Fermat’s principle

 

Fermat’s principle explantion 

You know how when you're at school and the bell rings for recess? You probably want to get to the playground as quickly as possible to have as much time as possible to play. So you would choose the path that gets you there the fastest, right? That might be straight down the hallway, or maybe you have to take a turn. But you wouldn't just randomly walk around, because then it would take longer to get there.

Fermat's Principle is kind of like that, but for light. When light is traveling from one point to another, it will always take the path that takes the least amount of time. This is why light travels in a straight line when it's moving through air or space. If it didn't, it would take longer to get to where it's going.

But what happens when light moves from air into water, like in a fish tank? It actually bends and changes direction. This is because light moves slower in water than in air. So the quickest path isn't a straight line anymore - bending allows it to spend less time in the slower medium (water), which makes the whole trip quicker.

So, Fermat's Principle is just a rule that light follows to get from one place to another as quickly as possible. Whether that's in a straight line or bending, light always wants to take the fastest route.