When a point source of light is far away
Shift the system of three slits further and closer from the point light source. Observe the position of the beams. What do you notice?
What we can conclude from this experiment
If we select some rays to study from the light of a point source, the angle between these rays near the point source is large. However, as we move away from the point source, this angle decreases. We can imagine that moving further away, these rays are almost parallel.