Hi, I'm reading N. Zettili Quantum mechanics. In the "double slit experiment", when the beam of the electrons pass the the two slits, they present an interference pattern on the screen. But when we have a light source there to trace the electrons, we see what we have expected to see of the particles in classical physics. Ok, we say that this goes back to Heisenberg uncertainty principle and light photons will affect the pattern of electrons. My questions is: why does the electrons show particle like behaviour when they are being observed by the light source? (esp concerned with the shape of the pattern on the screen) I mean why they choose the classical pattern? should we consider sth like compton scattering? Then why it is guaranteed that they show classical pattern on the screen? why don't we see a different pattern other than interference and classical?