Speed of light fibre optic signals which travel 30–40% slower at the speed of light through glass. There are different ways to determine the value of "c". One way is to measure the actual speed at which light waves propagate, which can be done in various astronomical and earth-based setups. However, it is also possible to determine "c" from other physical laws where it appears, for example, by determining the values of the electromagnetic constants "ε" and "μ" and using their relation to "c". Historically, the most accurate results have been obtained by separately determining the frequency and wavelength of a light beam,
Speed of light systems of natural units of measurement or the geometrized unit system where . Using these units, does not appear explicitly because multiplication or division by 1 does not affect the result. The speed at which light waves propagate in vacuum is independent both of the motion of the wave source and of the inertial frame of reference of the observer. This invariance of the speed of light was postulated by Einstein in 1905, after being motivated by Maxwell's theory of electromagnetism and the lack of evidence for the luminiferous aether; it has since been consistently confirmed by many experiments. It