Although the speed of light (and other electromagnetic waves) is constant in a vacuum, it changes in other media. It is faster in air than in water for example. It is convenient to define the refractive index as a ratio of the speed of light (c) to the speed of light in the particular medium. The refractive index is a useful quantity in refraction calculations, and it is important to understand how it is defined.
These slides briefly introduce refractive index and some common values of refractive index. Questions are also provided.