Geometrical Optics

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Light guidance in an optical fiber can be understood by considering a structure comprising of thin solid glasscylinder of refractive index n1 surrounded by a medium of lower refractive index n2. The light guidance in thestructure takes place due to successive total internal reflections at the interface of the media n1 and n2 as shown inthe figure. All rays with the angle of incidence i less than a particular value im are confined in the medium ofrefractive index n1. The numerical aperture (NA) of the structure is defined as sin im.