Although in everyday life it is rare for anyone to directly calculate what the speed of light is, the interest in this issue is manifested even in childhood. Surprisingly, we all come across a sign of the propagation rate constant of electromagnetic waves every day. The speed of light is a fundamental quantity due to which the whole Universe exists in the form that we know it.
Surely, everyone, watching in childhood a flash of lightning and the subsequent thunder, tried to understand what caused the delay between the first and second phenomena. Simple mental reasoning quickly led to a logical conclusion: the speed of light and sound is different. This is the first acquaintance with two important physical quantities. Subsequently, someone received the necessary knowledge and could easily explain what was happening. What is causing the strange behavior of thunder? The answer lies in the fact that the speed of light, which is about 300 thousand km / s, is almost a million times faster than the speed of propagation of sound vibrations in air (330 m / s). Therefore, a person first sees a flash of light from an electric arc of lightning and only after a while he hears a roar of thunder. For example, if 1 km from the epicenter to the observer, then the light will overcome this distance in 3 microseconds, but the sound will need as much as 3 s. Knowing the speed of light and the delay time between flash and thunder, you can calculate the distance.
Attempts to measure it have been undertaken for a long time. Now it’s quite funny to read about the experiments, however, in those days, before the advent of precision instruments, everything was more than serious. When trying to find out what is the speed of light, one interesting experiment was conducted. At one end of the carriage of a fast moving train there was a man with an accurate chronometer, and on the opposite side his team assistant opened the lamp shutter. According to the idea, the chronometer was supposed to determine the speed of propagation of photons of light. Moreover, by changing the positions of the lamp and the chronometer (while maintaining the direction of the train), it would be possible to find out whether the speed of light is constant or whether it can be increased / decreased (depending on the direction of the beam, theoretically, the speed of the train could affect the speed measured in the experiment ) Of course, the experiment failed, since the speed of light and registration by a chronometer are not comparable.
For the first time, the most accurate measurement was carried out in 1676 thanks to observations of the satellite of Jupiter. Olaf Roemer noted that the actual appearance of Io and the calculated data differed by 22 minutes. When the planets approached, the delay decreased. Knowing the distance, it was possible to calculate the speed of light. It amounted to about 215 thousand km / s. Then, in 1926, D. Bradley, studying the change in the apparent positions of the stars (aberration), drew attention to the pattern. The location of the star changed depending on the time of year. Consequently, the position of the planet relative to the Sun had an effect. You can give an analogy - raindrops. Without wind, they fly vertically downward, but if you run away, their apparent trajectory changes. Knowing the speed of rotation of the planet around the Sun, it was possible to calculate the speed of light. It amounted to 301 thousand km / s.
In 1849, A. Fizeau conducted the following experiment: between the light source and the mirror, located 8 km away, there was a rotating gear. The speed of its rotation was increased until, in the next gap, the stream of reflected light turned into constant (non-flickering). Calculations yielded 315 thousand km / s. Three years later, L. Foucault replaced the wheel with a rotating mirror and received 298 thousand km / s.
Subsequent experiments became more and more accurate, taking into account refraction in air, etc. Currently, data obtained using a cesium clock and a laser beam are considered relevant. According to them, the speed of light in vacuum is 299 thousand km / s.