Although in everyday life,directly calculate what is the speed of light, interest in this issue is manifested in childhood. Surprisingly, we all face the constant of the rate of propagation of electromagnetic waves on a daily basis. The speed of light is the fundamental magnitude by which the entire universe exists exactly as we know it.
For sure, everyone, watching as a child after a flashlightning and the subsequent thunder, he 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 got the necessary knowledge and could easily explain what was happening. What is the reason for the strange behavior of thunder? The answer is that the speed of light, which is about 300 thousand km / s, is almost a million times higher than the velocity of sound waves in the air (330 m / s). Therefore, a person first sees a flash of light from the electric arc of lightning and only after a time hears thunder of thunder. For example, if from the epicenter to the observer 1 km, then the light will overcome this distance for 3 microseconds, but the sound will need as much as 3 seconds. 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 is rather amusing to read about the experiments being conducted, however, in those distant times, before the appearance of precise instruments, everything was more than serious. When trying to find out what the speed of light was, one interesting experiment was conducted. From one end of the car of a fast moving train was a man with an exact chronometer, and from the opposite side his assistant on command opened the damper of the lamp. According to the idea, the chronometer should allow to determine the propagation speed of light photons. Moreover, due to the change in the positions of the lamp and the chronometer (with the train still moving), it would be possible to know whether the speed of light is constant or it can be increased / decreased (depending on the direction of the beam, theoretically, the speed of the train could influence the speed measured in the experiment ). Of course, the experiment failed, since the speed of light and registration by the chronometer is not comparable.
For the first time, the most accurate measurement waswas performed in 1676 due to observations of the satellite of Jupiter. Olaf Remer drew attention to the fact that the actual appearance of Io and the calculated data varied by 22 minutes. When the planets approached, the delay decreased. Knowing the distance, it was possible to calculate the speed of light. It was about 215 thousand km / s. Then, in 1926, D. Bradley, studying the change in the visible positions of the stars (aberration), drew attention to the law. The location of the star changed depending on the time of year. Consequently, the influence of the position of the planet relative to the Sun. You can give an analogy - a drop of rain. Without wind, they fly vertically down, but it's worth running - and 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: there was a rotating cogwheel between the light source and a mirror 8 km away. The speed of its rotation was increased until, in the following gap, the flux of reflected light did not turn into a constant (flicker) flow. Calculations gave 315 thousand km / s. Three years later L. Foucault replaced the wheel with a rotating mirror and received 298 thousand km / s.
The subsequent experiments became more precise,taking into account refraction in the air, etc. Currently, data obtained with the help of cesium clocks and a laser beam are considered relevant. According to them, the speed of light in a vacuum is 299 thousand km / s.
