 Newton proposed that light is made up of particles. So when you're looking at a bulb, for example, he would imagine, I mean, there were no bulbs back then, but if there was, then he would imagine bullets coming out of these bulbs, and that's what light was, particles, he called them corpuscles. On the other hand, a Dutch physicist named Christian Huygens, and that's not how you pronounce his name, turns out that his name is pronounced somewhat like Huygens or something like that. But I'm just gonna go ahead and call him Huygens. He proposed that maybe light is not particles, but instead light is made of waves, very similar to the waves in water. And I wanna explore, I mean, the goal of this video is to explore this wave nature of light. So one question we could be having is, when it comes to water particles or waves in water, the water particles are the ones that are doing the vibrations, right? When you disturb, you know, when you disturb still water, then the disturbance is carried away by the vibrating water molecules. But in light, what particles are vibrating? That's a question that you can immediately ask Huygens, because he's claiming light to be a wave. Well, back then, Huygens and a bunch of other folks assumed that there are particles everywhere in the universe, and this medium, which consists of these particles, they called it the aether medium. This was a very popular notion back then, aether medium. They assumed that this medium was every verse, sort of like air, you can't see it, but you can't even feel it, but it's there. That's what they said, okay? And Huygens says that light is vibrations, light is, you know, light waves are carried by the vibrations of the aether medium. So light is propagated by the vibrations of the aether medium. Now, if you want to visualize something like that, here's our animation. Here's our light bulb, and these particles are the particles of the aether medium that's present everywhere in the universe. And this is how Huygens imagined it. Light is basically disturbance propagated by the vibrations of the aether particle, ripples. Light is ripples in the aether medium. Beautiful, right? Well, now there's one difference, big difference between the ripples of light and the ripples of water. Water, these waves are on the surface, which is two dimensions. And so these ripples are indeed circular, and you may have seen them. But light is a wave in three dimensions. So although you can't see this in this diagram, it's in three dimensions. So these ripples are not circles, but these are spherical ripples. That's a major difference that you're finding. So this is a 3D wave. But now we can ask a question. We can ask what exactly are these ripples? Whether you consider light or whether you consider water, what exactly are those ripples? So let me get rid of this and talk about that. So what exactly are these ripples? Well, in physics, these ripples have a very technical and fancy name. We call them wave fronts. And you'll keep hearing this word over and over again in the future videos. So let's clarify what these wave fronts are. Well, wave fronts, let me just define them first and I'll show you. Wave fronts are basically set of particles, set of particles which are in sync with each other, which are vibrating in sync. And again, there is a technical term for particles vibrating in sync with each other. We call them in phase with each other. We just need to familiarize ourselves with these technical terms. So what do I mean by this? Well, if I come over here and look at one of these ripples or wave fronts and concentrate just on them, what I will find now is that every particle on that ripple or on that wave front is at the topmost position. And that's where that ripple is a crest. A little later in time, we'll find all those particles will be at the bottom most position for forming a trough. Let me show you one more, which is already a trough over here. We usually did not trough by dotted lines. A trough, if I look at that particular ripple, then you'll find all the particles on that are at their bottom most position. The important thing to note is that all the particles on these circles, they are always vibrating in sync with each other. They're vibrating together in phase with each other. Such a set of particles is what we call a wave front. And the same would be true for all light as well. Again, let me bring back that visualization. If I were to concentrate on one of those spheres and just look at those spheres, what you are finding now is, look, all the particles on that spheres are vibrating in sync with each other. You see that? Okay, one more time. All right, there you go. Look at that. They're vibrating in sync with each other. Such a set of particles, which are vibrating in sync with each other are what we call wave fronts. The last question I wanna explore is, what in general would be the shape of these wave fronts? Well, think about water first. If you were to disturb it at only one place at just one tiny spot, then you'll get these nice circular ripples or circular wave fronts. But if you were to disturb this all around with your hand everywhere, then the wave fronts are gonna be all messed up and will have weird shapes, right? The same would be the case with light as well. Again, looking at that animation. If the light source that we're dealing with is pretty tiny, or we're looking a little bit far away so that the bulb appears tiny, then the wave fronts are going to be spherical in nature, just like those ripples. But if you were to have a big source or if you were to go very close to that light bulb where you can no longer neglect the size, now very close to that light bulb, the wave fronts won't be spherical. They might have different shapes depending upon the shape or the size of this light source. But if you go far away, a little bit far away, where it appears to be a point, then the shape is gonna become spherical. But I want you now to pause the video and think a little bit about what would be the shape of the wave front if you were to go really, really far away from this light bulb. Think about that. That sphere is gonna become really, really big. What now would be the shape of that wave front? Can you pause the video and think a little bit about that? All right, if you were to go really far away, that sphere would become so big that the sphere would appear flat to us, just like how Earth appears flat to us when we look at it from a small distance, right? So let me show you that. If you were to go really far away from this, and we'll do that in a second now, there we go. If we go really far away, I go really, really far away, then look at this. This is a sphere, of course, but what we are seeing now is not a line. It's, remember, this is in three-dimension, is flat surfaces. So now we are getting flat ripples or flat or plain wave fronts. That's what we call them, plain wave fronts, okay? So usually, what would be the shape of these wave fronts? Well, if you're dealing with, say, point sources, point sources, then the shape is going to be spherical. As we saw in 2D, they are going to be circular. But if you're dealing with point sources or sources which are very far away, you can think of them as infinitely far away. So sources far away, sources far away, then the wave fronts, the shape are going to be, the shape is going to be plain. You're gonna be a flat, flat wave fronts. So the light that we're getting from the sun, for example, on the Earth, the light is so far away from the sun that we can assume that the wave fronts or the ripples of light coming from the sun are plain wave fronts. Now, before I end the video, remember that there is no ether medium. It has been proved in a beautiful experiment that that doesn't exist. And light is a much more complicated wave. You may have heard of it as an electromagnetic wave. But when we start learning about light as a wave, we like to start with Heigann's theory because it's simple and it can make some really, really good predictions.