The propagation of light and matter is governed by a wavefunction. This tends to spread out in space and diffract like a wave. However, it only describes the probability of finding a particle there. This duality applies to both light and matter, such as photons, electrons, atoms.
Although an electron wavefunction may be spread out in space. You will never measure just a bit of an electron. You will either measure it totally, or not at all, with a proability given by the wavefunction.
So you could say it becomes a particle when it is ‘measured’. They call this the collapse of the wavefunction. The next question is, what constitutes a measurement and why does it lead to wavefunction collapse?
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