The **Planck constant** (denoted * h*, also called

**Planck's constant**) is a physical constant that is the quantum of action in quantum mechanics. The Planck constant was first described as the proportionality constant between the energy (

*E*) of a photon and the frequency (

*ν*) of its associated electromagnetic wave. This relation between the energy and frequency is called the

**Planck relation**:

Since the frequency , wavelength *λ*, and speed of light *c* are related by *λν* = *c*, the Planck relation for a photon can also be expressed as

The above equation leads to another relationship involving the Planck constant. Given *p* for the linear momentum of a particle, the de Broglie wavelength λ of the particle is given by

In applications where frequency is expressed in terms of radians per second ("angular frequency") instead of cycles per second, it is often useful to absorb a factor of 2π into the Planck constant. The resulting constant is called the **reduced Planck constant** or **Dirac constant**. It is equal to the Planck constant divided by 2π, and is denoted *ħ* ("**h-bar**"):

The energy of a photon with angular frequency *ω*, where *ω* = 2π*ν*, is given by

The reduced Planck constant is the quantum of angular momentum in quantum mechanics.

The Planck constant is named after Max Planck, one of the founders of quantum theory, who discovered it in 1900. Classical statistical mechanics requires the existence of *h* (but does not define its value). Planck discovered that physical action could not take on any indiscriminate value. Instead, the action must be some multiple of a very small quantity (later to be named the "quantum of action" and now called Planck's constant). This inherent granularity is counterintuitive in the everyday world, where it is possible to "make things a little bit hotter" or "move things a little bit faster". This is because the quanta of action are very, very small in comparison to everyday macroscopic human experience. Hence, the granularity of nature appears smooth to us.

Thus, on the macroscopic scale, quantum mechanics and classical physics converge at the classical limit. Nevertheless, it is impossible, as Planck discovered, to explain some phenomena without accepting the fact that action is quantized. In many cases, such as for monochromatic light or for atoms, this quantum of action also implies that only certain energy levels are allowed, and values in-between are forbidden. In 1923, Louis de Broglie generalized the Planck relation by postulating that the Planck constant represents the proportionality between the momentum and the quantum wavelength of not just the photon, but the quantum wavelength of any particle. This was confirmed by experiments soon afterwards.

Read more about Planck Constant: Value, Significance of The Value, Dependent Physical Constants, Determination, Fixation, See Also

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“For me, it is as though at every moment the actual world had completely lost its actuality. As though there was nothing there; as though there were no foundations for anything or as though it escaped us. Only one thing, however, is vividly present: the *constant* tearing of the veil of appearances; the *constant* destruction of everything in construction. Nothing holds together, everything falls apart.”

—Eugène Ionesco (b. 1912)