Optical fibres are made from a core (cylinder) of glass of a certain refractive index, n₁, surrounded by a "cladding" of lower refractive index, n₂.

If the light meets the interface between core and cladding at the critical angle, c, then i is the maximum angle of incidence which will allow transmission of light down the fibre.

Speed of light in vacuum = v

The refractive index of light going from medium 2 to medium 1 is given by

and, as shown previously, this is equal to 1/sinc, where c is the critical angle of incidence for the two media.

and, from the diagram it is clear that

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