In the double-slit experiment, if you observe the photon after it passes through the slits, the act of observation or measurement collapses the photon's wave function. This collapse means that the photon's state becomes determined or "localized" to a specific outcome.
When the photon is not observed, it exhibits wave-like behavior, resulting in an interference pattern on the screen behind the slits. This pattern arises from the interference of different possibilities for the photon's path as it passes through the two slits.
However, when an observation is made to determine which slit the photon passes through, the wave function of the photon collapses into one of the two possible states: it takes a definite path through either one of the slits. This collapse destroys the interference pattern, and the photon behaves more like a particle, producing a pattern on the screen that corresponds to the two individual slits.
The act of observation disrupts the superposition of states that the photon was in, and it "chooses" one particular outcome. This phenomenon is often referred to as the "observer effect" or "wave function collapse" in quantum mechanics.
It's important to note that the process of observation or measurement can be indirect and can involve interactions with the photon without necessarily physically touching or disturbing it. The key factor is that some information about the photon's path becomes available, leading to the collapse of the wave function and the loss of interference.