"When a quantum "observer" is watching Quantum mechanics states that particles can also behave as waves. This can be true for electrons at the submicron level, i.e., at distances measuring less than one micron, or one thousandth of a millimeter. When behaving as waves, they can simultaneously pass through several openings in a barrier and then meet again at the other side of the barrier. This "meeting" is known as interference.

Strange as it may sound, interference can only occur when no one is watching. Once an observer begins to watch the particles going through the openings, the picture changes dramatically: if a particle can be seen going through one opening, then it's clear it didn't go through another. In other words, when under observation, electrons are being "forced" to behave like particles and not like waves. Thus the mere act of observation affects the experimental findings.

Source : https://www.pixiv.net/en/artworks/66435201"