 Hi, I'm Zor. Welcome to Nezor Education. Today we will talk about very interesting experiments which basically resulted in research and study of something which is called photoelectricity. Well, word photoelectricity means that it's related to light, photo, something related to light, and well, electricity, electrons, etc. Now, this lecture is part of the course called Physics for Teens, presented on Unizor.com. The website contains prerequisite course, mass for teens, and this one, physics for teens, and some other things. The website is absolutely free. There are no advertisements, no strings attached. You don't even have to sign in. I mean, unless you wanted to. It has, for every lecture, it has textual documentation, basically like a textbook, which explains exactly the same thing in a textual format. There are problems, there are exams, which you can take any number of times to check yourself. And obviously, this and the mass for teens are courses, which means there is a menu, there is a hierarchical menu, which basically put the whole material into a logical sequence. So, every lecture depends on something which was done before, and I do recommend you to take the whole course. Even if you just accidentally found this lecture on Unizor, on YouTube, for instance, go to Unizor.com and go with the whole course. Okay. By the end of 19th century, physics was pretty much in a very self-satisfactory kind of condition. The Newtonian corpuscular model of the light was, well, practically defeated because all these interference, diffraction, dispersion, etc., of the light, they explained everything from the wave standpoint. So, the light is the waves. And based on research of James Maxwell, who basically put everything into mathematical, onto mathematical background with four Maxwell equations, which basically describe the electromagnetic field and light as oscillations of magnetic field. So, that was done and everybody was pretty much satisfied. Well, and as usually happens, every model has its limits. And certain experiments showed these limits. So, the experiments are very simple. If you have something like a metal plate, let's say, whatever the material is, metal is usually they put something like silver so it doesn't oxidize. And you put a beam of light onto the surface. Under certain circumstances, people detected electrons flying off the surface. Well, by itself it was not surprising at all, because obviously the light carries energy. And the previous lecture was actually dedicated to calculating what is the energy density of the beam of light. With formulas, it's all based on, again, this wave approach to light as electromagnetic oscillations based on Maxwell's equations. We derived the formula for amount of energy per unit of volume, per unit of time, or amount of energy in one particular wave of a single ray of light based on wavelength, frequency, etc. Okay, considering light carries energy, obviously when light, the beam of light hits the surface of the metal, there are some electrons in this metal and the energy, which is carried by the light, whatever is not reflected is absorbed. And what does it mean absorbed? It's transferred to kinetic energy of electrons. They start moving faster and maybe atoms, the whole atoms maybe move faster. They're oscillating around some maybe neutral place. The whole piece of metal is supposed to be heated and that's fine. That's absolutely goes within the framework of electromagnetic waves as the theory of light. What was strange and that was discovered experimentally and that's what actually signified something which cannot be explained in the plane electromagnetic oscillation kind of theory. That unit for every type of type of metal, there is a special frequency, let's call it a zero, of the light. Unless light has this or greater frequency of oscillations, there is no electrons. So, for every metal, there is something like a threshold. If the light has this or greater frequency of oscillations, effect is observed. And if the light has less frequency or longer wavelengths, effect is not observed at all. That cannot be explained in terms of classical electromagnetic field as per the theory of electromagnetic field. It's presented by Maxwell on the top of this theory when electromagnetic field is basically continuous. Because there are differential equations, what does it mean differential equations? It means we are getting smaller and smaller piece of volume and we are calculating the density of light which is basically a derivative. The problem basically needs to be explained. Well, the first was Max Planck, German physicist at the end of the 18th century who was researching something not related to this particular experiments. He was talking about what kind of electromagnetic fields are emitted by a heated piece of matter. Because heat again, that's oscillations and when heat is radiating, that's electromagnetic oscillations. Maybe we don't see it unless we heat it really to a higher temperature when, let's say, the piece of metal becomes red because it was so hot. Then we see it. But if we don't see it, it doesn't mean that there are no emitted electromagnetic oscillations. There are, they are just an infrared spectrum and we just don't see it with our eyes. But we can see it with some kind of gadgets. He came up with some theory which was at least partially related to something which later on was called quantizing. So the energy emitted might not actually be a continuous energy. It might be in pulses. The real results, theoretical results towards that particular direction, changing of the model, was done by Einstein in 1905. He basically published something which was initially met by physicists with a lot of skepticism. Because what Einstein suggested was something that was something closer to Newton's corpuscular model of light. So he was suggesting that the light transfers energy in, not in the continuous flow when it's basically can be accumulated, et cetera, et cetera, and later on this would kick the electrons out. This does not observe these lower frequencies. And the classical electromagnetic theory doesn't depend on frequency. It just, you know, make it longer and even with a lower frequency it should supply enough energy for electrons to fly out. It did not observe. People did not observe it. So the theory which was offered by Einstein was completely different in the game. Closer to corpuscular model