In the chapter on partial wave analysis in Griffiths's Introduction to Quantum Mechanics, he considers a spherically symmetric potential and says that for large r, the radial part of Schrodinger's equation becomes,(adsbygoogle = window.adsbygoogle || []).push({});

[itex]\frac{d^{2}u}{dr^{2}}≈-k^{2}u[/itex]

with a general solution of

[itex]u(r)=C\exp{ikr}+D\exp{-ikr}[/itex].

He then says the first term represents the outgoing wave and the second term is the incoming wave. Why is that the case? These two differ by [itex]\pi[/itex] in the complex plane, but I don't see how that enables one to make the "incoming"/"outgoing" distinction.

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# Partial wave analysis - incoming/outgoing?

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