INTRODUCTION
We all are aware of what light is. Light as we know is a form of energy and an electromagnetic wave which can be perceived by our eye. Light is said to have a dual behavior or duality. Some say light is made up of particles or has particle nature whereas some say light behaves like a wave or has wave nature. In this argument, we can say that light has both natures and hence follows the concept of "wave-particle duality".
Well, for now let us take light to be a group of particles called photons. We can imagine light as a group of photons hitting our eye which makes us see the surrounding environment. Hence, light can be considered a ray.
CONCEPT OF REFRACTION
Now, the interesting thing about rays is that they travel in a straight line, don't they? As we've learnt in mathematics, a ray has one starting point and then it stretches to infinity in a straight line. Applying the same logic to light, it must also travel in straight line, which it does. But there's a caveat. Light will travel in a straight line only when it remains in the same medium it was travelling. What does this mean? Medium here refers to different places where light can travel, say air, water, glass etc. There are lots of media present where light can penetrate and travel. So does that mean if light changes its medium, will it bend? The answer is yes.
Say for example, light is travelling in air and suddenly it hits a glass slab. Now what do you think would've happened? Light must've bent? Yes, it did. But at what angle? To understand that we would need to understand the concept of normal. Normal is nothing but an imaginary line which is perpendicular to the surface where the light ray hits. The hitting ray is known as the incident ray and the ray that enters the new glass medium is known as the refracted ray, not the reflected ray.
Having understood what a normal is, we can proceed further in the concept of refraction. Now, which do you think is a more denser medium, air or glass? Of course, glass as it is a solid and solids are much denser than gases as their atomic particles are much closer to each other. So as glass is the denser medium here, naturally the less dense or the rarer medium is the air.
i) Whenever light enters from a rarer medium to a denser medium, the light ray tends to bend "towards" the normal ray (which is perpendicular to the surface).
ii) Whenever light enters from a denser medium to a rarer medium, the light ray tends to bend "away" from the normal ray.
If you noticed, both the points are reciprocals of each other. This shows that light follows the rule of reciprocity which is highly useful when we deal with subjects like ray optics and its math. This bending of light when there is a change of medium is what we call as "refraction". But now, a curious question that must've hit your mind would be, why? Why does light bend when it enters a new medium? Well if that question encountered you, you are now in a perfect position to understand the reason behind it.
FERMAT'S PRINCIPLE OF LEAST TIME
Having understood the concept of refraction, let us understand the reason behind it. But to understand that we need to know one of the basic principles that light follows. The Fermat's principle of least time. Now what is this? Well, Fermat's principle states that given any two points A and B, light will always take the quickest path between the two. This might seem obvious at first as you would say that a straight line is the shortest and the quickest path which light follows to reach from A to B. But that's not the case always. The shortest path may not be the quickest path always. How? Let me explain that to you with a quick example.
Say you live in a heavily crowded city. Now assume an ambulance has to reach the hospital in the shortest time possible. There are 2 routes to reach the hospital. One is the direct straight route connecting the hospital to the main city and two, is the indirect route which is a bit longer than the first. Now, the first route as we all know is always trafficked as it happens in the city. But the second route which is indirect is not that crowded as it is longer. If the ambulance driver is smart enough, he knows that if he follows the longer route, he will reach much sooner than if he drove on the smaller route. Hence the shortest route is not always the quickest route.
Now, here you may say that we never see light bending in real life if there is no change of medium, which is correct as there are no obstacles for light which causes to turn or bend. But hold on, why does light even want to take the quickest path? Can't light wait but still go through a straight line route if there're obstacles? Well, no. Is light in a hurry? Well, yes! Light is in a hurry. This is what Fermat's principle of least time boils down at the fundamental level. But at the end of the day, we don't know why light's in a hurry. So, you see, light bends in different mediums in order to follow the quickest path. But is that it?
ANALOGY
Let's assume light to be a car running on a road. It encounters a new medium, say clay. When light enters the clay, say its left tyre touches the medium first, hence it will slow down. But the right tyre is still at a faster speed. Now what happens in this case, turning happens and hence when the right tyre also touches then both the tyres are at the same speed and continue to run in a straight path again. And as a result we see light has bended a bit towards the normal. But what if both the tyres touch the clay at the same time? Then no turning or bending will happen, it will continue to run in a straight path.
Now this analogy might seem to confuse you but basically light bends because it encounters a new obstacle causing its speed to change. And this change in speed causes a disbalance in the photon particles which ultimately lead to a bending when it enters the new medium.
I hope you learnt something new. Thank you for reading.
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