 Hi, I'm Zor. Welcome to Unisor Education. Today we'll continue talking about light. The previous lecture was dedicated to corpuscular theory of light. Now today we will talk about the wave theory of light. Now this lecture is part of the course called Physics for Teens presented on Unisor.com. I do suggest you to watch this lecture from the website from the Unisor.com even if you found it somewhere else like on YouTube etc. First of all because this lecture is part of the course which means there is a prerequisite and there are some other lectures Which are related to this. There is a whole course which is prerequisite for this which is called Mass for Teens and the same website and you can get into this lecture from the website by choosing the menu Physics for Teens and then energy and then energy of light. Okay. Also I have to mention that every lecture on this website is complemented by the textual part. These are notes basically. You can read it as a textbook basically. Sometimes it contains some more illustrations or references. I mean the textual part. Sometimes it might be a little bit more precise as far as the numbers are concerned than I'm using in the lecture. So it's much more beneficial for you to use the website rather than just lecture found somewhere. And the site is free by the way. There are no strings attached. No advertisement. Nothing. All right, so let's talk about the light as the waves. Well, let's start with something which I have basically finished the previous lecture about corpuscular theory of light. There are certain phenomenon. There are certain properties of light which cannot be explained from the corpuscular standpoint. Primarily these are interference, diffraction and polarization of light. Well, let's just talk about interference probably as the kind of major manifestation of the short shortcomings of the corpuscular theory. Only the wave theory of light could actually explain the interference property. And primarily it's related to the name of Thomas Young, the brilliant English physician and physicist and well specialist in many different subjects, one of those old style universalists. So he made certain experiments which basically demonstrated this propitive interference. And here is how it was done. Consider, and I'm talking about the view from above, you have some kind of a screen with two slots and there is a source of light. Let's say it's parallel from some kind of a flashlight or sun. Yeah, sun probably would be better. Let's say this is the sun rays. They are almost parallel. You can consider basically them. So they go through these and this is a screen where you can see the image basically. So again, this is a view from the top. Now, if corpuscular theory is the one which explains the property of light, we will have two bright spots here and here, exactly where the light goes through this. Instead, it's observable and basically it's such a simple experiment. Anybody can actually do it at home. Instead, we had dark and light stripes. Now, if these are vertical slits, it's called double slit experiment, then you will have stripes parallel to. Let me just do it from the, if you will view it from this side, you will see the bright spot, then the dark spot, then the bright spot again, the stripe actually and then light and dark are actually interchanging. And the brightest one will be in the middle, which is here. This will be the brightest spot and then too dark around it, then too light again around them, but they are a little bit darker. So the brightness is decreasing as we are moving from the center. Isn't that interesting? Now, it's very important these lights to be, these two slits to be close to each other, then the closer they are, the more visible this type of a picture will be. So, we are talking about this particular observation, which basically renders the whole corpuscular theory invalid, so to speak. So we have to explain it somehow. Now, is it explainable from the wave theory of light? If light is the wave? Well, actually, you might observe similar interference picture when you are observing the water. Let's say you have a very still basin of water and you have two points very close to each other where some kind of an object is touching the surface of the water. Now, if these points are on some kind of a flexible, like a steel, for instance, rod, which can vibrate, and these are vibrating up and down. Let's say this is a steel and we are vibrating this, so they're touching the surface. Now, there will be waves around it, right? Each one will produce certain waves. Now, what's interesting is that you will see this kind of an interference picture, where you will see certain spots to be basically standing still without actually vibrating, and certain spots around them to be vibrating a little bit more, and then, again, not vibrating, etc. So how that can be explained? Well, from the wave theory, it can be explained, because these are waves and they are coming in crest and troughs, right? So you have a crest and then you have a trough. Again, crest and the trough, and then you have another one, and they are spreading in all different directions from these points, right? So there are certain points here and here and here and here, where both waves are coming, but they have different distance. Now, if this is straight in the middle, for instance, then the distance will be the same from both sources of waves, right? And if the distance is the same, then the crest of one wave will correspond to the crest of another wave, and they are actually