>>14163527>>14163548Changing the intensity of the source doesn't affect the diffraction (as demonstrated by the fact that you can do this with an e-gun firing one electron at a time and still get an interference pattern). What disrupts the diffraction pattern is introducing any interaction at the slits that can definitively determine which slit the light or matter is passing through.
Diffractions form because of waves interacting predominantly with themselves. Detection requires interaction, and if this interaction is non-negligible compared to the interaction the wave has with itself, it disrupts the diffraction.
The greater the interaction at the slits, the more precisely you know which slit each photon/electron/whatever passed through, but the more degraded the diffraction pattern becomes. They've actually done experiments with very *very* weak interactions at the slits and shown that you trade off getting a sharper distribution for which slit each thing passed through for a more degraded interference pattern.
