Man has always been interested in the nature of light, as evidenced by myths, legends, philosophical debates and scientific observations that have come down to us. Light has always been an occasion for discussion by ancient philosophers, and attempts to study it were made even at the time of the emergence of Euclidean geometry - 300 years BC. Even then, it was known about the directness of the propagation of light, the equality of the angles of incidence and reflection, the phenomenon of refraction of light, the reasons for the appearance of the rainbow were discussed. Aristotle believed that the speed of light is infinitely large, and therefore, logically reasoning, and the measurement of the speed of light is not subject to discussion. A typical case is when the problem is ahead of its understanding of the answer in its depth.
Some 900 years ago, Avicenna suggested that no matter how great the speed of light, it still has a finite value. This opinion was not only he, but no one was able to prove it experimentally. The ingenious Galileo Galilei proposed an experiment of a mechanistic understanding of the problem: two people standing at a distance of several kilometers from each other give signals, opening the lamp flap. As soon as the second participant sees the light from the first lamp, he opens his shutter and the first participant fixes the time of receipt of the response light signal. Then the distance increases and everything repeats. It was expected to record an increase in delay and, on this basis, perform the calculation of the speed of light. The experiment ended in nothing, because "everything was not sudden, but extremely fast."
The first to measure the speed of light in vacuum in 1676 was the astronomer Ole Roemer - he took advantage of Galileo's discovery: he discovered in 1609 four Jupiter satellites, in which for six months the time difference between two eclipses of the satellite was 1320 seconds. Using the astronomical information of his time, Roemer received the value of the speed of light equal to 222,000 km per second. It was amazing that the measurement method itself is incredibly accurate - the use of the now-known data on the orbits of the Earth, Jupiter and the delay time of the dimming satellite gives the speed of light in vacuum, at the level of modern values obtained by other methods.
At first, there was only one claim to Roemer's experiments - it was necessary to carry out measurements by earthly means. Almost 200 years passed, and Louis Fizeau built a witty installation in which a ray of light was reflected from a mirror at a distance of more than 8 km and came back. The subtlety was that it passed back and forth through the valleys of the gear wheel, and if the rotation speed of the wheel was increased, then there would come a time when the light ceased to be visible. The rest is a matter of technology. The measurement result is 312,000 km per second. We now see that Fizeau was even closer to the truth.
The next step in measuring the speed of light was made by Foucault, who replaced the gear with a flat mirror. This made it possible to reduce the dimensions of the installation and increase the measurement accuracy to 288,000 km per second. No less important was Foucault's experiment, in which he determined the speed of light in a medium. For this, a pipe with water was placed between the installation mirrors. In this experiment, it was found that the speed of light decreases during its propagation in the medium, depending on the refractive index.
In the second half of the 19th century, it was Michelson's time, who devoted 40 years of his life to measurements in the field of light. The crown of his work was the installation on which he measured the speed of light in a vacuum using an evacuated metal pipe more than one and a half kilometers long. Another fundamental achievement of Michelson was the proof of the fact that for any wavelength the speed of light in vacuum is the same and as a modern standard is 299792458 +/- 1.2 m / s. Such measurements were carried out on the basis of updated values of the reference meter, the definition of which has been approved since 1983 as an international standard.
The wise Aristotle was wrong, but it took almost 2000 years to prove it.