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