is a thin glass strand bilayer, each element (core and shell) has a different refractive index. The refractive index n of transparent material is the ratio of the velocity of light in vacuo (c - velocity of light) to the speed of light in this material (v), and defined by the following formula:
— respectively, relative dielectric and magnetic permeabilities.
Given that the relative magnetic permeability of the transparent material is usually constant and equal to one, the refractive index of the core for
, and for the shell