Question

We know light travels in a straight line. Why sun's light travel as a parallel beam towards the earth?

Answer 1

A light beam or beam of light is a directional projection of light energy radiating from a light source. Sunlight forms a light beam (a sunbeam) when filtered through media such as clouds, foliage, or windows. Some of the photons in the beam collide with particles in the atmosphere, which scatter them in all directions, and some of these scattered photons happen to hit your eye.