Heatwaves cause light distortion primarily due to refraction, which occurs when light passes through air layers of varying temperatures and densities. As the ground heats up, the air directly above it becomes warmer and less dense, bending the light rays. This bending results in a shimmering effect, making objects appear wavy or unclear. The phenomenon is particularly noticeable over hot surfaces like concrete during summer. Understanding refraction helps explain why visibility is affected in these conditions.