In the double-slit experiment, the act of observation or measurement can indeed affect the behavior of photons and lead to different results compared to when no measurement is made. This phenomenon is known as the "observer effect" or "measurement effect" in quantum mechanics.
When a measurement is made to determine the properties of a particle, such as its position, momentum, or polarization, it interacts with the particle and "collapses" its wave function to a specific value corresponding to the measured property. This collapse is often referred to as the wave function collapse or reduction.
Regarding your specific question about photon polarity, if the polarization of a photon is measured, the act of measurement generally collapses the photon's wave function to a particular polarization state. Prior to the measurement, the photon could be in a superposition of multiple polarization states, exhibiting wave-like behavior. However, once measured, the photon is found to have a definite polarization.
The involvement of detectors or measurement devices can affect the behavior of photons in the double-slit experiment. When detectors are placed to observe which slit a photon goes through, it disrupts the interference pattern. This is because the act of measurement collapses the wave function of the photon, determining its path with certainty and effectively removing the wave-like behavior responsible for interference.
The measurement disturbs the photon's state and introduces a distinguishing factor between the two paths (slits) that eliminates the interference. As a result, the pattern on the screen behind the double slits changes from an interference pattern to a pattern that resembles the distribution expected from two independent sources.
It's important to note that the specific details of how the observer effect manifests can depend on the experimental setup and the measurement being performed. In quantum mechanics, measurement is a complex and nuanced topic that continues to be the subject of ongoing research and investigation.