Here's why:
* The Sun is very far away. The distance between the Sun and the Earth is so vast that the light rays from the Sun, when they reach Earth, are essentially parallel to each other.
* Parallel rays create a plane. Imagine a sheet of paper with light rays hitting it perpendicularly. This creates a flat wavefront, like a plane.
However, it's important to note that:
* Sunlight is not perfectly collimated. There's some slight divergence in the rays due to the Sun's finite size.
* The Earth's atmosphere can distort the wavefront. Refraction and scattering caused by air molecules can change the direction of the light rays, slightly disrupting the planar wavefront.
So, in essence, direct sunlight can be approximated as a planar wavefront, but it's not perfectly planar due to the Sun's size and atmospheric effects.
