 Hi, I'm Zor. Welcome to a new Zor education. Today I would like to start a new topic which is related to electromagnetic waves, but in particular the visible light. So we will talk about visible light, about certain rules or laws. The propagation of this light actually is supposed to obey. So today's lecture will be very, very introductory. I'll just basically talk about light. There is no formulas or any kind of complicated calculations. Okay, this lecture is part of the course called Physics for Teens presented on Unisor.com. I recommend you to watch this lecture from the Unisor.com because every lecture is complemented by very detailed text, which is basically like a textbook for this particular lecture. Also the site contains a preliminary course, if you wish to call it, which is definitely necessary to study or to know actually before you study physics. It's called Math for Teens. Whatever the mathematical knowledge we need to study physics and there is a lot, it's all presented in that course, in preliminary course Math for Teens, which basically stands on its own obviously. Okay, now the website is completely free. There are no advertisements, there are no financial strings, there are no monetary benefits to anybody. Even signing is not really necessary unless you will get involved in a little bit more detailed study under supervision of somebody or maybe supervision of yourself. That's also possible. Then you might need actually the sign-on. But sign-on is just basically the name and password. Okay, so I have certain topics which I would like to talk about today. First, what is light? Well, right now we can say that light is the oscillations of electromagnetic field, electromagnetic waves. It's transversal and we were talking about before. And what do we need to know about waves? Well, first of all, we need to know the frequency, right? We might actually have to ask about what's the speed of propagation of these lights. And I can tell you, we are talking about only the light which is visible, which means our eye, which is a very sensitive instrument actually, can sense. Well, apparently not every electromagnetic oscillations can be sensed by our senses, in particular by our eyes. We are talking about certain range of frequencies of these electromagnetic waves which we can sense. And the frequencies are from 4 times 10 to 14 hertz, oscillations per second to 8 times 10 to 14 hertz. So, electromagnetic waves with the range of frequencies within these boundaries can be sensed by our eyes. I mean, obviously different people sense it differently. This is approximation or average, whatever it is. Average minimum and average maximum frequency. Usually children have more sensitifies and they might actually see even below and above this range. But this is kind of a greed upon range that we can actually call this is a visible light. So, frequency is number of periods per second. Okay, great. Now, the second parameter which we might actually think about is the, so this is F. Lambda is wavelengths. So, what's the wavelength? What's the length of one period in meters basically, right? Well, for this we need to know the speed of light. So, if speed of light is distance the front of the waves covers per, let's say per second. So, it's meters per second, for instance. And this is called C, usually abbreviated as C. Now, approximately, I will give you the exact number, but right now you can say that approximately it's 3 times 10 to the 8 meters per second. So, this is the speed. So, in one second speed covers the distance of 3 to the 10 to the 8 meters. Now, if frequency is certain number of oscillations or periods, if you wish, per second. So, if you will divide C by F, this is total distance which is covered in one second. And so, the length of one particular period would be this divided by number of periods per second which is frequency. So, lambda would be in the range of 375 times 10 to the minus 9 to 750 times 10 to the minus 9 meters. Now, 10 to the minus 9 meter is 1 nanometer. So, you can say that it's from 375 to 750 nanometers. These are wavelengths. So, whenever the frequency is less, we have longer wavelengths. Whenever the frequency is greater, we have shorter. So, this is basically what light is. I mean, you can also say something about period which is actually 1 over F. But this is rarely used. This is basically how much time actually to cover one particular period. That's rarely used characteristic. So, the frequency and wavelengths and the speed are major characteristics of visible light. And this is basically what we are talking about. One interesting detail, C does not depend on frequency. So, for any frequency, speed is the same. But that's a separate story. Okay, so, this is all about what actually light is. Now, we do sense the light, but we sense it differently. Depending on the frequency in this range, we see the light in colors. Well, at least those people who can do it. There are some people who cannot distinguish colors. This is an illness kind of thing. But generally speaking, people do differentiate the colors. So, the shorter frequencies, the higher frequencies, the shorter wavelengths. So, this part is towards the violet end of the spectrum. And the lower frequency and longer waves are for the red side of the spectrum. And everything in between actually has, generally speaking, we know which range of frequencies correspond to which color. But it's not really a discrete change of the color from one to another. It's a very gradual one. And in the text for this particular lecture on Unisor.com, I actually have a picture color from violet to red, represented with approximately boundaries where we consider end of the one color and beginning of another color. But again, this is just our view towards colors. Colors are not really like violet, blue, green, etc. Colors are gradually changing from one to another. It's our feelings how we sense these colors, which we are talking about right now. So, there is no abrupt change. This is red and this is yellow, for instance. No, it's very gradually changing. And you will see the picture which I put into the textual part of this course, of this particular lecture. You will see how the color is changing and where we are putting boundaries between the different colors. Now, everything less frequent than red is usually called infrared. And from our eyes it's basically black, we don't see it. And everything more frequently than violet is called ultraviolet and we also don't see it. So, it's also like black. And that's why that picture, which I'm talking about, which I present in the notes for