The refractive index (RI) is a characteristic value for the refraction of light in optical materials. It is a dimensionless material parameter, denoted by the Greek letter µ, used to determine the speed of propagation of light in optical media relative to the speed of propagation in free space.
The refractive index is the ratio between the speed of light (c0) and the speed of propagation (v) of light in denser media. The ratio is calculated from: µ = c0/v and is >1 for all media. The refractive index plays an essential role in optical waveguides, since the propagation of the modes depends on the RI values of the core glass and cladding glass. The difference is the refractive indices of core glass and cladding glass, it determines the numerical aperture ( NA).
Vacuum has a refractive index of 1; glass ranges from 1.4 to 1.6. For optical fibers, the refractive index values for the core glass, cladding glass, and coating are 1.48, 1.46, and 1.52, respectively. With an average value of 1.5, this means that light propagates about 50% slower in an optical fiber than in vacuum. The percentage difference between the refractive indices is only 1 % to 2 %. For single mode fibers, it is only 0.1% to 0.2%. For light waves to propagate within the optical waveguide, the refractive index of the cladding glass must be smaller than that of the core glass.