<Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (July 2017) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (July 2017) (Learn how and when to remove this template message) </Td> </Tr> <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> <P> The core is characterized by its diameter or cross-sectional area . In most cases the core's cross-section should be circular, but the diameter is more rigorously defined as the average of the diameters of the smallest circle that can be circumscribed about the core - cladding boundary, and the largest circle that can be inscribed within the core - cladding boundary . This allows for deviations from circularity due to manufacturing variation . </P>

Function of core and cladding in optical fibre
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