Question:How do optical fibers work?
Optical fibers use Total Internal Reflection to guide light along their lengths.
When light is introduced at one end of the optical fiber it is guided along the entire length of the fiber and exits from the perpendicular end of the fiber at the other end with almost no loss of light along the way.
The light enters at one end of the optical fiber and strikes the interface between the optical fiber core and the cladding (or the air) at a large angle to the normal. The light is reflected by TIR and carries on to the next interface on the other side, where it is again incident at an angle greater than the critical angle and TIR happens again. As long as the fiber is not bent too much in comparison to its diameter, then the rays will remain incident at angles greater than the critical angle all the way along the fiber.
To plot the path of a ray of light along an optical fiber you must draw a new normal line at every position where the ray is reflected and then apply the Law of Reflection to calculate the correct angles.
For Total Internal Reflection to occur at the boundary between the glass core of the fiber and the cladding layer, the cladding layer must be made of a less dense material. This is usually a thin polymer coating. Most polymers are less dense than most glasses. Remember the cladding is not necessary, a thin glass rod surrounded by air is actually a better guide for light, right up until the point where a finger print is put on it and the light spills out because the critical angle is much greater at a glass → finger grease interface than at a glass → air interface.