The speed of light

The speed of light (in vacuum), the speed of propagation of electromagnetic waves of any frequency, the limiting speed of propagation of any physical effects; one of the fundamental physical constants. There are two types of speed of light: c and g. The speed of light (c) is constant when moving from one frame of reference to another.
For the medium c’, the speed of light (c’) depends on the refractive index n of the medium, which differs for different frequencies v of radiation (dispersion of light). C'(v)=c/n(v). This dependence can be explained by the fact that it leads to a difference in the group and phase velocities, which are the same for a vacuum. When conducting an experimental study of the speed of light, the group velocity (i.e., the so-called signal velocity, energy transfer rate) is always measured.
It is established that the speed of light was first measured by O. Roemer in 1675 by measuring the time intervals between eclipses of Jupiter’s moons. Brown measured the speed of light in 1728. Based on observations of stellar aberration, Bradley was developed. Scientific studies of the speed of light were carried out in the 19th century by many scientists (Michelson’s experience, Fizeau’s experience, Foucault’s experience). A. Michelson obtained the value of the speed of light, which was the most accurate at that time: c=299796 ± 4 km/s. These measurements once again confirmed the wave theory of light. Produced in 1850 by J. B. L. Foucault’s comparison of the speed of light of the same frequency v, which is observed in air and water, showed that the speed of light in air u=c/n(v) corresponds to the predictions of the wave theory. Based on this, a connection was established between optics and the theory of electromagnetism: The speed of light and electromagnetic waves corresponded to each other.
Modern methods for determining the speed of light based on the use of laser radiation and a modified Foucault method using a laser made it possible to increase the accuracy of measurements and obtain a value of c = 299792.5 ± 0.1 km/s. By determining the speed of light using indirect methods, for example, determining the speed of light as a quotient of independently found wavelengths λ and frequencies of atomic or molecular spectral lines, even greater accuracy can be obtained. For example, since 1972, according to the caesium-139 standard, the spectral frequency of the CH4 laser radiation has been determined with an accuracy of 11 digits, and according to the Krypton frequency standard, its wavelength (about 3.39 microns). At the same time, the value of c was obtained by multiplying 299792456.2 by 0.8 m/s. The International Committee on Numerical Data for Science and Technology (CODATA) decided in 1973 that the speed of light in a vacuum should be 299792458 m/s. At the moment, this value is relevant.