 Hi, I'm Zor. Welcome to Unisor Education. I would like to start a new chapter in this energy section of this course. This chapter will be dedicated to light, light and energy which it carries. Well, first of all, this lecture is part of the course called Physics for a Team presented in Unisor.com. I do suggest you to watch this lecture from this website. You just have to go to Physics for a Team course, then energy, and that's one of the topics which is presented on the next screen. The reason for this is that, again, as a course, it has certain logical sequence, so there are prerequisites for this particular lecture. And also there is another course called Maths for Teens on the same website. And math is very important for physics. I mean, especially calculus and vector algebra, they are used everywhere in physics, so you definitely have to be proficient. And if you are not, take the Maths for Teens course on this website. The courses are completely free. There are no ads, so your attention will not be distracted at all. Okay, now back to light. You see, the problem with light is it's really a very difficult subject from the physics perspective, primarily because people did not really understand clearly what light actually is. You see, the Newtonian mechanics was primarily dedicated to like plain objects, which we can observe, and we can measure them, etc., etc. And it's all related to matter, something which we can feel, we can look at, etc. Now, the first problem with light, for instance, the light of the sun, is that there is no medium between sun and earth. How this light actually travels in the vacuum. And on the top of this, we are talking about energy which it carries. How can it carry energy if it doesn't seem to be any material substance in between sun and earth? Well, some time ago, people were thinking that there is something in between called ether. But then they basically completely negated this particular theory. There were other theories. So that's what we are talking about right now. These evolutions of different views onto light. So let me just start talking about light as carrier of energy. Well, first of all, I would like to state that light carries energy. Now, why is it obvious? Well, primarily because when you are leaving something under the direct sunlight, it will warm up. Now, warm means kinetic energy of the molecules inside the object. So, who provides this kinetic energy if it's just lying under the sun's light? Well, the light. So, light carries energy. Now, there is a very interesting experiment. There is a device called radiometer. And here is how it's arranged. So you have some kind of a axis and you have four perpendicular, small, very light rods or whatever. And you have a little plate on each rod. Now, this plate has two sides. One side is silverish, like mirror kind of thing. And another is black. So black absorbs light, as we know. And the silver surface reflects the light. Now, if you will, now, and it's supposed to basically rotate in this point. So, and also it should be in the vacuum. So, this is called radiometer. And I have a picture in one of the notes for this particular lecture. And on YouTube, which I have a reference to, by the way, in the notes, you even have a video, what happens. So, if you will put this device under light, these plates will start rotating. And if you will put it back in the darkness, for instance, it will start rotating, not under direct sun. Now, why? Well, obviously, if we put it under the light, it starts rotating. It means light has energy, obviously. So, why is it rotating? Well, primarily because the silver side of each plate is reflecting light. The black side is absorbing light, which means it absorbs the energy, which means the molecules on the surface of the black side start moving faster. So, if you have, this is your plate. This is a silver and this is black. So, the molecules here on this left side becomes agitated more because the sunlight energy is absorbed. And they're heating more from this side than from that side. And since these plates are arranged that the faces which are silver, they're all on the same side of the plate. So, the rotation starts as a main motion of this construction. Now, another proof that the light has energy is that we see it, we see the light. Now, what does it mean that we see the light? Well, obviously, light comes into the eye. It somehow affects certain receptacles over there, certain cells. And they are, again, being somehow agitated and they're sending electric signal to the brain and that's how we see the light. So, again, without energy, it would be impossible. So, I think I have convinced you that light carries energy. The question is why, how much? That's a very big question, actually. And people were suffering with this issue for a very, very long time. Well, and still suffer. Okay. So, let's just go back to the previous lectures related to gravitation. Now, we were introducing the concept of gravitational field. It has certain force, it pulls and it has, therefore, certain energy. But if you're just putting into an object into the gravitational field, if it doesn't move, it still has a potential energy. If you let it go, it will go towards the source of gravity, converting potential energy into kinetic energy. Now, so the concept of field is very important. What is gravitational field? It's a special kind of a state of a space. It doesn't have any material substance which actually carries the gravitational field. This is certain domain of the space which has certain forces and it has certain energy in it. Exactly the same thing we were talking, we can talk about the light. There is a concept of electromagnetic field. We will talk about electromagnetic field in details when we will talk about magnetism and electricity. However, as of right now, you just probably have to accept my word electromagnetic. This is a field and the light is going through this electromagnetic field in the same way as gravity goes through the gravitational field. I mean, they're completely different fields, obviously. But the concept of non-material substance, some kind of a special kind of a space which actually is a carrier of the energy and the forces, that's what the field actually is. This is just a separate kind of a field, electromagnetic field. Now, it was