Fermat's principle
- light takes the path of least time.
- Light bends at an interface so that it takes the quickest path
light actually goes to all directions, and your eye only sees the light in its direction. which happens to be the shortest time to get to your eye.
this causes refraction under water
if light is not on the fastest path to any point outside, it will refract. (no go zone)
light when changing material refracts through the go zone, but never the no go zone.
it is reflected back inside. Total internal reflection.