How come light is so efficient? Read on?

Question

If light travels from fixed point A tofixed point B, A being in air and B in water, the route the ray chooses is completely different to that which it chooses if I, say, replace air with alcohol and water with glass. Both routes will not look like a straight line, for that matter. But still, this route will always be the most efficient one- Not too much time spent on the slow side and not too little time spent on the faster side- the ray always arriving at the quickest time it can to point B under the limitations. How is that explainable?

Answer 1

f m's answer is correct. Your premise is not. If a ray (narrow beam) of light passes through points A and B, it's because it was carefully aimed to do so. It's like I threw a basketball at the wall and it bounced off and into the basket. The ball isn't 'choosing' the shortest path. It is obeying the laws of physics and responding to all the forces being applied to it. Light refracts when it passes between medial with differing refractive index. In general, if you change the media, you'll need to point the light in a different direction.

The terms you are using do have historical precedent. Read about Fermat's Principle (the principle of least time) and the related principle of least action. These principles are just an observation of how things seem to work, not an explanation of why. Read about them in their historical context and they will make sense. But look to more recent work which starts with how light works and deduces behavior from that.

Answer 2

Light always travels in a straight line, which in general is the most efficient path. If it launches at point A, then point B must coincidentally be on the path that the light is travelling. The light has no way of knowing where point B is, and thus does not magically determine the most efficient way to get there.

As light transitions from a medium with one refractive index into another ,it will refract (change direction). Upon exiting, it will refract back to the original direction, but the output beam will be offset from the input beam. So in fact, if light starts at point A and passes at an angle through an empty glass, it will strike at point B1. But, fill the class with water and it will now strike at point B2. it does not figure out how to change its direction to get back to point B1.