In quantum mechanics, the wave function collapse refers to the transition from a superposition of possible states to a definite state when a measurement is made. However, it's important to clarify that the wave function collapse does not directly involve photons "turning into" particles. Photons are already considered particles in the context of quantum theory.
When a measurement is made on a quantum system, such as the position or momentum of a particle, the wave function describing that system collapses to a particular state corresponding to the measurement outcome. For example, if the position of a photon is measured, the wave function describing its position collapses to a specific location.
After the collapse, the photon remains a photon, but its state is now determined by the measurement outcome. It will continue to propagate and interact as a photon according to the laws of quantum mechanics. It doesn't change into a different type of particle as a result of wave function collapse.
It's important to note that different observers can perceive the same photon differently based on their measurements and observations. Quantum mechanics describes the probabilistic nature of observations, where the outcome of a measurement is uncertain until it is made. Once a measurement is performed, the observer will obtain a definite result, but another observer might obtain a different result due to the probabilistic nature of quantum measurements.
So, photons remain photons even after wave function collapse, and their behavior and properties are determined by the laws of quantum mechanics. However, different observers may perceive and measure the properties of photons differently, leading to different outcomes and observations.