What is the name of the quantum constant denoted by the symbol h?

The quantum constant denoted by the symbol h is the Planck constant. It is a fundamental constant of quantum mechanics and sets the scale connecting microscopic energy with frequency and momentum.

In the photon-energy relation E = hν, a photon’s energy equals the Planck constant multiplied by its frequency. The constant also appears in the matter-wave relation linking a particle’s momentum with wavelength. These relationships express the idea that energy and other physical quantities are quantized rather than varying without limit at microscopic scales.

Max Planck introduced the constant in 1900 while studying black-body radiation. He treated energy as being exchanged in discrete amounts, a step that helped resolve a major problem in classical physics. Albert Einstein later applied the quantum idea to light and the photoelectric effect. Planck received the 1918 Nobel Prize in Physics for his discovery of energy quanta.

The closely related reduced Planck constant is written ħ, or “h-bar,” and equals h divided by 2π. It is common in equations involving angular frequency. The symbol h should not be confused with kB, the Boltzmann constant, or NA, the Avogadro constant.