IIT JEE Measurement of Time | JEE General Physics Measurement and Time


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Definition of Measurement of Time

 Measurement of Time

Time is defined as the period in which a process, action or an event takes place. Time is also called the Point or Period when something occurs. Time is an integral part of Physics and every single quantity in the physical world relies on it. Time has made the universe function in a simple and routine way. If time wouldn’t have existed, there would be many irregularities in schedule and timing of people. It is quite difficult to think of life when there is no time. There won’t be any physical quantities like acceleration, velocity and electromagnetic waves which depend on time for their existence.


National Physical Laboratory maintains the standard time of India using 4 atomic clocks

Image 1: National Physical Laboratory maintains the standard time of India using 4 atomic clocks

To measure a time interval we need a clock. Clocks in India works on the atomic standard of time, which is based on the periodic vibrations produced in a Cesium atom. That’s why Indian clocks are also called Cesium Clock. The standard unit of time is second.

Before the invention of atomic clocks, people used to measure time by observing daily durations of the day. People used to divide their lifestyle into three-time durations namely morning, afternoon and evening times. The need to develop clock came into form when people got confused with time-intervals and daily schedules as in winter and rainy seasons they can’t get their time durations assigned. Then after the invention of clocks, the time was divided into smaller sections to ensure quality work by people.


Earlier people used to see time using hourglass

Image 2: Earlier people used to see time using hourglass

Second is the universally accepted unit of time. 1 second is defined as the time taken for 9,192,631,770 vibrations of the radiation relative to the transition between two levels of ground state of Cesium-133 atom.

Symbol of NPL

Image 3: Symbol of NPL

The vibrations in atomic or cesium clock are same as those in quartz clock which is mostly used in wrist watches. The Cesium atomic clocks are very accurate and long lasting. The Indian standard of time “second” is conserved by four Cesium atomic clocks. NPL or National Physical Laboratory in New Delhi is authorized to maintain the Indian standard of time using Cesium clocks.

NPL in India manages time standards and address improvement in physical standards like frequency, time etc. The IST or Indian Standard Time is also linked to set of four atomic clocks in NPL to maintain the time of the country in a good accurate manner. The error in atomic size is of the order 10-13, which is very less.

Range of Time

certain units to measure time

Image 4: We use certain units to measure time

Just like other physical quantities, the range of time is also quite vast and varies fundamentally from the microscopic level to the macroscopic level. The table given below depicts various range and order of time intervals.

Event or Condition Time interval (s)

Lifespan of most unstable particle


Time to cross a nuclear distance


Period of atomic vibrations


Period of X-rays


Period of light wave


Life of atom in an excited state of an atom


Period of radio wave


Period of sound wave


Blink of eye


Time between two successive heartbeats


Time taken by light to travel from the moon to the Earth


Time taken by light to travel from Sun to the Earth 


Time period of a satellite


Rotation period of the earth 


Rotation and Revolution period of the Moon


Time taken by the Earth for 1 complete revolution 


Time taken by a light to reach earth from a star 


Average Human Life


Age of Egyptian Pyramids 


Age of the world


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