adding together. So if these are so-called in-phase, which means crest corresponds to crest and trough corresponds to trough, then the result will be like addition of these two. It will be significantly more. It will be double, basically. Much higher amplitude. Now, if, however, the difference in distances between this and this, between two sources to this point, are such that this wave is shifted by half wavelengths, so the picture is like this. The crest corresponds to trough. So what happens if they add together? They nullify each other. So here it's the same thing, basically. If light is wave, then whenever the distance between these two sources of light and the point is the same, that will be the bright spot, because the light plus light would be light, right? Now, however, if the distance is different, then it all depends whether these two waves are coming in-phase or out-of-phase. And obviously, if you are deviating from this point to any direction, if this is equal difference, then a little bit further, let's say up, the difference, this one will decrease and this one will increase. So there might be such a point where these waves are coming into counter-phases, so out-of-phase, and that will be the dark spot. And then again, the distance will be equal to even number of waves in both cases, and that will be the light spot. And then it will be, again, dark and light, etc. And same thing if you move this direction. And obviously, the brightness will decrease because the less and less greater distance will be and less visible light will be. So this is the so-called double slit experiment, which again is very very simple. It's explainable, it's explainable the same way as you see the waves on the water, and that's why it produces actually the opinion of the physicists that life is probably the wave. Now if this is a wave, then look at this these waves on the surface of the water. What exactly do we have as characteristics of the wave? Well, we obviously have the amplitude, the deviation from the middle, right? We have the lengths lambda, usually, the wavelengths, right? Well, actually a little bit further, from zero to zero. Yes, this is the wavelengths, and it repeats itself. And there is a speed of propagation of the wave. How fast this crest moving towards whatever, wherever, wherever it's moving. So we have these very important characteristics. We have speed of the crest or the wave propagating through space. We have lambda, which is the wavelength, and we also have the amplitude of the wave. Now, there is also another characteristic of of the wave, which is spreading along the space, along the straight line in the space. It's frequency. Now, the frequency is number of waves per second, so to speak. Now, what is speed? Speed is distance per second. Right? And what is lambda? Lambda is the wavelength. So if we divide speed by lambda, this is distance by distance which is covered in, let's say, meters. Lambda is the lengths of the wave in meters. So if we will divide one into another, we will have number of wave lengths per second, which are crossing any particular point. So the frequency and speed and wavelengths are related through this very, very simple equation, and obviously all the derivative of this equation, like this one, for instance. That's all the same. So these are characteristics of light, which, and we will talk about this when we will talk about optics, which can be measured by this picture of interference. So by knowing the distance between, let's say, the bright spots and the distance between these slits and the distance between this and this, we can actually determine these characteristics. Okay. So there are some other properties of the waves, like diffraction and polarization, which are also explainable through the wave theory. But I will reserve all these explanations towards the part of this course, which is dedicated to optics. For now, let's just come to an understanding that all these properties can be explained. Through the wave theory, more than that, even corpuscular properties, which is like the light is spreading along the straight line, for instance, and some other corpuscular properties, like photo effect, for instance. They also can be explained through the wave theory. So the wave theory at the end of the 19th century, beginning of the 20th century actually became a dominant opinion of the physicists. Okay. With corpuscular theory, it was kind of easy to imagine that certain corpuscles are propagating through the space. Let's say, Sun is the originator, the source, and they are emitted by the Sun and traveling through the space. That's fine. With wave, you see, the wave, for instance, needs water, right? So that's very important. We need the carrier of the waves and with light, although it displays very important properties of the wave, people really didn't know what actually carries these waves. So they came to an opinion that there might be some substance called ether, sometimes letter A in the front, they spell differently. So this ether is some kind of a substance, which is it exists like everywhere because the light goes everywhere, right? And the simulations of this substance actually makes the light to propagate. Now, initially, they were thinking that the light is propagating through the ether exactly the same way as sound waves, which is also waves. Sound waves propagate through the air. Now, how does that sound? It's it's the waves of compression. It's a longitudinal. So if sound is directed that way, then the waves are directed that way. We are