goes to the times of Newton. So he suggested that the energy is carried by light, not as a continuous flow, which can be infinitely divided into smaller and smaller pieces. No, there are some smaller pieces. Like matter has molecules, which is the smaller piece which basically carries the properties of the molecule, right? Let's say a molecule of salt, a regular salt. It's, I think a molecule is something like natural chlorus. Two different atoms together they make a molecule. If you break it, it's not a salt anymore, right? So there is a molecule as a minimum of matter which carries the properties of the matter. Same thing with light. He suggested that for every frequency of light the energy which is carried by light is not a continuous, but in pulses or pieces, whatever you can call it, each one carried amount of energy equals to, it proportional to frequency of the light. This f is a frequency, number of oscillations per second. So it does not really negate the wave theory because we're still talking about oscillations per second. We're talking only about how energy is carried by light. And h is just a constant, it's called Planck constant. Planck. That's the name of the physicist I mentioned before who basically researched the emitting of electromagnetic waves by heated objects. So he actually put the whole theory. It's a very big article of Einstein. And he was awarded Nobel Prize. But not immediately. Again, as I was saying, physicists were very skeptical about this in the beginning. And only additional theoretical research, etc., in 1920, like 122, I don't remember, at like 16, whatever, 17 years later, he was awarded Nobel Prize for his work, which basically was in the beginning of a revolution in physics of the 20th century called quantum physics. Because Einstein suggested that this is basically something which can be called a quantum of light, quantum of energy of light. So for every frequency, there is this amount of energy which is carried by pulses and there is no smaller piece of energy. It's not a continuous flow of energy. It's like pulsing. So you have one piece of energy, then another piece of energy, etc. That's how it's carried. It was an unusual theory to tell you the truth. Because on one hand he retained the wave properties of light because there is a frequency. It means it's oscillations. But on the other hand, he has suggested that there are some corpuscular properties, of the particles. Like particles, every particle carries certain amount of energy. You cannot divide it, basically. Now, this amount of energy is called photon. So that's how this particular term was invented. So the photon is a minimum amount of energy which light carries as a portion. Okay, now let's go back to our photoelectricity or photoelectric effect or photoemission sometimes it's called. Emission because electrons are emitted from the surface. So how can this be explained from this particular viewpoint? Okay, here is how. Electron consumes energy carried by a photon completely or not at all. So the energy gets consumed or rejected again based on these portions. So if this portion of energy is sufficient to basically give the electron enough kinetic energy to tear off the attraction of the nucleus of the atom, then it goes away, it flies away. If it's not, then it's not basically consumed or maybe it actually does have certain amount of energy consumed in this photon. But it's not enough for tearing off the attraction of the nucleus and it might actually shake a little bit and heat, maybe the whole thing is always heated but not sufficiently to produce electrons flying away from the surface. So that's basically how the existence of this particular threshold for the frequency can be explained. So again, if this frequency which is carried by a photon, if the frequency is high enough to give this portion of energy to an electron and electron would have enough energy to fly away from the nucleus, then we observe the effect, this photoelectricity effect. If this frequency is not sufficient, if the nucleus holds quite strongly the electron and it obviously depends on what kind of a metal we are talking about or whatever, it can be glass as well, it just holds it probably stronger. So if it's insufficient, then we don't observe this particular effect. By the way, effect can be very useful in practical purposes because if this particular flow of electrons depends on the light, we can construct, let's say, a counter of parts which are going on the conveyor. If you have a beam of light and the part as it goes along the conveyor can interrupt this beam of light, then while the beam of light is reaching the reception and reception can be something like this and there are electrons flying and there is some kind of a circuit which you can have plus here, minus here and electrons will go to this, making a circuit. So that's how you can count. So it will be either there is an electricity in this circuit or there is no electricity if it's interrupted and that's how you can count. Just a simple example, a count passenger which are going along some road, whatever. So existence of the protons was the most important achievement, if you wish, and the result of this is that the whole approach to physics of 20th century was changed towards quantum theory. After these ideas were supplemented by many, many experiments and skeptic physics physicists were convinced that Einstein was right, they started basically developing the whole quantum physics theory in this particular direction and it resulted in numerous successes. Okay, now I wanted to present you some kind of a common sense model which should give, well it gave me some comfort accepting that only a proper frequency is needed or greater is needed to kick the electrons out of the surface of this plate. And I have come up with some, as I was saying, common sense explanations for myself and I think I would like to share it with you and hopefully I will convince you that this is really not something which came out from the air, it has some common sense explanation. And here is my explanation. Consider you are in a car and you are going along the road and it's not smooth, it's bumpy. So you have some big bump, big in terms of, not in terms of height, but in terms of lengths. So there is some kind of a height and there is some kind of a length, okay. Now if this length is long enough and you are moving in your car with certain