this lecture, it has black top and black bottom. And in between, gradually changing from violet to blue to whatever, green, etc. To the red. So, this is all about colors. Now, and again, color is our subjective representation of different feelings, which we have based on different frequencies or different wavelengths of the visible light. Now, what about speed? Well, speed is an interesting thing. It's a separate story about how scientists were trying to measure the speed of light. Speed of light is very, very high. More than that, according to theory of relativity, speed of light in vacuum is the fastest possible speed to achieve. So, it's fast and it's very difficult, therefore, to measure. And there were many very ingenious experiments as a result of which we have an exact number for what is the speed of light. So, let me just put this number up and I'll talk about this a little bit. So, this is approximation. Exact value is 299,792,458 meters per second. That was determined, well, some time ago. And, yes, there are certain experiments which basically gave us this number. But then, physicists came up with a system of units which is called CSI, System International. Now, in this system of units, there are certain fundamental units, like meter for measurements of the distance, second for measurement of the time, and kilogram for measurements of the mass. Now, these fundamental units must be obtained from somewhere. For example, meter some time ago was basically the length of piece of metal, actually, which was stored somewhere at certain conditions, at certain temperature in Paris in some kind of an organization, government organization. But obviously, this is not as precise as contemporary measurements actually supposed to be. So, what physicists decided to do is to have some fundamental constant which is basically available in nature. And using this constant, define our main units of measurements. So, what they have decided with the meter, which is the unit of distance, they have decided that we will take the speed of light in vacuum, and one over this number would be by definition called the length of one meter. That's why the speed of light is exactly this particular number, because meter is defined through this particular number. It's not an approximation, this is exact. Similarly, when we are talking about second, how can you store some kind of equivalent of one second so people can measure against it? I mean, it's kind of difficult. So, they have decided to have some fundamental physical constant. And in this particular case, it was related to, if I'm not mistaken, radioactive decomposition of cesium, if I'm not mistaken. And again, the time during which certain radiation actually occurs or part of that particular time was called a second. So, basically, we do try to relate our fundamental units of certain physical characteristics to some constants which exist in nature. And this is one of the constants. Speed of light, speed of electromagnetic waves in vacuum is a constant. Well, according to our theories right now, that's another story. And that's how we define. So, this is the speed of light. Now, what's interesting is, I mentioned a couple of times that this is a speed of light in vacuum. And I would like to actually mention that speed of light in different other substances or environments is different. And just as an example, I think I have in glass, speed of light in glass is 2.25 times 10 to the 8 meters per second. You see? Less than this. It's about one and one-third approximately. So, speed of light is definitely related to substance within which it propagates. So, the glass or water about the same thing has less speed of light than in vacuum. Vacuum is the highest. And again, according to theory of relativity, it is by theory the highest possible speed which can be achieved. It's related to increasing of the mass, for instance, of the object which is moving with the speed. And mass goes to infinity. There are obviously certain mathematical equations where mass depends on the speed. And we might actually talk about this after this course at the very end, maybe, if I will have some time we'll talk about theory of relativity. But according to the theory, with increasing of the speed towards this particular number, the mass is increasing to infinity and that's why it's impossible to exceed to exceed. Okay, that's the speed. Now, what is the source of light? I have listed a few sources in my notes for this lecture. Well, obviously we all know about, for instance, burning. So, if you will burn the wood, now what is burning? Burning is a chemical reaction of oxidation. So, basically it's something like this. So, the molecules of carbon are connected to molecules of oxygen. And as a result, we have carbon dioxide. But during this, we have energy distributed. And this energy in the form of heat and light, which we see. So, that's why whenever we are burning the wood or coal, for instance, we see the light. Now, that's the chemical. Now, electricity can be the source. When you have an incandescent lamp, it's very intense flow of electrons through the metal, whatever metal spring is inside. Like, usually it's tangent or some kind of alloy, maybe it's tangent. And because of the intensity of electrons, which are going through this, I forgot the name of this, I'll call it spiral. Okay, the intensity of electrons is so big that it actually heats up this piece of tangent. And when it heats up, it actually emits energy again in terms of heat and visible light. So, that's electric. What else? Well, nuclear reaction, our sun. Now, there is a nuclear reaction which is happening on the sun constantly. So, it's a fission and it's a fusion, basically fission of some very heavy elements and fusion of light elements. And in both cases, we will produce a lot of heat and visible light, and invisible light as well, obviously. So, whenever you see the sun, for instance, you see only the visible spectrum. But yes, there are a lot of electromagnetic waves which are coming out from the same sun which you don't see. These are in ultraviolet or intra red parts of the spectrum of electromagnetic waves. Cosmic radiation, for instance, also is important, but we don't see it. Because, again, the frequency of these cosmic rays is much, much higher than the range which we can sense. Now, there is a luminescence process, whatever effect. You know that sometimes, if you look at the sea at night, you see that it lights up. Excuse me. That's basically some kind of animal, whatever. I don't know how they're called, but they emit the light. They absorb the light during the