Principle of least action. Computer artwork showing three possible routes a lifeguard could take to rescue a swimmer. The lifeguard is faster over sand than in water. A straight line (blue) would spend too long in the water and so would not be fast enough. Staying on the sand until right opposite the swimmer (green line) greatly increases the distance. The optimal route (red line) lies between the two extremes. This behaviour also explains why light is refracted when travelling through more than one medium. It is following the principle of least action, which states that when travelling between two points light will follow the path that takes the shortest time.