>>12191297Because a particle is not just a small pellet of stuff. Quantum particles behave extremely differently from other "small objects" like say a grain of sand. That alone could justify that a beam of these bad boys will not behave like your average stream of bullets.
On a macroscopic level, unless you want to prove the whole quantum effect experiment, light or beams of particles basically act as waves.
When a wave reaches a small slit, it creates a diffraction pattern, which means that the tiny slit will behave as if it was a source itself. However, the second slit also does the same thing.
Usually, when you shine a light on another light spot, light usually sums itself into a brighter spot, right ? That's because the phases are random (and the wavelengths too). What are the odds that two beach waves would cancel each other ? None.
Except when you create two light sources (through the diffraction process) from a single one. They therefore have the same phase and wavelength and can interact with each other. There are places where the light sources will meet at their highest or lowest phase together, which will create a maximum of light. There are also places where one will be at its max and the other at its lowest. That will create a minimum of light (pic related).
In optics, particles are usually analogues for oscillation modes. Hence the existence of "quasi particles" like plasmons and shit.
