Glossary / History of physics
Planck constant
The fundamental constant, written h, that sets the step size of energy for light and matter. Frequency times h is the smallest unit of energy for that light.
In 1900, deriving the blackbody formula, Max Planck assumed that light of frequency ν could have energy only in whole multiples of hν, introducing this new constant h[1]. His value from the data, 6.55×10⁻²⁷ erg·s, was already close to today's.
It appears in every equation of quantum mechanics, and because it is so small, energy looks continuous in everyday life. Since 2019 it has been used to define the kilogram (h = 6.62607015×10⁻³⁴ J·s, exact).
Related terms
- Blackbody radiationLight emitted by an ideal body that absorbs all light falling on it. Its spectrum depends only on temperature, not on what the body is made of.
- Light quantumThe particle-like packet of energy that light behaves as. Now called the photon.
- Bohr modelThe 1913 model in which electrons circle only on certain orbits and emit light of definite energy when jumping between them.