The Planck constant (denoted h), also called Planck's constant, is a physical constant used to describe the sizes of quanta in quantum mechanics. It is named after Max Planck, one of the founders of quantum theory. The Planck constant is the proportionality constant between the energy (E) of a photon and the frequency of its associated electromagnetic wave (ν). This relation between the energy and frequency is called the Planck relation or the Planck–Einstein equation:
Since the frequency ν, wavelength λ, and speed of light c are related by νλ = c, the Planck relation can also be expressed as
A closely related constant is the reduced Planck constant, sometimes called the Dirac constant. It is equal to the Planck constant divided by (or reduced by) 2π, and denoted ħ ("h-bar"):
The reduced Planck constant is used when frequency is expressed in terms of radians per second instead of cycles per second ("angular frequency"). The energy of a photon with angular frequency ω is given by
Planck hypothesized (correctly, as it later turned out) that some types of energy could not take on any indiscriminate value: instead, the energy must be some multiple of a very small quantity (later to be named a "quantum"). This is counterintuitive in the everyday world, where it is possible to "make things a little bit hotter" or "move things a little bit faster", because the quanta of energy are very, very small in comparison to everyday human experience. Nevertheless, it is impossible, as Planck found out, to explain some phenomena without accepting that energy is discrete; that is, it exists only in integer multiples of some base value.
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The Wave Structure of Matter explains Max Planck s Constant Energy E = Frequency f by Max Planck s Constant h E=hf Consequently there remains only the one conclusion that previous electron theories suffer from an essential incompleteness which demands a modification but how deeply this modification
