the sample is increased, the Becke line moves toward the material with the higher refractive index.