compressing the air and then the compressed air transfers this compression to the next molecules and this one becomes less compressed. And then again and again and again as far as the sound while I'm producing certain sound, then the air is compressing this way further and further from me. So it's longitude, though. Longitude oscillations. At the same time, there are certain aspects of light like polarization, which we will talk about. They kind of contradict this. So you see the concept of ether was actually challenged in so many ways, in so many ways, that eventually physicists had to basically discard it. It cannot be really like penetrating everywhere because it just, you know, we don't really feel it. We don't really have any kind of a tools which can measure it or check its density or something like this. I mean, it was so ephemeral, if you wish. So unreal that physicists just had to discard the theory, which left light by itself, basically. So it's waves, but we don't really know how it propagates. Well, then a little later, the famous physicist James Maxwell, he was experimenting with something and then he came with a theory of electromagnetic field. And all of a sudden he found that the speed of propagation of electricity in the wires is very much like the speed of light. And he was the one who basically suggested that maybe light is just the propagation of electromagnetic field. And we don't know much about fields. I mean, from the from the sense of understanding what exactly this is, but we do know the properties of the fields. We know the property of the electric field, of magnetic field. We know how they interact, et cetera, et cetera. So these are all properties which we can measure. And Maxwell actually came up with famous Maxwell equations which describe electromagnetic field. And basically describe the way how light can propagate as the oscillation of electromagnetic field. It's like a variable electric field produces variable magnetic field, which is in turn producing variable electric field, et cetera, et cetera. So it's self-propagating. Again, we will talk about this in more details when we will talk about electricity and magnetism. But for now, from this perspective of light as just what it is, it's probably enough to stop here saying that this electromagnetic theory of light really explained a lot. Practically all the properties of light, whatever we are observing, the wave properties of life, the corpuscular properties of light, they all can be explained in the framework of the oscillation of electromagnetic field. So, ether was rejected. Instead, we have electromagnetic field, which is the carrier of the light. Well, the last, well, nail, if I can say so, to the theory of ether was famous experiment of Michelson and Motrey. Now, this experiment proved that the speed of light does not depend on the speed of the source of light. So that's very, very important. So if you have here some kind of a recipient of light, and this is the source of light, whether the source goes this way or goes this way, the speed of light which light covers while traveling this distance, so if we go from here to here and at that moment we just emit the light, or we go from here to here and at this point, exactly the same point, we emit the light. In both cases, this guy will see the light at exactly the same time, which proves that the speed of light is the same, regardless of whether we move left or right with any speed. Well, this was an experiment of Michelson and Motrey. If ether was the carrier of light, we obviously had to observe different speeds, so something is not right. So this experiment was actually the last point on rejection of the ether in favor of using electromagnetic oscillations, electromagnetic waves as the explanation of the nature of light. Incidentally, this same experiment was the starting point for special theory of relativity developed by Einstein. We'll talk about this maybe sometime. So, what else is interesting? Well, basically that's all I wanted to talk about and introduce you to the concept of light as the oscillation of electromagnetic field, which basically carries itself. It does not depend on any kind of a surrounding substance where these waves are located. And another important thing is when we were talking about waves in the ether, which are like compression waves, like sound waves in the air, we were talking about longitudinal oscillations from the electromagnetic theory of light. It's actually the transversal, it goes across, so if light goes this way, the oscillation goes this way. It's more or less like on the surface of water. So when water waves are going, they are going up and down while they're propagating along the straight line, along the surface of the water. So it's perpendicular to the direction of propagating. So that's the same thing with electromagnetic field waves. And that basically completes my explanation of nature of light as the wave, as the wave theory. Now, obviously there is a continuation of this. There are lots of calculations which we can do, especially considering these major characteristics of light and how much energy is carried by the light, but that will be in some other lectures dedicated to this. Okay, so that's it. I do suggest you to read the textual part of this lecture on Unizor.com. Just go to Physics 14 course, find the energy in the menu and go to the energy of light. That's where you will find this lecture together with textual notes, which I do recommend you to read. That's it. Thank you very much and good luck.