Optical fibre is a glass thread with modulated refractive index in cross-section. It is larger in the center of the core and smaller on the surface of the fiber. Such a structure allows to get a total internal reflection for transmission of light.
A typical glass optical fibre has a diameter of 125 micrometres (μm), or 0.125 mm (0.005 inch). This is actually the diameter of the cladding, or outer reflecting layer. Layered structure is manufactured by alloying of the initial glass bar before exhaustion of a fibre. The core, or inner transmitting cylinder, may have a diameter as small as 10 μm. Through a process known as total internal reflection, light rays beamed into the fibre can propagate within the core for great distances with remarkably little attenuation, or reduction in intensity. The degree of attenuation over distance varies according to the wavelength of the light and to the composition of the fibre.