 Hi, I'm Zor. Welcome to Unisor Education. Today we will talk about something which is called Illuminescence. Now, Illuminescence is basically one of the things related to light, and we are basically talking about light right now. This is the part of the course called Physics for Teens, presented at Unisor.com. By the way, this website contains prerequisite course, Mass for Teens. Mass is absolutely necessary to study physics, but for a change, today's lecture will have absolutely no formulas. Yeah, for a change. It's a rare occasion. We'll just talk about certain properties of Illuminescence and we'll see how it goes. Now, the website is completely free. It contains not only the lectures like this one, but also a textual description notes for each lecture, which basically is like a textbook. So, we have lecture and the part of the textbook, if you wish, related to this lecture, right on the same page, if you will go to Unisor.com. Now, it's a course, Physics for Teens. It's a course, which means there is a menu. It's a hierarchical menu, which allows you to basically concentrate on something like a part or a chapter of the course, which contains a few lectures. And I do recommend you to study physics if you are using lectures like this. Studies from the website, not from any other source you might accidentally find it, like YouTube or somewhere else. Alright, so let's talk about Illuminescence. There are many different kinds of Illuminescence and there is a certain process which happens universal for all kinds of Illuminescence. Now, what is this process? Now, we're talking about emitting light. Okay, that's kind of a general common sense description of Illuminescence. Yes, emitting light. So, something becomes the source of light. The question is why and how, and what are these processes combined together into one particular term, Illuminescence, and then we will divide it into types of Illuminescence. So, in all the cases, emitting of light is related to basically one particular physical process. Imagine atom. It contains the nucleus, protons and neutrons, and it contains electrons which are somewhere surrounding, circulating whatever around this atom. This is our model, basically, which kind of works. Now, there are certain orbits, electrons are circulating around the nucleus. It's better to say shells, not orbits, because the word shell basically represents three-dimensional kind of way. So, there are certain shells and certain levels of energy which these electrons possess in a normal state of the atom. Now, what happens if electrons are excited? Well, excited means some other energy comes into this particular part of the object. And electrons are absorbing that energy. Now, what do they do? Well, they might actually jump from one orbit to another, from one shell to another, which basically changes the composition of atom, basically. Now, if there are certain amount of these electrons excited, accumulated, then they are actually trying to normalize and they are jumping back. And when they are jumping back to their proper place, proper shell, they emit extra energy which they have absorbed before. So, at the moment this energy is emitted, in many cases it's emitted as electromagnetic oscillations, which means light. And sometimes it's a visible light. So, the general approach to understanding luminescence means, basically, it actually means that electrons excited before, excited somehow, there are different ways of excitement. That's why we have different ways, different types of luminescence. But at the end, they give back this energy which they have accumulated before. And this extra energy is given as electromagnetic oscillation, and sometimes it's a visible light. So, now we can talk about just different kinds of luminescence, which basically means different kinds of initial accumulating of the energy. So, there are different sources of external energy which can be accumulated by electrons. And that's basically the types of luminescence. So, I actually have some cheat sheets here. So, one of the things is electric luminescence. So, electric luminescence means that we will put some electric current into something, and that something is supposed to be absorbing energy of this electric current. And then these extra energy will excite electrons to a certain degree, and when they try to normalize, because they cannot be infinitely excited. They cannot accumulate energy without giving it back. So, they will give it back as visible light, for example. Now, well, sometimes if we do this, we will convert this energy into light differently using the heat. For instance, in condensants lamps, the regular lamp which we were using before LED or other things. Now, these lamps were basically related to heat, because the tangent inside spiral had certain resistance. Resistance converts into heat when you connect it to electric potential. And the heat is the source of light which is emitted by this particular tangent spiral. Now, luminescence is not that. First of all, it doesn't really take as much energy. The object practically not heated up, practically. So, energy is not really used to, well, in case of heat, you basically make atoms oscillate. That's what heat is, and that's what basically results in increasing temperature and eventually it actually produces light. In this case, it's just electrons which are jumping from one orbit to another, from one shell to another, accumulating energy and then giving it back. So, it's different than in all these lamps, kind of thing. So, electric luminescence requires certain other substances. So, there are certain semiconductors. And the way how it usually done is the following. You have, for example, you can have something like a backlight. You have backlit screens, right? So, how is it arranged? You have two conductors. One is, let's say, plus, another is minus. And one of these conductors is transparent, and there are certain conductors which have this property. They are conducing electricity, but they're still transparent for a lot. Let's say it's this part, the right part. I don't remember actually which one. There's something here. It's kind of a semiconductor. So, something with certain additions. And this particular substance, it's a result of experiments, actually, has this property. So, whenever voltage is applied to these two conductors, one is transparent, another is not, the electrons inside becomes agitated, excited. And eventually they will give back this particular light. So, it goes through the transparent, transparent electrode or whatever you have, contact, and emit it outside. So, this is the way how flat screens are made, actually. So, this is electric luminescence. Now, other examples. Well, everybody is familiar with old style TV. When they have a CRT screen. TV and computers as well. You had a screen, and you had a source of electrons, and electrons are bombarding. Some composition which is on the screen, some kind of phosphorous or whatever, something is done based on phosphorous, I believe, primarily. And as a result of this bombardment, these are electrons, so