Planck's constant, denoted by the symbol h, is a fundamental constant in quantum mechanics that relates the energy of a single quantum of radiation (such as a photon) to the frequency of that radiation. It is named after the German physicist Max Planck, who introduced it in 1900 as part of his groundbreaking work on the quantization of energy.
The value of Planck's constant, 6.626 × 10^-34 joule-seconds (J·s), is determined through experimental measurements. Planck originally derived the value of his constant based on fitting the experimental data for black-body radiation, which is the electromagnetic radiation emitted by a perfectly absorbing and emitting body (such as a heated object) at thermal equilibrium.
Planck's constant appears in Planck's law, which describes the spectrum of radiation emitted by a black body as a function of its temperature. By introducing the concept of energy quantization, Planck was able to explain certain aspects of the black-body radiation spectrum that classical physics failed to account for.
The precise value of Planck's constant was refined over time through further experimental measurements and advancements in quantum mechanics. Today, the value is known to an even greater degree of precision due to modern experimental techniques.
It is important to note that Planck's constant is a fundamental constant of nature and does not have a known theoretical explanation from first principles. Its value has been determined through experimental measurements and is accepted as a fundamental constant in physics.