The formation of shadows does not, directly, prove that light travels in a straight line. Shadows arise because light rays traveling in certain directions are blocked by a solid object, resulting in a region of darkness. However, this observation alone doesn't provide conclusive evidence about the path taken by light.

It was a through systematic experimentation and observation that scientists, such as Alhazen and Galileo, determined that light typically travels in straight lines. Various phenomena like the rectilinear propagation of light, the ability to form sharp shadows with well-defined boundaries, and the creation of straight-line paths in optical instruments, were crucial pieces of evidence leading to the understanding of light's rectilinear motion.