How Would you Save a Drowning Girl?
So when we told you that light always travels in straight lines, we already know the light bounces off in surfaces. In one sense, it is bending, right? In another sense, also it also bends. We kind of______, we told you right? Light always travels in a straight line. It does travel in a straight line, until and unless something stops it, it either gets absorbed or bouncing off, which is called reflection, or goes through. But when it does go through, does it is continues to go in the straight line? Or does it do something else..called bending. So, we like bend, when it goes from one medium to another? Now we know one thing. Everything in this chapter comes out of one fundamental principle and what is that? Light is in a hurry. Light wants to go from one point to another point as quickly as possible, so it takes the quickest path. Now how do we know this or why is this true, we don’t know. But we know all this from a principle of least time. So let’s see if we can extend the same analogy and understand if the light will bend. If it will bend, how it will bend, which direction it will bend in..all those things. So to do this, let’s take a very very interesting analogy. Let us say 3 friends. Right? 3 of you, guys in this case, walking along the shore of a river or of a sea…whatever it is. And all of a sudden if you watch a gorgeous woman…sinking, right? She is screaming for help…somewhere in the middle of water. And all 3 of you know how to swim. And you decide this is your chance…that you get her attention and become the hero. Because all of us watch these movies. So what you decide to do? Three of you. or one of you is extremely enthusiastic. You are a kind of guy who think that you know..Oh my..I have to do this. So you start running right at her. So what kind of path you take? You start running right towards her, so you reach the water at some point, then start swimming. But if you are little smarter, what would you do? Because you know that most people can run faster than they can swim. Right? So you probably run a little bit more and then arrive at an angle and then start swimming towards her. So who is going to win really? Is that the guy who is really enthusiastic who runs right at her, reaches the water, starts swimming? Yes, he has to swim a lot more, right? As you can see. He starts off, he goes straight at her and then he starts swimming that much, right? Do you think he is going to reach first or this guy is going to do that? He is going to reach first? Right? So one of those paths are going to be more than…or more efficient than the other path. It means that the quickest path need not always be the shortest path. And already know this. If you were to go…if you stay in a metro in India or any other part of the world, you know that the straight line path that connects two points in the city might through a main road and you will never ever reach on time. It will take a little short-cut. Of course, it is a little longer, but you reach faster. Because there is not traffic. A very similar analogy here. So what…what do you observe? The shortest path need not be the quickest path. But what path light…does light always take? It takes the quickest path. So let us now begin to notice. What is the quickest path? It’s not a simple problem, right? You might also ask how does light know it has to take the quickest path? We don’t know. How does light find the quickest path? That also we don’t know. Or rather if you really want to know about this, we will have to go and understand what really light does and behaves in this way. But in one sense here, we really don’t know how light figures this out? But it does figure out. Doesn’t’ it? So light always picks up the quickest path. And as you can notice, if this is land and this is water, the light starts here and its not going to go straight that way, but it is going to go here and then take that path because it wants to spend as much as possible on land where it is faster and as little time as possible in water where it is slow. So people would have asked you why does light bend? So typical answer would be because the speeds are different. But much better answer is to say speeds are different and light wants to take the quickest path. Therefore it takes that way. Of course the light won’t do something as stupid as to go and come back like this, right? That will be really stupid. Because the worst case path is ….and down there straight. Right?