How do fiber optic cables work?
(Lansing State Journal, March 13, 1996)
Light moves at different speeds in different media. It travels fastest through a vacuum, and slower through a medium like Earth’s atmosphere.
As a result, where there is a boundary between different media, the direction of a light beam can be altered. We can measure this change, called the index of refraction.
A fiber optic cable is made from a special medium that has two important properties: light travels easily through it, and the index of refraction is such that very little light can escape. To communicate using a fiber optic cable, a light source produces pluses of light that travel through the cable. These pulses carry information that can be translated into a picture or voice.
If a pulse encounters the boundary between the cable and its neighboring medium, its direction is changed so it goes back in the cable. This effect, called total internal reflection, means those light pulses will be received at the other end of the cable without leaking out.