When a single plane-wave photon/electron passes a slit/orifice, its direction of travel becomes random. Although there is the well-known Uncertainty Principle, it is not a replacement of the law of conservation of momentum for the phonon/electron before and after passing the slit. Question 1: If the photon/electron having pasted the slit is measured as traveling along a direction different from that of the original plane wave, where does the momentum difference go? Does the difference go to the screen of the slit? (I totally have no idea on this.) Question 2: A deterministic understanding would be that the direction of the photon/electron after the slit is determined when the photon/electron passes the slit, albeit the determination of the direction is random. (So the wave of the photon/electron after the slit is a new plane wave pocket. ) A non-deterministic understanding would be that the direction of the photon/electron is NOT determined until the photon/electron is measured. (So the wave of the photon/electron after the slit is a spherical wave?) Is the above description correct? And which of the two understandings is more "correct" (or accepted by the mainstream)?