Larry sees Beautiful Betty struggling to stay afloat. He is 100 meters from the water, she is 100 meters into the water and 200 meters down the shoreline. Lifeguard Larry swims only half as fast as he runs and he wants to get to Beautiful Betty as quickly as possible. If Lifeguard Larry goes straight toward Beautiful Betty he will go the shortest possible distance but will have to spend so much time swimming that it will take too long. If Lifeguard Larry chooses to minimize the amount of swimming then he will travel much further than he needs to and again will be too late. Lifeguard Larry wisely chooses something in between the two extremes.

 

 

It turns out that Lifeguard Larry is doing exactly the same thing that light does as it travels from point A to point B when points A and B are in different materials (or even in the same material). What the light does is travel the path that minimizes the time it takes to get from A to B. Comment on the connection between your book's description of the refraction of light and this different, yet completely equivalent, understanding.