2 Measurement of Length, Mass and Time
Measurement of Length, Mass and Time
Measurement of Length
- Measurement of Large Distances
- Method used − Parallax method
- Parallax − Name given to the apparent change in position of an object with respect to the background, when the object is seen from two different positions
Far-away object (S) is observed from two different positions (A and B).
Distance between the two observation points is called basis (b).
The angle θ is called parallax angle. When θ is very small, we can consider the line segment AB as an arc of length b having radius D. By geometry,
- Size of an Astronomical Object
Size of a far-away planet can also be determined by using the same method.
- Estimation of Small Distances
- The size of small microscopic particles cannot be estimated by optical microscopes.
- Optical microscopes use visible light. The wavelength range of visible light is 4 × 10−7m to 7 × 10−7 Hence, the distances smaller than this cannot be measured by using visible light.
- Instead of visible light, electron beams can be used to measure very small distance. Electron microscope uses such electron beams.
- Electron microscope with a resolution of 0.6
can be used to measure subatomic particles.
Measurement of Mass
- SI unitof mass − Kilogram
- While dealing with atoms and molecules, we use unified atomic mass unit (u or amu) as standard unit.
1u = 1/12 part of the mass of C12Â atom
Or, 1 u = 1.66 × 10−27 kg
- Masses of commonobjects can be measured by balance.
- Large masses can be measured by using gravitational method.
- Masses of sub-atomic particles can be measured by using mass spectrograph.
- Range of variation of mass− from 10−30 kg to 1055 kg
Measurement of Time
- Atomic standard of time→ Periodic vibrations produced in a cesium atom
- 1 second→ time required for 9, 192, 631, 770 vibrations of the radiation of cesium 133 atom
- In our country, the National Physical Laboratory (NPL) has the responsibility of maintaining Indian standard time.