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The optical fiber passes through a horizontal cylinder with refraction, n sub f, surrounded by another cylinder of refraction, n sub c which is the cladding layer. The cone attached to the left of the cylinder is the acceptance cone, from which optical fiber enters the cylinder. Angle theta sub i is measured in acceptance cone, angle theta sub 2 and angle theta sub 3 are measured in the cylinder and angle theta sub i is measured on right side of the cylinder.