Here's why:

* Point source: This means the light is originating from a single, infinitesimally small point.

* At infinity: This implies the source is incredibly far away.

* Wavefronts: These are imaginary surfaces connecting points of equal phase in a wave.

When a point source is very far away, the wavefronts become essentially flat and parallel to each other. This is because the curvature of the wavefronts becomes negligible over the large distance.

Think of it like this: imagine dropping a pebble into a still pond. The ripples created are circular wavefronts. As the ripples spread further out, they start to appear more and more like straight lines. A point source at infinity is analogous to the ripple having traveled a truly enormous distance.