# Cycle of radiation

The second is defined as the duration of 9,192,631,770 cycles of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom. So what is meant by a cycle of radiation?

• A complete cycle of the sine wave. – PM 2Ring May 29 at 15:45
• But won't any radiation of same frequency as that emitted due to the transition between the two hyperfine levels of the ground state of the cesium-133 atom take the same time? – bo habib Jun 4 at 9:44
• Yes, by the definition of frequency. What EM source do you have in mind that has exactly the same frequency? ;) Note that the caesium atoms should be at 0K to produce the desired frequency, so a practical caesium clock has to make a compromise. – PM 2Ring Jun 4 at 9:58
• – PM 2Ring Jun 4 at 10:07

## 1 Answer

Taken a point in the space, hit by the radiation, the electric field in that point will oscillate periodically, as $$\sin(\omega t)$$ (where $$t$$ is the time). The oscillation will repeat exactly equally after a period $$T=2\pi /\omega$$.

The part of oscillation taking place during a time interval of length $$T$$ is "one cycle". It is, for example, the part of oscillation between two minima (or between two maxima).

A second is defined as the time needed to perform 9,192,631,770 cycles, i.e. 1 s=9,192,631,770 $$T$$.

• But won't any radiation of same frequency as that emitted due to the transition between the two hyperfine levels of the ground state of the cesium-133 atom take the same time? – bo habib Jun 4 at 9:43
• Any radiation with the same frequency will have the same period, thus a given number of cycles will take the same time. But I do not know any other physical process giving rise to the same frequency, spontaneously, without the need of an accurate tuning. – Doriano Brogioli Jun 4 at 10:16