actually very difficult for people to come up with this particular idea and to prove that this is really electromagnetic field. And there is a lot of very interesting historical books and articles about this, how the whole theory of light was developing. So, from the very simple kind of a concept that there is substance, material substance called ether and light is just the waves inside of that ether in as much as you have the water waves on the surface of the water. Well, that was an idea. But then again, it was kind of disproved differently and certain other ideas were introduced. And electromagnetic field as a carrier of the light is the idea which is considered to be right now the most important kind of a thing and true to the moment, let's put it this way. And the light is basically the waves in this electromagnetic field. We are replacing the term ether which is kind of a material substance with the word electromagnetic field, but we are still retaining the term waves. So, the light is basically the waves of electromagnetic field. Now, the wave has certain parameters. What's the most important parameters of the wave? Well, there are basically two. If we imagine the wave as just a sinusoidal kind of oscillations, let's say waves on the surface of water, right? So, what's the most important characteristics? Well, one is called amplitude. Well, that's basically the deviation from the neutral state. So, on this, this is basically amplitude from this point to this point. This is amplitude. Now, the greater the amplitude, well, the more energy this wave actually carries, right? Another very important characteristic of the wave is frequency. So, you can have these waves with this amplitude or you can have this wave with the same amplitude. So, there is a frequency. Now, what is frequency? Number of oscillations per unit of time. It's measured, by the way, in the units called Hertz. Hertz is one Hertz is one oscillation per second. So, if we observe the waves on the surface of the water, we can always check how many of these waves are moving per second. So, that's what basically our frequency actually is. Now, here is a very important thing, energy. We are talking about energy, right? So, it's kind of obvious that the amplitude of the waves is directly related to the energy. Well, it's less obvious what is the relationship between frequency and the waves and the energy. Well, let me put it this way. If you know something about photo labs, which were developing the old-fashioned film, not the contemporary digital one, but the old one, with chemicals, etc., etc. So, you probably know that usually these labs were lit with a red light. So, why red light? Why not white? Well, for obvious reason, we don't want to overexpose our film, right? So, it means that the red light doesn't really expose our film. So, what does it mean? It means that the red light carries less energy than, let's say, white light, right? Less energy because, obviously, again, the film developing the whole procedure was based on the properties of the light to somehow affect the chemical composition of the film. And then, using certain chemicals, we can make it stable and the image actually is retained on the film. But the red light doesn't do this. So, red light, the only explanation you can have is that the red light doesn't have as much energy to change the chemical composition of the film. Okay. So, what's the difference between red light and white light? Well, we all know what is the rainbow, for instance, and we all know that white light is actually a combination of many different monochromatic, as we are saying, lights. Red is a color, as well as green and blue and violet, etc. So, it looks like the light has certain characteristics which we, the people, view as color. Again, why do we have different kind of understanding, different view and different lights? Well, again, because certain lights are affecting our receptacles in our eye, our cells, which are responsible for accepting this light, they affect it differently. And what's the difference? Again, energy. So, certain lights are affecting more, certain less. And it appears that something like the red light, it affects less than, let's say, green light. Or white light, which is a combination of red, blue, green, etc. So, what exactly is the source of this difference? What's the difference between red and green? Well, yes, the energy which they carry. But from this perspective, from perspective of amplitude and frequency, amplitude is basically the brightness of the light, right? So, if the brightness is exactly the same, what's the difference between red and green? The frequency. And it was actually determined through many different very sophisticated experiments that, yes, every different color of light which we observe has different frequencies. And red has the lower and the violet has the higher frequency. Actually, right now we know that red is something around 0.4 times 10 to the 15 hertz. And violet is something like 0.7 times 10 to the 15 hertz. So, 10 to the 15 is more or less where these lights which we can observe are concentrating. Now, but we also know, well, at least you remember, like ultraviolet lights or infrared lights, right? Infrared, for instance, is something which our remote controls are using to control the TV. UV ultraviolet lights is something which we're trying to protect our eyes using special sunglasses. Now, why do we have to protect our eyes? Well, again, violet has the highest frequency and therefore the highest energy. Ultraviolet has even more. So, my point right now is that not only the visible light is basically a manifestation of the waves of electromagnetic field. There are some other electromagnetic field waves which are not really perceived by our eyes as visible light, but nevertheless they exist and we can use them. Well, again, significant number of important experiments and technical developments basically have established that we have the whole set of different phenomena related to electromagnetic waves, waves of electromagnetic field rather. Now, the most energetic with the highest frequency, electromagnetic waves, are related to so-called gamma rays which we're actually getting, we're bombarded from space and our atmosphere is, well, partially shields us from these rays. Now, then there was a discovery