<P> The core of a conventional optical fiber is a cylinder of glass or plastic that runs along the fiber 's length . The core is surrounded by a medium with a lower index of refraction , typically a cladding of a different glass , or plastic . Light travelling in the core reflects from the core - cladding boundary due to total internal reflection , as long as the angle between the light and the boundary is less than the critical angle . As a result , the fiber transmits all rays that enter the fiber with a sufficiently small angle to the fiber 's axis . The limiting angle is called the acceptance angle , and the rays that are confined by the core / cladding boundary are called guided rays . </P>
the core of an optical fiber has a
cylinder of glass or plastic that runs along the fiber 's length