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definition of second is given as It is the duration of 9192631770 periodsof the radiation corresponding tothe transition between the two hyperfine levels of the ground state of cesium. Please explain this.

definition of second is given as
It is the  duration of 9192631770 periodsof the radiation corresponding tothe transition between the two hyperfine levels of the ground state of cesium.
 
Please explain this.

Grade:10

1 Answers

Aarti Gupta
askIITians Faculty 300 Points
7 years ago
The second is the base unit of time in the International System of Units (SI).Earlier second was defined as 1/86,400 of a mean solar day and then was defined in terms of the period of the Earth's orbit around the Sun in 1900,but it is now defined more precisely in atomic terms.Astronomical observations of the 19th and 20th centuries revealed that the mean solar day is slowly but measurably lengthening and the length of a tropical year is not entirely predictable either; thus the sun–earth motion is no longer considered a suitable basis for definition. With the advent of atomic clocks,second is defined based on fundamental properties of nature and defined as-
‘The duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.’
.Cesium 133 is the element most commonly chosen for atomic clocks.
To create a clock, cesium is first heated so that atoms boil off and pass down a tube maintained at a high vaccum and first they pass through a magnetic field that selects atoms of the right energy state; then they pass through an intense microwave field. The frequency of the microwave energy sweeps backward and forward within a narrow range of frequencies, so that at some point in each cycle it crosses the frequency of exactly 9,192,631,770 Hert.The range of the microwave generator is already close to this exact frequency, as it comes from an accurate crystal oscillator. When a cesium atom receives microwave energy at exactly the right frequency, it changes its energy state.At the far end of the tube, another magnetic field separates out the atoms that have changed their energy state if the microwave field was at exactly the correct frequency. A detector at the end of the tube gives an output proportional to the number of cesium atoms striking it, and therefore peaks in output when the microwave frequency is exactly correct. This peak is then used to make the slight correction necessary to bring the crystal oscillator and hence the microwave field exactly on frequency. This locked frequency is then divided by 9,192,631,770 to give the familiar one pulse per second required by the real world.This is how second is defined.

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