 Hi, I'm Zor. Welcome to Inizor Education. I would like to have some kind of theoretical basis for whatever we were discussing before. In a few previous lectures, we were talking about certain interaction between electromagnetic oscillations and matter. For example, luminescence, photo emission, photochemistry. These are basically what happens when electromagnetic field oscillations are somehow interacting with matter. And I would like to talk about what happens underneath. What are certain fundamental properties of light or electromagnetic oscillations and matter, which basically enable these type of activities. Now, this lecture is part of the course called Physics for Teens, presented on Unizor.com. I suggest you to watch this lecture and all other of these courses from the website, from Unizor.com. For many reasons. Number one, Physics for Teens is a course, which means lectures are arranged in certain logical order. They are interrelated, so I'm using a subsequent lecture, whatever we were talking about previously. The second reason is every lecture on the Unizor.com is supplemented with detailed notes, which basically is like a textbook for each lecture. A paragraph for whatever chapter of the textbook. They are parallel to the video and the textual presentation notes. There are certain problems which are discussed during the lectures. And there are exams, which you can take as many times as you want. Plus, there is a similarly arranged math, routines course on the same website, which is kind of prerequisite. You cannot learn physics without knowing mathematics, especially calculus, vector, algebra. And the final, probably very important reason, the site is completely free. There are no advertisements, no financial strings attached, no strings attached at all. You don't even have to sign in if you don't want to. There is a certain functionality of the website, which is related to supervised study, in which case you need the teacher who is signing and you are a student who is signing. There is a relationship, so the teacher can actually guide you, assign you, etc. So, back to Photons and Matters. So, this lecture is about certain fundamental properties of electromagnetic oscillations and matter. Well, let's start with electromagnetic oscillations. Sometimes I will use the terminology electromagnetic oscillations, sometimes radiation, sometimes just light. Well, light usually implies visible light, but you can have ultraviolet, you can have infrared, you can have x-rays, etc. So, light. So, light. The fundamental property of light was first researched by Newton, who basically suggested that light is basically particles, which are flying from the source to whatever they're flying to. Then, it's called corpuscular theory, and these particles were called corpuscles. Then there was a wave theory of light, developed to a very, very good extent by James Maxwell with his four Maxwell equations, which describe basically the electromagnetic oscillations as waves. And actually, that was the dominating theory up until the end of the 19th century. Then there were certain experiments which showed that maybe sometimes corpuscular properties of light do exist under certain circumstances. For example, the photoelectricity, when the light actually kicks off the electrons from the metal plate, let's say. So, researches were done, and two very important physicists contributed to contemporary view of the light. Max Planck, at the end of the 19th century, basically came up with the idea of quantization. Let's put it this way. Which means that energy, which is carried by electromagnetic field, is not infinitely divisible. It has a certain minimum part, which basically depends on the frequency of oscillations. Now, that was done with heat waves primarily. But heat radiation is still electromagnetic oscillations. And Einstein in 1905 came up with a very important article or research where he suggested that photoelectricity is also... Well, it happens what happens because electromagnetic field oscillations carries energy in certain chunks, not infinitely divisible. So there is a minimum chunk. And this minimum chunk is dependent on the frequency. And the energy is equal to h times f, where f is frequency and h is Planck's constant. The same Planck, who basically was researching heat radiation. And he came up with something similar idea of quantization. So quant means basically a chunk, a piece of. This is the plural form. And singular form is quantum. So sometimes... I don't remember who did it, maybe Einstein. Instead of quantum of light, he suggested to use the term photon. So photon is basically a quantum of light, the minimum part of energy carried by electromagnetic oscillations. And it's equal to h times f. So h is a constant. There is basically a numerical equivalent of this. And f is frequency. Now, what's important is that energy of this minimum amount, which can be carried by light, by radiation, by electromagnetic oscillations. This amount of energy depends only on the frequency, not on amplitude of oscillations. And what depends on amplitude? Well, the number of photons, which are emitted by the source of electromagnetic oscillations. Which means that if you are talking about electromagnetic field, then the amplitude of the source affects the density of photons. Basically amount of number of photons per unit of space, per unit of time. That's what basically amplitude is important about. But the frequency is important about the energy carried by the minimum particle part of light. Well, in some way it's similar to matter. If you're talking about, let's say, metal, or plastic, or anything, there is the smallest amount of that matter, which basically still carries the properties of this matter. It's a molecule, remember? Now, same thing basically here. Molecule for matter has basically the same role as photon for electromagnetic radiation for light, basically. So we have to really kind of make sure that we understand it 100%. Energy of light is not infinitely divisible. It's carried in chunks called photons. And every photon has this particular amount of energy. Number of photons per unit of space, per unit of time, whatever, depends on the amplitude of electromagnetic oscillations at the source, whatever produces the electricity, and electromagnetic waves. So that's all about quantization of electromagnetic oscillations. So light is distributed, carried, the energy of light is carried