Fig 2
Fibers in bundles are clad by a material that has a lower index of refraction than the core to ensure total internal reflection, even when fibers are in contact with one another. This shows a single fiber with its cladding.
Source
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"OpenStax College, Total Internal Reflection. January 29, 2013."
http://cnx.org/content/m42462/latest/
OpenStax CNX
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