What is the speed of light

Although in everyday life, few people have to directly calculate what the speed of light is equal to, interest in this issue is manifested in childhood. Surprisingly, we all encounter the sign of the rate constant 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 in childhood a flash of lightning and the subsequent thunder, tried to understand what caused the delay between the first and second phenomenon. 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 is happening. What is the reason for 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 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 a thunder of thunder. For example, if from the epicenter to the observer 1 km, then the light will overcome this distance in 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 experience 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 have allowed to determine the speed of propagation of photons of light. Moreover, due to the change in the positions of the lamp and the chronometer (with the train still traveling), 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, because the speed of light and registration by the chronometer is not comparable.

For the first time the most accurate measurement was made 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 regularity. 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 the 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: between the light source and a mirror, 8 km away, there was a rotating cogwheel. The speed of its rotation was increased until, in the following gap, the flux of reflected light did not become permanent (flicker). Calculations gave 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 accurate, taking into account refraction in the air, etc. Currently, the 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.