A fiber optic is a glass "hair" which is so thin that once light enters one end, it can never strike the inside walls at less than the critical angle. The light undergoes total internal reflection each time it strikes the wall. Only when it reaches the other end is it allowed to exit the fiber.
Fiber optic cables are used to carry telephone and computer communications. Advantages over electrical wired include:
- Fiber optics can carry much more information in a much smaller cable.
- No interference from electromagnet fields result in "clearer" connections.
- No electrical resistance.
- No hazard of electrocution if cable breaks.
A further application is in binoculars and stereo microscopes. A pair of 45-45-90 prisms are arranged so that the incoming light totally reflects off the inside surface of the prisms. This increases the optical path while keeping the size of the instrument relatively compact. And the alignment of the mirrors stays constant.