11.2: Spherical Coordinates

The solution to Schrödinger's equation in three dimensions is quite complicated in general. Fortunately, nature lends us a hand, since most physical systems are "rotationally invariant", i.e., \(V(x)\) depends on the size of \(x\), but not its direction! In that case it helps to introduce spherical coordinates, as denoted in Fig. \(\PageIndex{1}\).

 

The coordinates \(r, \theta\) and \(\phi\) are related to the standard ones by

\[\begin{aligned}x & =r \cos \varphi \sin \theta \\
y & =r \sin \varphi \sin \theta \\
& =r \cos \theta\end{aligned}\]

where \(0<r<\infty, 0<\theta<\pi\) and \(0<\phi<2 \pi\). In these new coordinates we have

\[\Delta f(r, \theta, \varphi)=\frac{1}{r^2} \frac{\partial}{\partial r}\left(r^2 \frac{\partial}{\partial r} f(r, \theta, \varphi)\right)-\frac{1}{r^2}\left[\frac{1}{\sin \theta} \frac{\partial}{\partial \theta}\left(\sin \theta \frac{\partial}{\partial \theta} f(r, \theta, \varphi)\right)+\frac{\partial^2}{\partial \varphi^2} f(r, \theta, \varphi)\right] .\]
 

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