 Hi, I'm Zor. Welcome to Nezor Education. Today we will talk about lasers, so about what kind of device this is, how it was made, etc. Well, obviously not in all details because it's kind of complicated issue and a few physicists have received a Nobel Prize for basically making this particular device. However, certain general ideas about what's inside we will talk about. Now, this lecture is part of the course called Physics 14 presented on Unisor.com. I suggest you to go to this website Unisor.com to watch this lecture. If you found it somewhere else, you will have only this particular lecture video, let's say on YouTube. The website contains the whole course, which means the resume menu logically related to each other, different parts of the course, chapters and individual lectures. Every lecture on the website has textual explanation, which basically is like a textbook, piece of a textbook for that particular lecture. There are exams and there are certain other functionalities which you might find very helpful over there. And some other courses, like for instance, there is a prerequisite course, Mass 14s. You can do physics without mass, so basically that's one of the important components of the website, plus something else. And the website is completely free. There are no advertisement, no financial strings attached. You don't even have to sign in if you don't want to. And there are certain pieces of functionalities which require sign-in. Like for instance, you would like to do a supervised study with your parents or teachers or something else. Back to lasers. Well, I'm sure you know a lot about lasers. I mean, you've seen lasers, like for instance, in the supermarket you see the scanners, the price scanners. There are pointers, like a small little tube and it points like green or red light very, very far away. Laser printers. There are some developments in the military industry, like a weapon, laser as a weapon, to hit the drone, for instance, if it's not far away. So there are many different applications which you might or might not be familiar with it. And obviously, everybody knows that lasers are the favorite weapon in all the science fiction movies, starting from the Star Wars. So, we will talk about lasers in this lecture. And again, it's not about all the technical details, but rather about ideas which are at the foundation of this device. Now, let's start with something which you might be familiar with from other lectures of this course. I'll just remind it to you. So, the first thing which I would like to talk about is structure of the atom and how photons and electrons are acting with each other. There is a special lecture about this, but if you did not watch that lecture, I'll just very, very briefly repeat it. So, the contemporary view onto atom is that, first of all, there is obviously a nucleus which contains protons and neutrons. And there are electrons which are around it. The number of electrons is equal to the number of protons. That's the atomic number of every element, like hydrogen has atomic element one. There is one proton and one electron. The number of neutrons can be different, but anyway, helium is two. So, there are two protons, two electrons. Now, in case of hydrogen and helium, one or correspondingly two electrons are on the same orbit around the atom, the first one, the first orbit. Now, what's very important, and this is the contemporary view onto structure of the atom, that every element has more than one orbit. And there is certain maximum number of electrons per orbit. For instance, there is two on the first layer, eight on the second layer as a maximum, 18 on the third one, etc. And different elements have different elements of different number of electrons in different layers. Now, every layer has certain number of electrons for whatever element we are talking about. And all the electrons on the same orbit, or the same shell, as we are now more appropriately, I think, call it, because we are talking about three-dimensional things, so it's a shell basically. So, all electrons in the same shell, whether it's the first shell or the second shell, etc., they share exactly the same amount of energy, because they are on the same distance from the nucleus, so the same forces between nucleus and electrons are working and keep them on this shell. So, all electrons on any particular shell of any element have the same energy. Now, what's very important is that for each element, the radiuses of these shells, and you could consider them as concentric spheres, just as a model. So, the radiuses cannot take any value. There are specific distinct value for every element. So, this is, let's say, the radius and what's important energy level of the first shell around the nucleus. This is an energy level for the second, the third, etc., etc. So, again, every element has distinct values for energies on every shell. Now, can this electron jump to the shell, which is at a further distance from the nucleus? Yes, it can, but this shell has a different energy level. So, just by itself, it's unlikely that it will jump there. We need certain push. So, if there is certain exchange of energy, and in particular case radiation, electromagnetic oscillation, between something from outside and electrons, these can actually give a push to the electron to jump from one shell to another. Now, there are certain positions of electrons which we call stable, or ground positions, or ground level of energy. Now, this is the stable element. If a particular electron jumps from a shell where it's stable, but supposed to be, so to speak, in a neutral environment, without any kind of interaction with outside energy. So, if it jumps to a different shell, let's say we supply some energy and push it to the outside. Well, energy is supposed to be conserved, right? So, if this particular electron is pushed, there is certain conservation of energy law, which says that it will supposed to have more energy, excited, as we speak right now. So, it moves to a higher orbit, a higher shell from the nucleus. Now, by the way, the potential energy is negative of all these electrons, and we