Light travels in a straight line due to a fundamental property known as the rectilinear propagation of light. This property is based on the behavior of light waves and is a consequence of the principle of least action.
Light is an electromagnetic wave, composed of oscillating electric and magnetic fields. When light propagates through a medium, or even in a vacuum, it follows the path of least resistance, which is a straight line. This principle can be understood using Fermat's principle of least time.
According to Fermat's principle, light takes the path that requires the least amount of time to travel between two points. When light encounters different mediums or objects, it experiences interactions with particles or atoms, causing the direction of its propagation to change. These interactions can lead to phenomena like reflection, refraction, and diffraction.
However, even in these cases, the overall behavior of light can still be explained by the principle of least action. For example, when light reflects off a mirror, it follows the path that minimizes the total distance traveled between the source, the mirror, and the observer. Similarly, when light passes through a transparent medium like glass, it refracts (bends) due to the change in its speed, but it still travels in a straight line.
It's important to note that the straight-line propagation of light is an idealized description and assumes an absence of external influences like gravitational fields or strong curvatures of spacetime. In reality, the path of light can be influenced by various factors, but under normal conditions, light travels in straight lines as a result of its wave nature and the principle of least action.