Light does not "know" when to stop or have any inherent mechanism to cease its travel. The reason light appears to stop or doesn't travel infinitely is due to various factors and interactions it encounters along its path.
Absorption: Light can be absorbed by certain materials or substances. When light encounters an object or medium that absorbs it, the energy carried by the light is transferred to the absorbing material, which can convert it into heat or other forms of energy. This absorption process can effectively stop the light from continuing its propagation.
Scattering: Light can also scatter when it interacts with particles or structures in its path. Scattering occurs when the incoming light interacts with the particles or structures, causing it to change direction and spread out in various directions. This scattering can cause the light to disperse and lose its coherent propagation.
Reflection: When light encounters a surface, it can be reflected back. Reflection occurs when the incident light strikes the surface and bounces off, changing its direction. The reflected light can be redirected and continue to propagate, but it may follow a different path.
Refraction: Light can change its direction when it passes from one medium to another with a different refractive index. This phenomenon is called refraction. When light enters a different medium, such as from air to water or from air to glass, its path bends, which can affect its travel direction and speed.
It's worth noting that in the vast expanse of space, where there is a near-vacuum and negligible interaction with matter, light can travel extremely long distances. The observable universe is filled with light from distant stars and galaxies that has been traveling for billions of years before reaching us. However, even in space, light can encounter obstacles, such as interstellar dust or gas clouds, which can cause absorption, scattering, or other interactions that may eventually diminish or alter its propagation.