talked about the reason why. So, when it moves further, it decreases the absolute value of this potential energy. But since it's negative, it's an increase. It's closer to zero, and where is zero, zero is on infinity. Infinity is zero potential energy. And the closer we are, since this is positive and this is negative, it's the field itself which pulls it. It's not we who spend energy. So, that's why the potential energy is considered to be negative. We don't spend energy. The field actually attracts it. So, it decreases absolute value, but since it's negative, it's increasing the potential energy. And it's possible only if we supply certain amount of positive energy to this, and since the energy is supposed to be conserved, whatever it has right now, plus our positive, will give it a less negative, so to speak. But that's an increase in energy of this particular electron. Now, when it moves to another orbit, to another shell, this electron becomes less stable, and it has a tendency to jump down by itself, or maybe with certain push from outside, again, some energy may be supplied. But in any case, when it just by itself, let's consider by itself, when it goes from this level more energetic level to this level, which is less energetic, the piece of energy is supposed to be going somewhere. So, that's usually electromagnetic oscillation. It's a quantum of energy, which we call photon. So, whenever an electron jumps from here to here, it actually gives away certain amount of energy as a photon. Well, it can be any kind of electromagnetic oscillation. It can be in a visible spectrum, and then you will see the light. Okay, so this is basically a preamble about how the atom is structured and how it interacts with energy. The question is, can we use it to create something like laser? Well, by the way, laser is an abbreviation. It's a light amplification by stimulating emission radiation. Emission radiation. Emission radiation is basically the light which goes out. But we have to amplify it somehow. To have something like a Star Wars laser guns, you need to amplify this. So, that's why it's a light amplification. Now, what S stands for is stimulating. So, by itself, whenever, let's say, we are supplying some electric energy to the gas. And I will talk about particular kind of lasers which are based on the gas, like helium and neon, for instance, mixture. If you put two electrodes, then the gas, at certain moment, if the voltage is sufficient and the voltage is supposed to be really sufficient because the gas, generally speaking, is isolating substance. But if sufficient amount of energy is supplied, high voltage. Then the electrons will move from cathode to anode. And meanwhile, they will hit electrons of the gas itself and excite them. They will bump to another level. After a while, when this other level becomes overpopulated, it will jump down by itself because it cannot really hold anymore so much energy. And the whole gas becomes, actually, like emitting certain light. This is not the laser, but light will be emitted because the energy which is supplied through these two electrodes which are going into the gas. And since the energy is actually supplied to the whole gas, which means into electrons of the atoms, it should go somewhere. It cannot infinitely consume this amount of energy, basically doing nothing. Because, yes, after certain amount of time, accumulated on the outer shells, electrons in a non-stable condition will start jumping back and emitting light. That's how it goes. Alright. However, as I said, this is not a laser yet because it's not like a one coherent ray of light. Coherent means it's the same frequency or the wavelength and the same phase. That's what makes it amplification. That's what makes amplification. You have to really have all these rays of light emitted by all the electrons to have the same phase and the same frequency. Only then they will increase each other. So if you have one oscillation and another oscillation exactly the same, then the result will be the oscillation of higher amplitude. So that's what makes it stronger. So the question is how to do this type of synchronization between electrons jumping from here to here. That's what's very important. And here comes the idea. Here is the idea. How to stimulate, and what happens if you do, how to stimulate electron jumping on our command. Not by itself. God knows when and God knows how. But on our command how to make it jump from here to here and emit light of a specific frequency and specific phase. And here is the idea. Again, this is idea. It's not implementation yet. Let's say you have supplied a certain amount of energy and this electron jumps to a higher energy shell. It's less stable, but it's still there. And then another one and another is set. Now what happens if there is one particular photon which hits this particular electron and amount of energy which is carried by this photon is exactly equal to the amount of energy which is different between the ground level, the normal, the stable level where this electron lives and the energy where it is right now on a higher level shell. So what happens? That's very interesting and there were a lot of experiments actually about it. What happens is the following. This becomes even more excited. And it does not really consume this energy which is carried by this photon. Now it actually forces this particular electron to jump back to whatever its ground shell is, the stable. And since it emits certain amount of energy, its energy will be from the higher to the lower level, right? So it's supposed to emit certain amount of energy. But it does not consume this one. This is just like in certain chemical reaction you have to add something to force this chemical reaction but it's not really participating in this chemical reaction. Same thing here. This photon is just giving it a stimulus. That's what stimulating S is. It gives a stimulus to jump back. But then it just goes away further. So I don't know. You can say that if it's two balls for example and this ball hits it at some angle, well this will continue working, moving further and this