This video relates a phenomenon that we have taken for granted to the study of quantum physics (more specifically, photon absorption) and atomic structure.
I find this video easy to understand and it may be useful for students to appreciate the wave property of matter and how it is observed via interference. The video ends with a mind-boggling problem that when an attempt to detect the path of the electron, it goes back to behaving as a particle.
There’s a whole series of “What the Bleep” videos that you might want to check out also. Be careful though, the rabbit hole is pretty deep.
Quoting from another website on what could have happened to each electron and to make the problem clearer (and hence more confusing):
The possibilities are: 1) the electron went through the left slit; or 2) the electron went through the right slit; or 3) the electron went through both slits. For the sake of logical rigor, we should add the possibility that 4) the electron went through neither slit (that is, it found some other way to get to the back wall). Now, one problem with possibility number 3 — a single electron going through both slits — is that, in nature, there is no such thing as half an electron. So if we found half an electron at both slits, we would have something really new; but that has to be a distinct possibility, considering that, in order to create the apparent interference pattern, something would have to radiate from both slits.
How are we going to find out? Well, we are going to put an electron detector at each slit. The electron detectors at the slits will be devices to keep watch over the passage through the slit. Every time an electron (or part of an electron) goes through, the detector will give a holler, “Hey, an electron (or part of an electron) just went through.” In this way, we will be able to learn something about how the electrons get through the barrier in a double slit experiment.
As it turns out, when you put the electron detectors at the slits, the result is that the electron is always detected at one slit or the other slit. It is never found going through both slits. And it is never found going through neither slit. You send one electron through, you find it at one of the slits. We have eliminated possibilities number 3 (both slits) and number 4 (neither slit). The only results we find are possibilities number 1 (left slit) or number 2 (right slit), in equal proportions.
They call this phenomenon the measurement effect. When we measure something at the quantum level, the very act of measurement will have an effect on the thing itself.
This is a phenomenon that still has no classical explanation.
Even Richard Feynman called it “a phenomenon which is impossible, absolutely impossible, to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery.”