Guys, I'm suddenly confused by what the de Broglie formula actually says:
$$
p=\frac{h}{\lambda}.
$$
Griffiths is explicitly talking about the wavelength of $\Psi$ here, so how can we speak of "the" momentum of the particle, when the momentum is undefined? Why isn't the de Broglie formula about $\langle p\rangle$ or something like that?