Imagine you're at the beach and see your friend struggling in the water. You want to go help him quickly. The shortest path is a straight line, but you can run faster than you can swim. So, you go down the beach to minimize the distance through the water. But this path is longer than it needs to be. The best path is somewhere in between. It depends on how fast you can run and swim.
This is the same law that governs light passing from one medium to another. Light takes the fastest path from point A to point B. But how does light know which path to take? You might say there's nothing strange about this. Light just sets out in some direction and changes direction when it hits an interface. But that's not what's happening. Light explores all possible paths all at once. The final path we see is one of the greatest illusions there is.
The idea that light takes the fastest path is not true. It's an illusion. Light actually explores all possible paths at the same time. This is a strange idea, but it's what happens in the world of physics.
