More fundamentally, I think this problem should be explained from the perspective of light source. Light travels in a straight line in a uniform medium. But "light" is ideal and has no dimension. We can only see a certain dimension of "light beam" at ordinary times, because there is no ideal point light source. Real light sources have a certain size, and the beams emitted by these light sources cannot be strictly parallel. The principle of light source luminescence is that the energy level transition of atoms releases photons. The direction of photon emission is random. This makes it impossible for the light source to emit light in one direction strictly in principle. The unidirectional light sources made by human beings all use some optical equipment to shape the light in one direction, which is difficult to be strict. Therefore, strictly parallel beams cannot be manufactured. So it will get thicker and thicker. Even the most collimated light that human beings can make at present-laser is not straight, and the wavefront of light wave is not flat. The actual laser is a hyperbola.