In a double-slit experiment, the behavior of photons can be described using the principles of quantum mechanics. When a photon is emitted and travels towards the double slits, it exhibits characteristics of both particles and waves. This is known as the wave-particle duality of light.
When a photon interacts with the slits, it does not transfer energy to the physical material of the slits in the classical sense. Instead, the photon's interaction with the slits can be understood as a probability distribution of finding the photon at different positions along the slits.
The wave function of the photon describes this probability distribution, and it evolves according to the Schrödinger equation. The wave function spreads out and interferes with itself as it passes through the slits, resulting in an interference pattern on a screen placed behind the slits.
The collapse of the wave function occurs when a measurement is made to determine the photon's position or some other observable property. In the case of the double-slit experiment, the act of detection or measurement is typically performed by observing the pattern on the screen. When this measurement is made, the photon's wave function collapses, and its position is determined with a certain probability at a specific location on the screen.
The key point to understand is that the interaction of the photon with the slits is probabilistic and does not involve a direct transfer of energy to the physical material of the slits. It is the measurement or observation process that causes the wave function to collapse and determines the outcome of the experiment.