So in 1927 Davisson and Germer showed that electrons shot at a crystal do indeed have a DeBroglie wavelength inversely proportional to their momentum (h/p)? That would mean that their wavelength is a function of their velocity, the voltage used to accelerate them, etc. But I seem to remember that Dirac showed that electrons always move at c, and the macroscopic velocity we measure is just the average of all those back and forth trips over some finite period of time. Are these two ideas compatible? Math Questions: I am interested in doing my own calculations, so what velocity do the electron wavelets have after being scattered by the atoms? Do they interact with only the outermost electrons of the atoms, or also some of the inner ones, or even the nucleus?