# Difference between revisions of "Planck's constant"

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".