My name is Alexander FufaeV and here I write about:
Fermat's Principle: How a Ray of Light Takes the Fastest Path
Fermat's principle states that light takes the path between two points that is reached fastest by light.
Since light propagates at different speeds in different media (e.g. water, air), the light does not take the smallest geometric path \( s \) between two points A and B, but the smallest optical path \( L ~=~ s \, n \). (\( n \) is the refractive index of the medium under consideration).
If the ray of light were a lifeguard, it would not necessarily rush to a drowning person in a straight line, but would first run across the beach, as it can run faster than it can swim, and then swim the shorter route to the drowning person.