 Hi, I'm Zor. Welcome to Unizor education. I would like to continue talking about mysteries of light. In this case, we are talking about quantum properties of light, light as quants. We were talking about corpuscular theory, about wave theory of light. Now it's a quantum theory of light. They're all related in some way or another and this lecture basically is about common properties of On one hand, we will have corpuscular theory. On the other hand, we have wave theory and the light has properties of corpuscles and and the waves and this particular quantum theory is kind of getting things together it combines basically all the known properties of light into one theory which seems to correspond to all the experiments known to us at the time. So we think that this theory is better explaining actually the nature of light. Okay, so this lecture is part of the course called physics for teens presented on Unizor.com. I suggest you to watch this lecture from the website. You just go to physics for teen course on the website and then to energy and energy of light in this particular case. There you will find this particular lecture. There are also benefits to use the website because it has exams for those people who want to take them. Now this course has prerequisite course, which is math for teens also on the same website, which I do recommend you to take before you go to into physics because physics is really based on mathematics, especially calculus and vector algebra. And the website is completely free. There are no advertisements, so it's just for everybody free for all for free. Okay, let's talk about light. By the end of 19th century physicists more or less switched from corpuscular theory of light into the wave theory. Because the wave theory was able to explain everything which was in the domain of corpuscles and at the same time it was able to explain certain other phenomena like interference of light. So the wave theory actually took over by the end of this time. At the same time there was development in the field of electricity and primarily the very very important discovery was by Sir Thompson in 1897, if I'm not mistaken, the discovery of electrons as carriers of electricity of electric charge basically. So it was possible to talk about electric charge, positively charge, negatively charge. They're connecting. There is a spark between them, etc. So there were certain experiments related to electricity. Which uncovered a very interesting effect where electricity and light are related to each other. The experiment was very simple. If you have charge two poles, negative pole and positive pole, and they are relatively close to each other, at some point you are accumulating the charge, well, you're accumulating the number of electrons in the negative pole and decreasing the number of electrons in the positive pole. So there is an axis and deficiency of electrons. And at a certain time there is a spark between them. So everybody knows, everybody saw the spark, everybody saw the lightning, for instance, which is basically the same thing. It's a spark, giant spark. So the spark is basically a flow of electrons from the negative pole to the positive pole. So wherever it's an axis of electrons these electrons are going to the positive, where there is a deficiency, basically neutralizing them both. Now the experiments were conducted with the light, which is actually directed towards the negative pole. And what had been discovered is that the spark appears earlier if the negative pole, negative electrode, is lit with the intense light. So basically it's a very simple thing. So if this is negative electron and this is a negative pole and this is positive, negative positive. So there is an axis of electrons here. So when the axis of electrons is really significant, then they just travel from here to here. But if there is a light, directed towards this negatively charged plate, the spark appears faster, like easier. So you don't have as much charge to have the spark as if you do without the light. And obviously by that time the experiments were sophisticated enough to measure basically how much electrons are moving, even the speed of the electrons, which are moving from one plate to another. So experiments were quite precise. And first, they have explained basically this effect very simply in the framework of the wave theory. If light is the electromagnetic field oscillations, the oscillations carry the energy with it. So this energy is transferred into the electrons. They are agitated more. And if they are agitated more, they're moving more intensely. They are easier going from negative to positive poles and the spark comes earlier because the electrons not only they have their own ability to basically arrange a spark, but also they are agitated, they're moving much faster. And that's what makes the spark easier. Good enough. However, there were certain properties of this effect, which is called photoreffect. Photoreffect of electricity. So there are certain properties which are not exactly fall well into this particular explanation. There are three major properties which physicists observed in the photoreffect. Okay, the property number one is if photoreffect is present, then the number of electrons depends on intensity of light. So the more intense light causes more intense flow of electrons from negative to positive. Well, you can say great. It falls into the contemporary wave theory, the classical, contemporary at the time, the classical wave theory, very nicely because obviously the intensity is basically an amplitude of oscillations. So it's more energetic because the amplitude is basically the measure of the