Ch 20.1 A semiclassical approximation valid when the potential varies slowly compared to the de Broglie wavelength: $\psi(x) \approx \frac{C}{\sqrt{p(x)}}\exp\left(\pm\frac{i}{\hbar}\int p(x')\,dx'\right)$, where $p(x) = \sqrt{2m(E - V(x))}$. Connection formulas handle the transition at classical tu