In quantum mechanics, the behavior of particles, including photons, is described by wave-particle duality. This means that particles can exhibit both wave-like and particle-like properties, depending on the experimental setup and whether they are observed or not. However, it's important to note that the language of "turning into particles" or "being in wave form" can be misleading when describing the behavior of photons.
When photons are not observed or measured individually, they behave according to their wave nature. In the double-slit experiment with photons, for example, if a stream of photons is directed at a barrier with two slits and a screen is placed behind it, an interference pattern is observed on the screen. This pattern arises from the superposition of the probability amplitudes associated with different paths that the photons can take. It indicates that photons exhibit wave-like interference phenomena, similar to how waves in classical physics interfere.
On the other hand, when individual photons are observed or measured, their behavior is described in terms of particle-like properties. For instance, if we try to determine which slit each photon goes through in the double-slit experiment, the interference pattern disappears. This is because the act of measurement or observation disturbs the photon's wavefunction, collapsing it into a particular state corresponding to the measurement outcome. As a result, the photon behaves more like a localized particle, with a definite position and trajectory, rather than exhibiting wave-like interference.
It's important to note that the language of "observation" or "measurement" in quantum mechanics does not necessarily require a conscious observer. Rather, it refers to any interaction or measurement that extracts information about the particle's properties. The key point is that the act of measurement disturbs the system and affects its behavior.
In summary, the behavior of photons, as with other quantum particles, is described by wave-particle duality. Whether they exhibit wave-like or particle-like behavior depends on the experimental context and whether they are observed or measured, with the act of observation disturbing the system and collapsing its wavefunction.