The light in a fibre-optic cable travels through the core by constantly bouncing from
the cladding (mirror-lined walls), a principle called total internal reflection.
Because the cladding does not absorb any light from the core, the light wave can travel great distances. However, some of
the light signal degrades within the fibre, mostly due to impurities in the
glass. The extent that the signal degrades depends on the purity of the glass and the wavelength of the transmitted light. Below
is a diagram of how total internal reflection works in an optical fibre.