The understanding that light travels in straight lines is based on centuries of scientific observation and experimentation. The principle that light travels in straight lines is known as the rectilinear propagation of light. Here's how we know:
Experimental evidence: Scientists have conducted numerous experiments to investigate the behavior of light. One of the earliest experiments was performed by Ibn al-Haytham (known as Alhazen) in the 10th century. He used a pinhole camera (also known as a camera obscura) to demonstrate that light travels in straight lines. By allowing a small hole for light to enter a dark room, he observed that the light formed an inverted image of the outside scene on the opposite wall.
Shadows: When an object obstructs a light source, it casts a shadow on the opposite side. Shadows are sharp and well-defined, which suggests that light travels in straight lines. If light did not travel in straight lines, shadows would not have such distinct edges.
Reflection: The law of reflection supports the notion that light travels in straight lines. According to this law, the angle of incidence (the angle between the incident ray and the normal) is equal to the angle of reflection (the angle between the reflected ray and the normal) when light reflects off a smooth surface. This behavior is consistent with the idea that light travels in straight paths.
Refraction: When light passes from one transparent medium to another, it changes direction. This phenomenon is called refraction. The laws of refraction, such as Snell's law, demonstrate that light bends as it passes through different materials. However, even in cases of refraction, the light still follows a straight path within each medium, and only the overall direction changes due to the change in speed.
Interference and diffraction: When light passes through narrow slits or around obstacles, it exhibits interference and diffraction patterns. These phenomena can be explained by the wave nature of light. However, the interference and diffraction patterns themselves indicate that light travels in straight lines between the source and the observation screen or detector.
The collective evidence from these experiments and observations supports the conclusion that light travels in straight lines. This understanding forms the basis of geometric optics, which deals with the behavior of light in terms of rays and straight-line propagation.