Ch 5.3, Ch 12.1 A vector observable associated with rotational symmetry. In quantum mechanics, the orbital angular momentum operators satisfy $[\hat{L}_i, \hat{L}_j] = i\hbar\epsilon_{ijk}\hat{L}_k$. Eigenvalues of $\hat{L}^2$ are $l(l+1)\hbar^2$ ($l = 0, 1, 2, \ldots$); eigenvalues of $\hat{L}_z$ a