energy of light and so the more energy goes into the electrons and obviously more electrons are flying across. Okay. But now there is another property. Speed of electrons does not depend on intensity, but depends on frequency of light. Now, this is a very, very important thing. So no matter how intense the light is, the speed of electrons and therefore their kinetic energy of each electron is the same. So they measured somehow, I don't know how they measured the speed of electrons or their energy, but whatever the way they did it and what's important is that the energy of each electron does not depend on intensity. With increasing intensity, you have increasing number of electrons, but not their kinetic energy. The kinetic energy of each electron was exactly the same regardless of intensity, which doesn't seem to be, you know, nicely falling into this picture of classical wave theory. In a classical wave theory, the more energy you pump in, yes, obviously the more electrons you bump out, but at the same time, their speed probably also should go because the speed is basically the measure of energy. Energy is coming and therefore energy is supposed to go out somehow. Not that it completely contradicts. You can always say, well, maybe the speed of electrons is the same, but their number is greater and that's why the more energy, the more electrons are coming, but with the same speed. But how would you explain the dependency of the frequency of light? So a certain frequency produces certain speed. The greater frequency produces greater, of light, produces greater speed of the electrons. The lesser produce lesser. But this dependency on the frequency doesn't really fall well into this classical wave theory. And then there is a third property, which is completely unexplainable for each type of material, this electrode, this pole is made of, there is certain minimum frequency of light below which light does not affect at all the number of electrons, no matter how intense the light is. So if the frequency is less than certain level, which is specific for each material, like it's a steel or copper or something else, I don't know. Depending on the material, there is some minimum frequency below which light does not affect the flow of electrons between them, between the electrons. This is completely unexplainable by this classical wave theory because, well, even the lower frequency, if you compensate the lower frequency with a higher intensity of the light, then it should actually produce exactly the same effect, which means electrons should be bumped out from the surface of this plate, negative plate. So what do we do? Classical wave theory does not really explain this. Well, if you remember, the whole wave theory appeared from certain experiments which could not be explained by corpuscular theory, so they switched the wave. Now the wave theory doesn't explain certain experiments, we have to switch to a new theory. New theory was first suggested by Planck, famous physicist, but then it was developed significantly further for the specifics of the Potter Electric by Einstein, who actually, he has received the Nobel Prize in 1921, primarily for his research of the Potter Electric, not for theory of relativity. In any case, so what Einstein has suggested was the following. Yes, light is electromagnetic oscillation, electromagnetic field oscillations, that's right. What he suggested was that it's not just a continuous oscillation, but light comes in certain groups of oscillations, like one, then something, there is a pause, then another, and then another. So these pieces of oscillations, they are called quants or photons. Now, what also was important, that each particular quant, each particular photon of light carries a certain amount of energy which depends on the frequency. So energy is the frequency. So there is no such thing as amplitude of a classical wave theory. The concept of amplitude of oscillation doesn't really exist in the same sense in the quantum mechanics, in the quantum explanation of lights. Well, you can imagine that they all have exactly the same amplitude and just different frequencies. And the more frequency is related to more energy, which particular photon actually carries. So mentally I have imagined it myself as oscillation, then no oscillation. Oscillation, no oscillation. So this is photon. This is a photon. Whether it's really like this, I'm not trying to impose this particular model on you, but you can actually think about light as a sequence of individual quants of light or photons, and each one carries a certain amount of energy which depends on the frequency of oscillations of electromagnetic field. Now, what does it explain better than the previous model? Well, let's just think about explanation of these particular qualities, whether they, whether this particular model explains or it doesn't explain them all. Well, with the first one it's easy because the first one is basically depends on the intensity. Now, what is intensity in this particular case? Well, intensity in this particular case is the number of quants of light, the number of photons per unit of time falling on this particular surface. So the more photons are coming, not the greater amplitude of these, but the more individual photons. So it's not like a classical energy carrier waves. The energy is carried by one particular photon and the energy depends only on its frequency. So each particular photon has certain energy and intensity of light is the number of photons which are crossing whatever falling on this surface per unit of time. And obviously this particular dependency is explainable by this because if there is a photoelectric