Hello! I've been reading up about Quantum Mechanics, and I'm trying to understand various aspects of the double slit experiment. This is one of among many videos I've watched: Dr Quantum - Double Slit Experiment Here's my understanding so far of the basic setup (where the gun is shooting out particles one at a time): The source sends out a waveparticle (photon or electron), which reaches the barrier with the two slits. The waveparticle passes through both slits, and the result that emerges from each slit interferes with the result from the other slit. The waveparticle is in a superposition state with a probability distribution defined by its Schrödinger wave function The wave function apparently collapses at the film (the plane where measurement is being made) into a single spike of 100% probability at a specific coordinate, and we finally know where that electron ended up. I understand there is still a lot of discussion about the "collapse" phenomenon, and there is not a consensus yet (Copengahen interpretation vs many worlds, etc). But what I'm trying to wrap my head around is the Observer Effect as mentioned in the video at 3:40, where a detector is placed at the barrier to see what is actually happening at the slits. Apparently this causes the wavefunction collapse to happen early. So my questions: Is there any real life video / pictures out there showing the setup with such a detector at the barrier? I've seen live videos/demos of the basic setup that results in the interference pattern, for example this one and this one . But I have not been able to find a similar video that shows the result of the "detector at the barrier" scenario. What is the nature of the detector, or what is the detection mechanism? How were they trying to determine which slit the particle went through? Was there some kind of electro-magnetic detector that generates a signal when the photo/electron flew by? So the gun fires off one photon, which eventually ended up at the back film. I'm trying to grasp how that could occur if that photon was measured earlier and should either be absorbed or deflected by the measuring mechanism.