According to the principles of quantum mechanics, if a photon is observed or measured to determine through which slit it passes in a double-slit experiment, the act of measurement disrupts the interference pattern that would otherwise be observed on the detection screen. This phenomenon is known as the "observer effect" or "wavefunction collapse."
In the double-slit experiment, when photons are sent through a barrier with two slits and allowed to hit a detection screen, they exhibit an interference pattern, indicating their wave-like behavior. However, if a measurement is made to determine which slit the photon passes through, such as by placing detectors at the slits, the interference pattern disappears, and the photons behave like particles, creating two distinct patterns corresponding to each slit.
The act of measurement, whether direct or indirect, interacts with the quantum system, causing the wavefunction of the photon to collapse into a specific state. This collapse destroys the interference pattern because the photon is now localized to one of the slits, losing the wave-like behavior necessary for interference. Consequently, the ability to determine through which slit the photon passes prevents the observation of interference.
It's worth noting that there have been variations of the double-slit experiment, such as delayed-choice experiments and quantum eraser experiments, where information about the which-path (through which slit) can seemingly be obtained after the photon has hit the detection screen. However, these experiments involve complex setups and considerations, and the final interpretation is still a subject of debate and ongoing research in quantum mechanics.
In summary, in the standard double-slit experiment, if you attempt to determine through which slit a photon passes, it disrupts the interference pattern, making it impossible to tell through which slit the photon went based on the final detection result.