speed, you might feel that you are going a little bit up and a little bit down, but it's no big deal because the steepness of this is really minimal. Let's consider you have exactly the same height but the road contains bumps of much shorter wavelengths or larger frequency, right? Because as the car goes with certain speed, the frequency of these bumps, the long bumps would be significantly less than the frequency of this bump. Now what happens in this case? Where would you feel much more bumpy in this case or in this case? Well obviously in this case. Why? Because this is steeper and whenever the car moves it should go up steeper, which means during the same amount of time the car goes with certain speed, right? So the time is, the time needed to cover this distance, the car goes up. In this case, during the same time, car goes up only much less. So the acceleration, upward acceleration and downwards acceleration is significantly greater in this case and what if acceleration is greater? Well, it will just basically kick you upwards, right? So you might even be in the air above the seat of the car if the bump is really kind of steep, right? So it means a specific frequency of these oscillations to provide you enough acceleration so you basically are jumping in the air and you go above the seat. In this particular case you will not be in the air. You will be on the seat maybe with a little less or a little more pressure but you will not be in the air. You will not hit your head on the car's roof, right? But in this case you might. So that's exactly my kind of common sense explanation that with a higher frequency, even of the same amplitude, but with a higher frequency you have more chances for electrons to be kicked out from the surface of the, the electrons are like you in the car and the surface of the metal was basically the level of the seat of the car. In this case you might be up in the air. In this case you will not. Another common sense example. Consider you just wash your hands and you would like to shake off the excess of water. What do you do? If you do very slow movement the water will not come out from your hands, right? But if you shake it, if you jerk it basically, then the water will come out. So this acceleration is very important. So it looks like in our world, not in the world of electrons and the electromagnetic waves, in our world this frequency is also very, very important. It basically gives you the same kind of effect which you observe when the frequency of light is even from the standpoint of pure wave theory, right? Now it's a different question that Einstein formulated this thing using basically the concept of quantum of light, the photons. This is, I would say, a theoretical physics which I right now I don't want to go into. But the common sense explanation that you need certain minimum frequency of oscillation to kick electrons out from their orbits around the nucleus, it's kind of making sense with this type of explanation or with the shaking of hands to shake off excess of water. So that was kind of my, again, common sense attempt to explain that there is nothing very strange about existence of minimum frequency the electromagnetic waves should have to start kicking off the electrons. Now what did I not cover yet? Yes, one more thing, yes. So energy of the photon depends on frequency and the plane constant. It's a constant basically. It has certain value in textual notes for this particular lecture, contain the value. I don't remember it. So what if the frequency is significantly greater, the frequency needed to kick off electrons with that particular material the plate is made of? So every material has certain minimum frequency needed to start photoelectricity, photoelectric effect. So what if f is greater? So if h times f is more than sufficient to kick off an electron, and again one electron consumes one photon. So where is the energy, excess of energy going? So if you have certain energy, I call it energy free, which is needed for electron to free from the orbit around which it's circulating. So what if my energy of the photon, which is this, is greater? Well, since the whole energy is consumed by electrons, part of this energy, meaning this part, is consumed to basically tear off the orbit. And the rest of it goes to kinetic energy of this electron as it comes out from the surface. So this is a kinetic energy of electron after it spent this amount of energy to basically tear off the nucleus. And whatever the excess goes into kinetic energy of the electron, with m is mass and v is speed of electron as it comes out from the surface. Now what if I will increase the intensity of the light of the same frequency? Which means I will increase the number of photons per second, so to speak, emitted by the source of light. What happens in this particular case? Well, more electrons, well if the frequency is sufficient to kick off electrons from its orbit, then if number of photons per second is increased, then the number of electrons picked out will increase. So this number of photons per second is basically an intensity of the light. And that increasing intensity with the same frequency sufficient to kick off one particular electron, so increasing intensity will increase the number of electrons. So intensity of the light, if the light is sufficiently strong, if f is sufficiently great, then it will increase the number of electrons. Increasing the frequency will increase kinetic energy of every electron kicked out from its orbit. So these are two very interrelated things, intensity of the light and frequency of the light. Frequency goes into kinetic energy of the electrons, kicked off the orbit, and intensity goes into the number of these electrons. So all electrons will have exactly the same kinetic energy, but their number will increase. So kinetic energy of the electron depends only on the frequency of the light and properties of the material, how much energy electron needs to basically rip itself off the orbit. So these are two very important things. Frequency increases kinetic energy of kicked electrons, intensity increases the number of these electrons per unit of time. Okay, that's it. I do suggest you to read the notes for this lecture. It's maybe a little bit more concise than what I was talking about. But anyway, it's very good to have it in writing as well as this lecture type of things. Thank you very much and good luck.