daytime, like an energy is absorbed, and then release it later at night. So, this is the luminescence. Now, we have a phosphorescence, we have fluorescence, and we have chemiluminescence. These are all different kinds of the same thing. Now, what's another very important way of producing light is relatively recent invention called LED, light emitting diodes. So, all the streets, for instance, in New York, where I live right now, are lit by the lamp which is based on LED technology. It's something related to electronics. There are special kinds of light emitting diodes. They're very low energy consumption, but a lot of light, so they're very good from the energy consumption standpoint. So, these are all different sources of light. And the last thing, which I wanted actually to talk about, and I'm not really very comfortable in this, quite frankly, because it's a very, very complicated manner, the history of our view onto light. In the beginning, it was relatively simple. In the beginning, light was considered as basically a set of small corpuscles or particles, basically. That's the view of Isaac Newton, for instance. So, they were viewing, he and some other people, they were viewing light as just a flow of certain particles. And there were experiments, actually, which can confirm it. For example, something like a reflection from the mirror. Well, the reflection of the mirror, obviously, we see it's something like this, if this is the mirror. So, it's basically exactly like if you will have tennis balls and you will throw it in this direction, it will go that way. So, that's why the particle theory is the simplest one. And, well, kind of the most natural to come up with if you don't know anything about electromagnetism or anything like that. Later on, there were certain experiments which basically could not be explained within the framework of the corpuscular theory. And these experiments are related, for instance, like this. If you have source of light here, and this is just a small hole, well, you would see that if it's just a set of particles, now these particles will not go and these particles will go. So, who will see these particles? Well, the person who is here. What about the person who is here? Well, actually, the person who is here actually sees something like light, not as bright, but still sees it. How can it be explained if the particles are going only this way within whatever this small hole actually is? And there are some other experiments, defraction, interference, etc., which people observed but could not explain within the framework. But they are very, very well explained within the wave theory. So, if light is a wave, like sound, for instance, consider this is a source not of light, but a sound. And these are soundproof walls with a small hole. Well, the sound will go here, but these people will hear the sound. Why? Because the oscillation of the air here will be propagated in all directions. So, that's why other scientists like Huygens, for instance, they have suggested that, well, maybe the light is just oscillation of some substance, which is oscillating like air is transmitting the sound. This substance, which we don't really see, we don't really know what it is, but it transmits the waves of the light. This substance was called ether, and I think it's spelled like this, ether. And that was also a very plausible theory. I mean, it explained a lot, including the properties of reflection, for instance, and some other properties. So, this theory actually dominated for quite some time. Well, but then the problem was with ether, because there was no experiment which can prove that ether actually exists. Like air, for instance, is the substance which transmits the sound waves. We know what air is. I mean, we definitely feel its pressure, etc. With ether, there was absolutely nothing. At the same time, Maxwell, who was actually, Faraday actually, and Maxwell, they were investigating electromagnetic waves. At that time, the electromagnetic waves theory was actually developed. And, for instance, Maxwell came up with equations which basically explained how the whole electromagnetic field behaves. And they were theoretically actually coming up with something like speed of electromagnetic waves, which by that time approximately was equal to the speed of light propagation. And that's why they have suggested that maybe the light is actually the oscillation of the electromagnetic field. And oscillation of electromagnetic field. Now, if you remember back to concepts of magnetic induction, etc., the electric field, variable electric field is producing variable magnetic field. Variable magnetic produces variable electric, etc., etc., and that's how the light propagates. In vacuum, you don't really need any kind of ether to transmit these waves. They are self-transmitting, so to speak. They are moving themselves. And so that was the period when people kind of agreed that light is actually the oscillation of electromagnetic field, electromagnetic waves. Well, and the last component of this was the quantum theory, which actually explained some corpuscular properties of light. So these properties, light as a particle, do exist. But to explain them from the purely wave theory was very difficult. So the quantum mechanics actually introduced certain concepts like photon, for instance, which without contradicting the wave theory, added the component of particularity or corpuscularity of the light. So there are certain characteristics which are significant from the particle standpoint. They were explained in quantum theory, and that's kind of a contemporary way where it is. Again, I'm not actually comfortable right now to get into all the details of this. Well, number one, because I probably don't know the details myself. And secondly, it definitely doesn't belong to this course, which is basically kind of introduction to the real physics. But nevertheless, it's a very interesting theory, and you might actually read about how people were measuring the speed of light, how one theory changed another theory. There are many famous names involved like, for instance, Newton, Huygens, Einstein, Max Planck, etc. So all these stories are very interesting from the history of science standpoint. And I do actually recommend you to read. You might actually like it. Alright, so that was an introductory to what basically light is all about. And the real properties of light we will start addressing in next lectures. So thank you very much. I do suggest you to read the notes for this lecture. Look at the picture with colors, which I put into notes. And good luck. Thank you.