one will go this way, right? But this particular thing is exactly the same. So this photon goes further but since it moves this electron or it stimulates this electron to move from higher energy level to lower energy level and it actually emits certain amount of energy as radiation, as a photon, as electromagnetic oscillation it actually emits the second photon. So now we have two photons and what's very important is, and again that was kind of experimentally discovered, proved, I don't know how to say it, that these two photons have exactly the same frequency and phase. Now the fact that they have exactly the same frequency is obvious because as you remember the amount of energy which is photon is Planck constant times frequency. This is a quant of energy. This is a photon. It depends on the frequency. And as I said, the amount of energy this photon carries which means its frequency is exactly the same as the amount of energy between these two shells which means that amount of energy of this photon which is emitted when the electron moves down the orbit again is the same as amount of energy here which means its frequency is exactly the same because frequency and amount of energy are related only by a factor in Planck's constant. So these are exactly the same frequency. That's obvious by design because again we have stimulated with this particular amount of energy carried by this photon. About phase, well, I don't know. Maybe it just happens. I don't know why it happens, but maybe there are certain theoretical foundation why the phase of this particular photon emitted by the electron is exactly the same as incident photon. But whatever it is, it is the same phase. Now we have two photons. So what happens? For one photon, we have two now. What do we need for this? All we need is one excited electron. And we have to make sure that the energy of this photon corresponds to the difference between energy levels of these two shells. Okay, great. So this is idea. How can we amplify, so to speak? Because these two photons will meet another atom. Will meet another atom. And it has two electrons on a higher, on a non-stable energy level. Each one of them will do exactly the same as this one. So this one will produce two and this one will produce two. Now we have four. So we have a chain reaction, actually. This is a chain reaction and we can maintain this chain reaction if number of electrons on the outer orbits, the high energy orbits, will be supplied constantly. And we can do it if, for instance, if this is a gas and we have two electrons with high voltage, it's constantly moving electrons to higher orbits. Because energy is supposed to be electric energy, which we are supplying from the battery, will be converted into excited electrons. So we will have a flow of high energy electrons. Okay, great. Now these photons, let's say we just hit one photon, but one photon will produce two and two produce four, etc. So we will have a chain reaction and that's how all the energy supplied by the batteries will be converted into light. Number of photons will be greater and greater. And it's important that all of them will repeat exactly what this particular photon has, the frequency and phase. And that's what makes it actually, already this idea kind of closer. Now my question is only how to direct all these photons to the same place, because they all emphasize each other, they amplify each other, and they are the same frequency, the same phase, so that would be a blast, so to speak, right? And now the last part of this lecture is a very, very primitive schematic description of how this might, for instance, work in case of gas as a medium. As I was saying, something like helium and neon mix is used in certain lasers and what they do, they put it, let's say, in a tube, they put a reflector here and they put a partial reflector here. Now what is reflector and partial reflector? Reflector reflects all the light. Partial reflector reflects only the big light, but the strong light will break through. Now if you have two electrodes here connected to a battery, and this is the gas, so with high voltage it becomes actually a conductor and the electric current will be established between these two electrodes and as it goes it excites all the electrons here and now all we need is this. Well, we don't really have to have it as externally supplied in this particular case because every once in a while electrons are just jumping down by themselves and they emit exactly the photon, let's start with one photon, so we don't have an external photon, we have one spontaneously emitted photon when this electron jumps here and this one spontaneous now stimulates already two, four, etc. So that's how we have the whole avalanche of photons. But now how do they move? Well, those who are moving randomly in these directions are basically disappear or maybe we can put some reflection here as well, but those who move here will be reflected here and while it's still weak signal it will be reflected back so all these photons which are going along this direction will amplify each other greater and greater until they will break through. Now, again this is extremely primitive picture of how the lasers, gas lasers are arranged but nevertheless looks like it works. There are lasers based on solid body medium like ruby for example. There are liquid based lasers. There are many kind of lasers by now but initially the whole idea and that's exactly what I wanted to talk about today the idea of the laser that hit by a photon of a specific energy which corresponds to the energy needed for this particular electron to jump from higher orbit to lower orbit will generate the chain reaction and now we just force these photons which are exactly the same frequency and phase to go to our direction and amplify each other. Well, that's basically it about lasers. Read the notes for this particular lecture it might actually be a little bit better explained even I was trying to be very specific in these notes and there are a couple of pictures much better than these ones. Well, that's it. Other than that, thank you very much and good luck.