The interference pattern observed in the double-slit experiment, even when using light as the source, is a result of the wave-particle duality of light. Light can exhibit both wave-like and particle-like properties depending on the experimental setup.
When light is treated as a wave, it passes through the two slits and behaves as an electromagnetic wave. The waves from the two slits can overlap and interfere constructively or destructively, creating an interference pattern on the screen behind the slits. This interference pattern is a characteristic of wave phenomena and is observed even when the light intensity is low, indicating the superposition of waves.
However, when light is observed as particles (photons), each photon can only pass through one of the slits at a time. This behavior is apparent when the intensity is reduced to such an extent that only one photon is present in the experiment at a given time. However, over time, as more photons are detected, they collectively create an interference pattern that corresponds to the wave-like behavior of light.
In summary, the interference pattern in the double-slit experiment with light arises due to the wave-particle duality of light. While light can exhibit particle-like behavior at the level of individual photons, the cumulative behavior of many photons results in an interference pattern consistent with wave-like properties.