Planck's constant

Planck's constant was first discovered by Max Planck in experiments that helped lead to the formation of quantum mechanics.[1] He formed the Planck Hypothesis that radiation could only come in discrete packets now called photons. Planck's constant is usually written as h, but it's often useful to divide Planck's constant by $2\pi$, and then it is written $\hbar$, which is called h-bar:

$\hbar = \frac{h}{2 \pi}.$[2]

The value of the Planck constant[3] is:

$h = 6.626\ 069 \times 10^{-34}$ J·s $= 4.135\ 667\ 5\times 10^{-15}$ eV·s

The value[4] of $\hbar$:

$\hbar = {{h}\over{2\pi}} = 1.054\ 571\ 726(47)\times 10^{-34}$ J·s $= 6.582\ 119\ 28(15)\times 10^{-16}$eV·s.

4. NIST (2015, June 20). Plank constant over $2\pi$. [Online]. Available: http://physics.nist.gov/cgi-bin/cuu/Value?hbar%7Csearch_for=h-bar