# The SI unit of time is defined in terms of _____________ frequency.

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The SI unit of time is defined in terms of _____________ frequency.
1. Magnesium
2. Caesium
3. Uranium
4. Thorium

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Correct Answer - Option 2 : Caesium

Concept:

SI Unit

• In 1960 Eleventh general conference of weight and measures SI UNIT was accepted as an international system of units.
• It is consists of seven fundamental units and two supplementary units.

Explanation:

• Time: To measure any time interval we need clocks. We now use the atomic standard of time i.e Atomic clock OR Cesium Clock for that.
• It is based on the periodic vibrations produced in the cesium atom.
• Frequency is no of cycles per second. The unit of frequency is Hz which is s-1 i.e  $\frac{1}{s}$
• In the cesium 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 states of the cesium 133 atoms.

The SI unit of time is defined in terms of cesium frequency.

Seven fundamental units are given below

 QUANTITY NAME UNIT NAME SYMBOL QUANTITY Length meter m l Mass kilogram kg m Time second s t Electrical Current ampere A I, i Thermodynamic Temperature kelvin K T Luminous Intensity candela cd lv Amount of substance mole mol n

Two supplementary units are given below

 QUANTITY NAME UNIT NAME SYMBOL QUANTITY Plane Angle radian rad $\Theta$ Solid Angle steradian sr $\Omega$