In Young's experiment, whether it involves a single slit, two slits, or multiple slits, the assumption that the waves emanating from any point on the slits are in the same phase when they leave the slits is based on the concept of coherent sources.
Coherence refers to the relationship between the phases of two or more waves. In the context of Young's experiment, coherence is necessary for the interference phenomenon to occur. When waves interfere constructively or destructively, their phases must be related in a specific way.
When we assume that the waves emanating from any point on the slits are in the same phase, we are assuming that the slits themselves act as coherent sources. This means that the oscillations of the individual points on the slits are synchronized, and they emit waves that have a fixed phase relationship with each other.
In reality, achieving perfect coherence is challenging because various factors can introduce phase differences between different points on the slits. These factors include variations in the path length from different points on the slits to the observation point, differences in the angle of incidence, and imperfections in the slits themselves. However, these factors can be minimized through careful experimental design and by using high-quality slits.
By assuming coherent sources, we simplify the analysis and focus on the interference effects. It allows us to treat the waves as if they were emitted from two or more point sources that are in phase. This assumption is crucial because it allows us to describe the resulting interference patterns mathematically and explain the observed behavior of light in Young's experiment.
It's important to note that the assumption of coherent sources is an idealization for theoretical analysis. In practice, there are often limitations and practical considerations that affect the coherence of the waves, and these factors can impact the observed interference patterns.