When a beam of light passes from one medium into another it changes direction. This bending is called refraction, and it happens because light is obeying a deeper rule of nature: the principle of least action. According to this principle, light always follows the path that takes the least time, not necessarily the shortest distance. That’s why a beam entering plastic from air bends, because it moves slower, and it adjusts its path to keep the total travel time as short as possible. This behavior