# Formula Light Propagation in Medium Speed of light in medium    Refractive Index    Vacuum light speed ## Speed of light in medium

Unit
Speed of light in medium is the speed at which light propagates for example in air, water or glass. Since the refractive index is $$n \ge 1$$, the speed of light in the medium is smaller than in vacuum.

## Refractive Index

Unit
Refractive index is characteristic of a transparent medium through which the light is transmitted. Vacuum has refractive index $$n = 1$$. Air has approximate value of $$n = 1.0003$$. Water $$n = 1.33$$. Quartz glass $$n = 1.46$$.

Refractive index is not always constant, but can depend, for example, on the frequency of the light used; but also on light intensity, light polarization and angle of incidence. In general, the refractive index is complex-valued, where the imaginary part of the refractive index, describes light absorption. The imaginary part can be neglected only for transparent media, because transparent media let the light pass without absorbing it.

## Vacuum light speed

Unit
Vacuum light speed is a physical constant and indicates the speed of light propagating in vacuum ($$n = 1$$). Vacuum light speed has the value $$c = 2.99 \cdot 10^8 \, \frac{\mathrm m}{\mathrm s}$$.