**Planck's constant**, denoted , is a fundamental constant in physics. The current value of **Planck's constant** is Joule-seconds (numbers in brackets represent uncertainty in the last two decimal places).^{[1]}

## History

It was first used by Max Planck around 1900 to explain the radiation curves of black-bodies. He supposed that light could be modelled as a series of particles (now called photons), each with an energy proportional to its frequnecy, . The constant of proportionality was Planck's constant. Mathematically this can be expressed as:

In 1924, Louis De Broglie proposed Wave-particle duality, the idea that matter (protons and electrons for example) had a wave-like nature.^{[2]} Here too, is Planck's constant:

where

- is the wavelength of the particle
- is the momentum of the particle

The Planck constant also occurs in equations fundamental to quantum mechanics, such as the Schrodinger equation.

## Reduced Planck's Constant

Planck's constant often occurs in equations with the mathematical constant . As such, a factor of may be taken into Planck's constant as

The resulting constant is pronounced "h-bar".