An electron represented by a matter wave has at the beginning only one momentum $$p = p_{\text y}$$ in $$y$$ direction. It points exactly in the direction of the single slit. After passing the single slit, the electron wave is diffracted. The electron momentum $$p = p_{\text y}$$ has got a part $$\Delta p_{\text x}$$ in $$x$$ direction. This $$x$$-component represents the momentum deviation $$\Delta p_{\text x}$$ from the original momentum $$p_{\text y}$$.