= Reduced Planck constant
{title2=$\hbar=h/(2\pi)$}
The <reduced Planck constant> is $\hbar=h/(2\pi)$, with the same <action> units as the <Planck constant>. Since <angular frequency> is $\omega=2\pi\nu$, a <photon>'s <energy> can be written $E=\hbar\omega$ instead of $h\nu$.
The <reduced Planck constant> occurs naturally in the <Schrödinger equation>, the <canonical commutation relation> $[X,P]=i\hbar I$, and the <Heisenberg uncertainty principle> $\Delta X\Delta P\geq\hbar/2$. Comparing a characteristic <action> with $\hbar$ helps assess a <semiclassical approximation>, although a large ratio alone does not guarantee that all <quantum interference> becomes unobservable.
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