Explanation
Light year and wavelength both represent the distance, so both have dimensions oF length not of time.
Addition and subtraction can be done between quantities having same dimensions.
Density is not always mass per unit volume.
Avogadro number (N) represents the number of atoms in 1 gram mole of an element, i.e., it has the dimensions of mole-1.
As the distance of star increases, the parallax angle decreases, and great degree of accuracy is required for its measurement. Keeping in view the practical limitation in measuring the parallax angle, the maximum distance of a star we can measure is limited to 100 light years.
The last number is most accurate because it has greatest significant figure (3).
Because representation of standard metre in terms of wavelength of light is most accurate.
As surface tension and surface energy both have different SI unit and same dimensional formula.
Dimensional constants are the quantities whose values are constant and they posses dimensions. For example, velocity of light in vacuum, universal gravitational constant, Boltzman constant, Planck’s constant etc.
Surface tension is defined as the force per unit length and hence its unit is newton metre-1 or Nm-1.
Pyrometer is used for the measurement of temperature.
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