Units and Measurement

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For the measurement of length.

For example, a metre scale is used for lengths from 10^-3m to 10² m.

A vernier callipers is used for lengths to an accuracy of 10^-4 m.

A screw gauge and a spherometer can be used to measure lengths as less as to 10^-5m.

Standard metre is defined in terms of wavelength of light and is called the atomic standard of length.

The metre is the distance containing 1650763.73 wavelengths in vacuum of the radiation corresponding to orange-red light emitted by an atom of krypton-86.

Nowadays metre is defined as the length of the path travelled by light in vacuum in 1/299,7792, 458 part of a second.

Large distances such as the distance of a planet or a star from the earth cannot be measured directly with a metre scale. An important method in such cases is the parallax method.

Certain special length units for short and large lengths. These are
1 fermi = 1 f = 10^-15 m
1 angstrom = 1 Å = 10^-10 m
1 astronomical unit = 1 AU (average distance of the Sun from the Earth) = 1.496 × 10¹¹ m
1 light year = 1 ly= 9.46 × 10¹⁵ m (distance that light travels with a velocity of 3 × 10⁸ ms^-1 in 1 year)
1 parsec = 3.08 × 10¹⁶ m (Parsec is the distance at which average radius of the earth is orbit subtends an angle of 1 arc second)