Planck Constant - Significance of The Value

Significance of The Value

The Planck constant is related to the quantization of light and matter. Therefore, the Planck constant can be seen as a subatomic-scale constant. In a unit system adapted to subatomic scales, the electronvolt is the appropriate unit of energy and the Petahertz the appropriate unit of frequency. Atomic unit systems are based (in part) on the Planck's constant.

The numerical value of the Planck constant depends entirely on the system of units used to measure it. When it is expressed in SI units, it is one of the smallest constants used in physics. This reflects the fact that on a scale adapted to humans, where energies are typically of the order of kilojoules and times are typically of the order of seconds or minutes, Planck's constant (the quantum of action) is very small.

Equivalently, the smallness of Planck's constant reflects the fact that everyday objects and systems are made of a large number of particles. For example, green light with a wavelength of 555 nanometres (the approximate wavelength to which human eyes are most sensitive) has a frequency of 540 THz (540×1012 Hz). Each photon has an energy E of hν = 3.58×10−19 J. That is a very small amount of energy in terms of everyday experience, but everyday experience is not concerned with individual photons any more than with individual atoms or molecules. An amount of light compatible with everyday experience is the energy of one mole of photons; its energy can be calculated by multiplying the photon energy by the Avogadro constant, N_A ≈ 6.022×1023 mol−1. The result is that green light of wavelength 555 nm has an energy of 216 kJ/mol, a typical energy of everyday life.