Physics of Total Internal Reflection

When light passes from a medium with one index of refraction
(m1) to another medium with a lower index of refraction (m2),
it bends or refracts away from an imaginary line
perpendicular to the surface (normal line). As the angle of
the beam through m1 becomes greater with respect to the
normal line, the refracted light through m2 bends further
away from the line.

At one particular angle (critical angle), the refracted light
will not go into m2, but instead will travel along the
surface between the two media (sine [critical angle] = n2/n1
where n1 and n2 are the indices of refraction [n1 is greater
than n2]). If the beam through m1 is greater than the
critical angle, then the refracted beam will be reflected
entirely back into m1 (total internal reflection), even
though m2 may be transparent!

In physics, the critical angle is described with respect to
the normal line. In fiber optics, the critical angle is
described with respect to the parallel axis running down the
middle of the fiber. Therefore, the fiber-optic critical
angle = (90 degrees - physics critical angle).

In an optical fiber, the light travels through the core
(m1, high index of refraction) by constantly reflecting from
the cladding (m2, lower index of refraction) because the
angle of the light is always greater than the critical angle.
Light reflects from the cladding no matter what angle the
fiber itself gets bent at, even if it's a full circle!

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 fiber, mostly due to
impurities in the glass. The extent that the signal degrades
depends upon the purity of the glass and the wavelength of
the transmitted light (for example, 850 nm = 60 to 75
percent/km; 1,300 nm = 50 to 60 percent/km; 1,550 nm is
greater than 50 percent/km). Some premium optical fibers show
much less signal degradation -- less than 10 percent/km at
1,550 nm.