of X-rays, if you remember, by the person called Rand-Gen. X-rays are also electromagnetic waves of the frequency less than gamma rays. Now, this is on the level of 10 to the 24 hertz. So, X-rays is less frequency. Then we have ultraviolet lights. Still, we are not seeing these, but gamma rays actually, or X-rays actually, you know, it can actually damage because they're very energetic. That's why when we are getting an X-ray in the dental office, for instance, they usually cover with some kind of a protective piece, all the body, right? Now, ultraviolet light, it's not really adversely affect our body, but it can affect our eyes and that's why we're trying to protect against it. Well, then we have the visible spectrum. The visible spectrum is from violet down to the red and then even less frequent oscillations have infrared. Then we have microwave. Yeah, the regular kitchen microwave, it's also using electromagnetic fields. Magnetron is something which is inside, which is a source of electromagnetic field of this particular frequency. And then we have radio waves. Now, radio waves are also different. We have AM, FM, we have long, short, medium range waves and the longest waves, the longer, obviously, the less frequent these waves are. They have actually about like three or four hertz, three or four per second. So you see from the range of different wave frequencies of electromagnetic field are from something like this, three, four hertz to 10 to the 24 hertz. It's an extremely wide range and obviously extremely wide energy which they carry. And again, we will talk about how to measure it, etc. Right now, this lecture is only kind of introduce you to a concept of light. And everything, whatever people have learned about light, which I was just trying to convey right now, turned upside down with development of the quantum theory. Well, I am not the person who can explain what is quantum theory is right now, at least not at the moment. But believe me, it was a revolution. I mean, the light also, electromagnetic theory, electromagnetic field theory of the light still is the one which we are using as the theory right now. However, quantum mechanics and quantum theory of light brought a little bit more to this. Apparently, according to the quantum theory, light is not really like this plain sinusoidal wave, like we have on the, let's say, on the surface of the water. It actually comes in packets. And it's difficult for me to explain right now what packet is, but maybe you can imagine it as high frequency and then low frequency and high frequency again and low, maybe. I mean, you just have to have some kind of a model what is a packet. So you can view the packet, for instance, like this one. And only these guys are actually carrying the real energy, nothing in between. Now, this packet is called photon. So there is a concept called photons. And this is the most elementary, so to speak, part of light which still is the light. Remember what molecule is? The molecule is the smallest piece of matter which still carries the chemical characteristics of this particular matter, like molecule of water or molecule of nitrogen or something. Again, the photon is actually the smallest unit. We can measure the energy which is carried by the light. Well, anyway, it seems to be representing certain other view to the light, the view to the light as the number of certain particles which are flying. Now, it's still a wave, but it behaves like a particle. Let's put it this way, because if you have something like this pulsation or, I don't know, maybe it's a pulse or whatever. Anyway, packets. Each packet, while being still a wave, might be viewed as a particle. And this is the contemporary view towards light. The energy can be measured for one particular photon, of particular frequency, obviously, etc. So this is the smallest part of the light. There is no smaller part. Let's put it this way, because, again, if you kind of try to think about the waves in the water, you can basically divide it as much as possible. It will still be a wave. But here, we have this smallest part, like in matter, we have a molecule. Now, in light, we have this photon, which is the smallest part, a smallest particle. You can say it's like, it behaves like a particle. Let's put it this way. It's still a wave, but it behaves like a particle. And basically, the number of photons is the characteristic of the brightness of the light. Now, the frequency each photon is oscillating is the frequency of light. So these two characteristics are expressed in a slightly different way through the photons. Now, again, I will not go into any details of the quantum mechanics, but I will still employ a regular wave-like properties of the light, which is amplitude and frequency. And they basically define the light. Maybe sometimes I will do something related to characteristics of the photons, but that's not right now part of this particular lecture. So, again, this lecture, I wanted to introduce you to a concept of light. And I don't know if I succeeded, but I wanted to impress you with how difficult it is to really go into the very root of what actual light is. It was a very painful kind of process of development of this physics of light. And certain mathematical calculations, which are related very important and very involved mathematical calculations like Maxwell's equations. So anyway, and then theory of relativity actually also related to light, which says the speed of light is the maximum speed possible to achieve. So all these are components of contemporary view on what is actually light. And again, my purpose was to basically convince you that light as a source of energy as a carrier of the energy is a very sophisticated object of research and development and science. But there is something which we call contemporary level. So that's it. I would suggest you to read the notes for this lecture because they might contain something which I have missed. And if you're interested, actually, there is a video in the notes for this lecture. There is a video of how radiometer is working. So you put it in light and it starts spinning and you put it back into the darkness, it stops. So that's a very interesting experiment just to show you how energy is carried by light. All right, that's it for today. Thank you very much and good luck.