What happens when a collimated beam of light hits a narrow slit? For the slit that is only slightly narrower than the beam, it appears that the slit cuts out part of the beam. When the slit opening decreases, the light at the output starts to spread and covers wider and wider angle. This diffraction of light demonstrates famous Heisenberg’s uncertainty principle. The intensity distribution (probability of photon occurrence) is spatially more localized when the slit gets narrower (delta x smaller). At the same time the momentum uncertainty of the photon increases (delta p larger), which is obvious from the increased angle spread of the output pattern.
The photos were taken using green laser pointer (en.wikipedia.org/wiki/Laser_pointer), variable-width slit (Thorlabs:VA100) and haze machine (en.wikipedia.org/wiki/Fog_machine). Made by QOLO and Point Film.