Читать книгу Stigmatic Optics - Rafael G González-Acuña - Страница 21

When light is not travelling in a vacuum its speed is modified by the refraction index of the medium in which it is travelling.

The refraction index of a homogeneous medium is constant, and it is a dimensionless number that describes how fast light travels through the medium. The refraction index is defined by

where, c is the speed of light in vacuum and is the phase velocity of light in the medium.