in photons in quantum. Great. So that's about photons, photons of this letter. Now, in this lecture, let's talk about matter. So what about matter? Is there a similarity? Well, apparently, yes there is. We all kind of have in mind the orbital structure of atom. Now, classical way, basically, is that electrons are circulating around the nucleus on some orbits. Well, first of all, let's use, instead of the word orbit, let's use the word shell, because it's a three-dimensional structure, so it's not really flat. So it's not really just orbit, but it may be this orbit, maybe this orbit, maybe this orbit, and maybe orbit is actually not exactly a circle. Maybe it's just some kind of a traveling around a nucleus on a certain level, a certain distance. That's an idea. Actually, it does have something to do with reality because of quantum theory, which is not actually part of this course. But basically, it's one of the things which tells that you cannot really tell exactly where, at some moment in time, where is the electron and what's its speed. There is certain principle behind this. But in any case, let's just consider the word shell. So electrons are on shells. This is kind of more appropriate, I would say, contemporary terminology. Now, based on which exactly shell the electron or electrons are, well, basically, their potential energy depends on it, right? It's like you have a satellite, for example, around Earth. It's a similar situation. It has certain potential energy. Okay. Now, potential energy of electron, by the way, is supposed to be negative. And that's because, you see, if we are putting something together and we have to spend energy, that means the potential energy is increasing. If it's the other way around, if we don't have to spend energy to put pieces together, but we have to really spend energy to separate them, that means the potential energy is supposed to be negative. Now, protons inside the nucleus are positive, electrons are negative. We don't really have to do some kind of work putting electrons on the orbit around the nucleus, right? Electron will be attracted. So that's why energy, potential energy of electron, which is circulating somewhere in a shell around the nucleus, is negative. But that's all right. It doesn't really matter. Most important is, and that is, again, contemporary view on the structure of atom, which we will talk about in the next chapter of this course, which is dedicated to atoms, but I'll just have to talk about this because it's very much related to how light interacts with matter. So what's most important about these shells is that every shell is at certain, well, let's say, distance, but it's better to say on certain energy level relative to the nucleus. And these energy levels are not anything, basically. There are certain distinct shells where electrons can actually be, and nothing in between. So it's a discrete, shells can only be on discrete energy levels. So the further you are, since potential energy is negative, so the further you are, the closer you are to zero, basically, which means it's increasing potential energy being negative, it's increasing towards zero the further you are from the center, from the nucleus. So, again, the most important part is these shells can be at certain energy levels, but obviously in infinity it's energy level zero, potential energy. The closer you are, the more absolute value will be, but it's negative. So it's growing as you go further and further from minus something towards zero. So it's distinct energy levels and every atom, every element actually has atoms which have certain distinct levels. Electrons which belong to this atom. So you have, let's say, hydrogen. It has one proton in the nucleus and one electron. So this electron can be at certain energy levels, a little further or a little closer to the nucleus. So there is certain, which is called ground level, which is like a normal state of this electron, normal shell. It's called ground level shell. And then if somehow we excite this electron, we'll give it the certain energy, it can jump a little bit further from the nucleus on the next level shell. So there are certain shells, shell number one, which is ground level, shell number two, that's basically the first excited level, shell number three, etc., etc. And again, as I was saying, for every element, there are certain distinct shells with distinct levels of energy. Now, let's talk about interaction between electromagnetic oscillations and atoms. Let's talk about one photon and one electron, photon from the electromagnetic oscillations, the minimum chunk of energy, which is this, and we will have electron, which let's consider initially on the, it's circulating somewhere in the shell with a ground level energy, whatever that ground level is. So what happens? Okay, let's consider that this amount of energy is equal to E j minus E i. i and j are two indices, two shells. So this is i, this is j. So there are distinct shells, discrete shells, and they are numbered from one to basically to infinity. And each of them has certain energy level. Now, there is a formula for atom of hydrogen that energy of the level n is equal, if I'm not mistaken, 3.6 times 1 over n square. So the energy level is absolute value. The absolute value is diminishing, but considering this potential energy is negative, so it goes basically up to zero, all the way up to zero from negative. But let's put minus here, so it will be kind of more understandable. So it's increasing from n is equal to 1, so it's minus 13.6 electron volts. We did talk about what electron volt actually is in the previous lecture. It's the amount of energy. Now, so this is for n is equal to 1, this is the ground level, this is the energy of the ground level electron. But next one will be what? 1 over 2 square, so there is a difference. And the difference is minus 13.6 times 1 over i square minus i over j square. So if electron is on number i, on the shell number i, it can jump to shell number j if it's supplied with energy equal to exactly this. So if this is true and our electron is on the i's shell, it will jump to j's shell. So it looks like there's supposed to be some kind of agreement between the energy of light, between the photon and as arrangement of the shells of particular element. They must match. So if one particular photon has this energy and our electron is on i's shell, it can jump to j's shell if this is a true equation. It