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 |
\(\Omega \) |
|