it's still kind of electric luminescence. Now, these electrons are bombarding, and what does it mean? Well, they carry energy, this energy is absorbed by electrons inside this phosphorous, and eventually they give it back. So, if you direct your electrons, direction of electrons to this particular point, this point will emit light. And that one. So, that's how the first televisions and computer screens were made. Computer screens were green and black, old ones. So, green is basically the light which is emitted by phosphorous under the influence of energy. Which came with electrons. The first televisions were black and white. So again, it was some kind of other substance which covered the screens of televisions on the inside. Outside was just a glass, and from the inside it was with this particular substance. Electrons bombarded, and they lit up different points on this particular screen. So, that's how the first televisions were. It's all kind of electric luminescence. Now, contemporary electric luminescence examples is LED. LED is light-emitting diodes. Well, diode is some kind of a semiconductor usually. Light-emitting means exactly what luminescence is all about. The electric potential was connected to some kind of substance which is this particular semiconductor, LED. And LED becomes the source of luminescence, source of light. So, that's how we, so that's what electric luminescence is. Now, what's all electric about it? Because some external electricity was applied either as a flow of electrons in CRT, or just contact with two electrodes which are connected to source of electricity. And all this allows whatever the substance is used to start emitting light. Okay, next. Next is chemical luminescence. Now, chemical luminescence is, obviously as you understand, this is the result of chemical reaction. Now, certain chemical reactions produce energy, and this energy in many cases is the light. Now, again, everybody probably experienced the light stick. You know, you just break it a little bit, and it starts glowing, right? Sometimes in a party, sometimes as a source of light, movies, when some kind of a cave or a well, and people just break this particular, not break, they just slightly bend this particular stick, which means something is broken inside, some chemicals mix together, start chemical reaction, they throw it in the well or in the cave, and it lights whatever is there. That's an example of chemical luminescence. Alright, then there is something which is called luminal. It's just a substance which is mixed with hydrogen peroxide, just as an example. Hydrogen peroxide, which is H2O2. H2O is just regular water, H2O2 is peroxide, hydrogen peroxide. So, mixed with this, this luminal glows basically. So, again, it's a chemical reaction. Some heat also is emitted, but what's most important for us right now, what's most important is that it emits lights as well. And why exactly the same story? Electrons of luminal in this particular case are excited as a result of the chemical reaction and emit light. Okay, bioluminescence, yeah, bioluminescence is certain life organisms, many of them live in ocean like bacteria, whatever. They are producing electricity, very weak light, but still it's light, and it's a result of some chemical process which is occurring inside these bacteria. That's how they live. Alright, so bioluminescence is also another jellyfish, yeah, but jellyfish sometimes is producing electricity. Okay, now, photo luminescence. Well, photo luminescence sounds like a strange thing. Photo means it's caused by light, but luminescence means it emits light. So first you have to absorb the light, and then you emit the light a little later probably. So that's what photo luminescence is. Now, something like my watch has some phosphorous paint on each number and on the hands. So during the sunlight it accumulates the light, and if I will go to dark room after that I will see my digits in my hands on the watch. So that's photo luminescence. So again, light, regular light is the source of energy which excites electrons, and after this electrons are, well, it's probably done immediately as well. So the sun goes in and my phosphorous actually is agitated. Electrons are agitated and give it back, but I don't see it when the sunlight is on. But if I will go to another room, there is no sunlight, but the energy is still emitted for some time, short time, but still. Well, actually, how long it will emit energy depends on the substance. I mean, different substances probably can accumulate more, and they light up a little bit longer after the source of energy, the sunlight or something stopped. So that's photo luminescence. First exposure to light, and then when later on when electrons are relaxing from their excited state, they will gradually return, emit the light back. What else? Mechanical luminescence. Well, mechanical luminescence is sometimes, certain substances, if you do something mechanical with them, they emit light. Very interesting example is diamonds, for example. When you cut diamonds, when you make these flat surfaces to shape the diamond into something beautiful, well, sometimes they do have this effect of emitting light, blue or white. And it's, again, very big light, but still, you can see it. What other things? Yes, piezo elements, that's another interesting thing. There are certain substances, if you press them, they emit light. So piezo crystals, sometimes quartz crystals, if you will rub them, they will emit light. So that's another source of... What else? I don't remember all these cases. Okay, and the last one is thermal luminescence. Now that's an interesting thing. Thermal luminescence is basically a combination. First, you have to have a source of energy, for example, some kind of electromagnetic oscillations, radiation. You are radiating some substance. Now, it accumulates this energy, but it does not immediately release it. So it's accumulating and the electrons are in this excited state. Well, up to an extent, of course. Now, but they do not immediately release this energy as visible light, for example. But if you will heat it a little bit after the fact of radiation, then they will start giving back this energy as light. So that's thermal luminescence. I don't think I have any example of this. I just read about this. Quite frankly, I'm not familiar with all these little cases. I just decided to read it because it's kind of appropriate for this particular course. I think that's the last example of luminescence I wanted to talk about. I promised no formulas and I hold my promise. So I do suggest you maybe to read the notes for this lecture. So you go to Unizor.com. It's Physics 14's course. The chapter, not the chapter, it's part of the course called Waves. And in the Waves, if you will open the next screen, you will see one of the topics to be luminescence. I didn't mention it, but the website is totally free. There are no advertisements, no financial strings attached, no monetization in any kind. So just use it as you will. All right, that's it for today. Thank you very much and good luck.