will consume basically the energy of that one photon and jump to the next level. Now, what if there is no match? Well, then electron cannot actually jump in between the shells because again, this is our fundamental theory that shells are at certain energy level. So electron cannot be in between the shells. It can be either here or there. So if it cannot be there, it stays where it is and the photon doesn't really supply this energy to this particular electron in this particular way to make him jump from one shell to another. What happens with energy? Well, most likely it's dissipated somehow in heat. Some atoms will probably start shaking or whatever it is, the heat is manifesting itself. So this match is extremely important. Now, let's say it did happen. An electron being, let's say, in the beginning at ground level where i is equal to 1, it jumped to another level, let's say from 1 to 2, from number 1 to number 2. It means that minus 13.6, 1 over 1 minus 1 over 4, so it's 3 4's, it's something like minus 10.2 if I'm not mistaken, for hydrogen. So its potential energy from minus 13.6 jumped to minus 10.2 and now it's excited. Potential energy is greater. I mean it's negative but it's less by absolute value so it's greater. And now at certain point electron wants to normalize its condition. So it jumps back down and it emits that same photon which it accepted in the first place. And it emits exactly the photon which has the frequency which satisfies this same equation. So it looks like whenever you have a certain element you have certain shells around the nucleus. Every shell has certain specific for this element amount of energy. So let's say we will consider first five shells. 1 2 3 4 5 from the ground up. So what does it mean? It means that there are many different ways the electron can jump. It can jump from number 5 to number 1. It can jump from number 5 to number 2 or 3 or 4. From number 4 it can jump to 1 or 2 or 3. And all these jumps are emitting energy because its potential energy is diminishing. And for each jump some kind of similar equation depending on i and j would give you a specific frequency of light which is emitted. So whenever electron jumps back to lower energy it emits certain amount of energy as the electromagnetic oscillations as light if you wish, radiation. And the frequency is related again. Now the 13.6 is hydrogen. We are talking about hydrogen right now because there is no such formula for more complicated atoms. But the idea is still the same. Every atom no matter what it is whatever element has certain shells every shell has certain energy levels and the difference between energy levels is something which h times f should be equal when electron jumps down from the higher potential energy to lower potential energy. Which means every element has certain well actually finite which is distinguishable. I mean I understand that shells can be up to infinity but it's impractical. In practicality there are certain limited number of shells and limited number of frequencies which this particular element can emit when its electrons are reducing their energy by jumping down from a higher energy to a lower energy shells. So there is certain spectrum if you wish for each element which can be distinguished and we will talk about the spectroscopy that will be another lecture which will actually talk exactly about how to find out what exactly the element was the source of the light it has this particular spectrum of light emitted by this particular element when it's excited. Alright, now one more small basically part of this is what if electrons are on relatively high shell already and we are still bombarding them with a significant amount of energy in terms of photons. If photon is powerful, so the frequency is high and electron is already on a relatively far away shell it doesn't jump any higher because the difference between layers, between the shells, difference in energy levels is diminishing. Between first and the second the difference is something, for instance, for hydrogen. Difference is 13.6 times 3 quarters. The difference between let's say 5th and 6th so between 1 5th and 1 5th square minus 1 6th square the difference will be much, much smaller. So basically if electron is already on a high shell within high shell and there is a powerful photon with a high frequency there is nothing it can move actually it will just be completely bumped out and that was the photo emission which we were considering before, photoelectricity in which case whenever the electron is completely bumped out from the atom it has certain kinetic energy which is equal to a difference between the energy of the photon and as much energy it's needed to basically be completely free. So that happens too. Well, I think all I wanted to talk about so for hydrogen atom I actually have a numerical example in the notes for this lecture what is exactly the frequency of light emitted if I jump in from whatever level to whatever level the whole table actually put from 1 to 2 from 1 to 5 well actually it's the other way around well in one case it's positive amount of energy which is needed to really consume to be consumed and another is negative amount giving it back. Alright, so that's basically it about how photons and matter are interacting. Now, partially it's related to quantum theory of quantum physics basically which I'm not going to talk about in this particular course partially it's about structure of the atom there will be a next chapter of this course about atoms and I will probably talk about this a little bit more but right now since we are in between basically we are talking about interaction because matter and the electromagnetic oscillations I had to really touch on both bases so what's important is energy is quantized there is a minimum amount of energy which cannot be less than that carried in chunk basically and the structure of atom is also quantized so to speak the shells where electrons can actually live or circulate are discrete on discrete radius on discrete energy levels around the nucleus these are two most important things quantization of energy in electromagnetic oscillations in light and radiation and quantization of the shells around the nucleus so, read the notes for this lecture go to Unisor.com it's Physics 14's course the part is called waves and that's where you will find this photons and matter part ok, that's it, thank you very much and good luck