effect, obviously the more intensity of the light is, which means the more photons are falling per unit of time, the more electrons are bumped out. So the first one is great. Second one, speed of electrons does not depend on the intensity, but only on frequency. Here's what happens. One particular photon falls onto this plate and since the energy of this particular photon is transferred to an electron, which it falls onto, this energy it goes to electron and obviously the more energy it carries, which is the frequency, the more energy is absorbed by electron and this energy is released as a kinetic energy of photoelectrons, those electrons which are bumped out, the more kinetic energy they have. So they're faster, speed is more greater. So the second one really explains quite well why it doesn't depend on intensity, because intensity is just the number of photons and the number of photons will correspond to the number of electrons which are bumped out, but not to their individual speed, because each particular photon is capable of hitting a particular electron and bump it out from from the surface and the amount of energy depends only on the frequency of this photon. The more photons, the more intense is the light, the more electrons will go out, but each electron will have exactly the same speed which depends only on the frequency of the light. So intensity does not contribute to the speed of electrons, intensity of light, only the frequency of light, because the frequency defines the energy which is carried by individual photons. So one photon with certain amount of energy which is determined by its frequency hits the electron and gives this electron certain amount of energy which depends on the frequency and that energy is, if it bumped out, it goes out as a kinetic energy. So the second one is also explainable. How about this one? There is a minimum frequency below which electrons are not affected. The flow of electrons are not affected. Again, very simply. There is certain energy which is needed to leave the surface. It's like we need certain amount of energy, certain amount of speed, if you wish, but speed is actually energy. If you will throw a stone from the from the ground to go all the way into the space and leave the gravitational field of the Earth, right? So there is certain minimum speed which is needed. Otherwise it will return back. If we will throw the stone up and the speed is not sufficient, then something like 11.2 kilometers per second, then the stone will fall back. We did some calculations when we were talking about gravitational field. Same thing here. There are certain ties which need to be broken for electron to go out. So it needs certain minimum amount of energy to leave the surface and travel all the way to another pole, another positive pole. Which means, since our energy is basically measured by the frequency, it means that unless our frequency is sufficient enough to supply this minimum amount of energy for an electron to leave the surface of the negative pole, there is no photo effect. And that's what actually an explanation for the third property. So there is always certain amount of minimum frequency, which means certain amount of energy which each photon carries with itself, which is needed to make the photo effect happening. So basically this particular explanation, again as I was saying it was developed mostly by Einstein and he got the Nobel Prize for it, is a contemporary view onto the light. And what's important is that this division of the electromagnetic waves which basically light is, into individual photons binds together corpuscular theory which we used to have in the very, very beginning and the wave theory. So it's still waves, but the waves divide it into certain quants, into certain photons. And the energy of light depends only on the frequency of oscillation of individual photon. Intensity of light depends on the number of these photons per unit of time, crossing certain border, whatever. Well, okay, so this is contemporary view onto the light, which is basically as I was saying developed in the beginning of the 20th century, primarily by Einstein. And this is what we are currently thinking about the nature of light. Now I presented it in this particular schema. I'm not really trying to impose this scheme on you, but it's really difficult to understand how the whole thing is happening and why. Nobody knows actually that. But it presents a model like everything else. I was saying it many times. We are not really finding out what's really, how it's really done in nature. We are building a model which corresponds to whatever observation we are making. We first build the corpuscular theory of light and it corresponded to certain things. Then we build the wave theory to explain certain things which were not explainable by the corpuscular theory. Then we build right now the quantum theory of light based on photons, which explains basically everything, whatever we observe right now, which means that as of today this model seems to be corresponding to whatever we observe in the nature. And that's probably as much as we can do basically. Again, we don't know how it's done, but we do know that these principles produce certain results which correspond to our experiment, which means we consider them as a true model of the nature of the light in this particular case as of today. But we'll be tomorrow, I don't know. But as of today this seems to be the front end of our understanding of what light actually is. And on this I basically finish my lectures on light. There will be an exam for those people who want to take it. And I wish you good luck. Thank you very much.