A clock is used to measure time interval. An atomic standard of time is based on periodic vibration produced in a cesium atom. This is the basis of the cesium clock, sometimes called as atomic clock, used in the national standard. In the cesium atomic clock, the second is taken as the time needed for 9,192,631,770 vibrations of the radiation corresponding to the transition between the two hyperfine levels of the ground state of cesium-133 atoms.
The cesium atomic clocks are very accurate. In principle, they provide portable standard. It is used at the National Physical laboratory (NPL) in New Delhi to maintained the Indian standard Time.
Range Of Time
The time intervals of events which can be occurred across in Physics vary over a very wide range from order 10⁻²⁴ s (life spam of most unstable particle) to 10¹⁷ s (age of the universe).
Some of the clocks developed are as-
(I) Electric oscillators
(II) Electronic oscillators
(III) Quartz crystal clock
(IV) Solar clock
(V) Atomic clock
(VI) Decay of elementary particles
(VII) Radioactive dating