 Hi, I'm Zor. Welcome to Ungezer education. I would like to continue talking about light From the perspective of energy, which it carries So this is the second lecture in this category The first one I was trying to introduce you to a concept of light Among other related to radiation, etc. concepts. Now I will talk about light from how we Understand the nature of light from historical perspective. So this will be lecture about the very first attempt to understand what actual light is Now this lecture is part of the course presented On Ungezer.com. The name of the course is physics 14 14s Yes, there is a prerequisite course, which is math for teens on the same website. All courses are free. There are no ads so you can freely use them and If you have found this lecture somewhere else like on the YouTube for instance searching it I would still suggest you to go to the website and approach the whole thing from the whole course Perspective because this lecture is part of the course Which means there is some logical connection between this lecture and the previous one and the following one, etc And again as I was saying there is a prerequisite course, which is also on the website. So go to the website Ungezer.com Okay, so Today we will talk about the first attempt to understand what light actually is Well, let me just make a little philosophical X curse How do we build our understanding of anything which is in in the nature around us? Well, we built the model So we are assuming that things are such and such and then we're trying to make some logical Relations relations between this model and some other models. We are trying to analyze whether this particular model explains qualitatively and quantitatively certain realities which we observe and if it does well, it means that the model kind of works and We are using this model to predict even certain qualities of whatever the subject of our research is and Again, if later on we will realize that yes these qualities which we have predicted Really do exist as well. Well, that's the confirmation that the model is right But again, we're only talking about the model How it is in reality? It's it's a big question We don't really know but our models might represent quite well in some cases What the reality is for example Newtonian mechanics? Represented quite well, whatever we are observing as far as motion around us. Well, when we were approaching certain Boundaries we have found out that not not exactly Things not exactly the same as predicted by Newtonian mechanics The relativistic mechanics came along then when we went to microscopic Structure of the matter. We also have found out that our model is licking certain things Which we observe and we needed some explanation. We built new models So we are talking about modeling of what actual light is in the first one the model which Was developed by Newton. Well develop. Maybe it's a strong Word he just Suggested that maybe the light is such and such and then he was trying to explain whether certain qualities of light can be Basically related to whatever the model he has built So what's his model? Well, the model is that the light Represents it is basically consists of very small particles. So if this is the source of light So these small particles are Caring basically the light and they are coming into human eye for instance and They hit the the back of the eye the retina whatever. I don't know if they understood what retina actually is at the time Anyway, they hit the eye Which agitates nerves and then it goes to the brain and we feel it We sense it as as the light now It was a natural kind of a way of thinking about light at the time. Don't forget It was before we knew anything about electricity for instance or The the electric bulb or whatever so So that was basically his hypothesis now all these tiny Pogicles of light he called them corpuscles corpuscles and the whole theory was called corpuscular corpuscular theory of light corpuscular based on corpuscles All right, so corpuscular Theory of light is basically a very mechanical view onto light. So if every particular Particles, I'll rather use particles instead of corpuscles So every particular Particle of light it has certain mass it has speed It has size It has kinetic energy. I mean that was all very as I was saying Mechanistic type of type of approach which Newton has suggested All right, so now he has to explain with this model Different things which we are observing about the light. So our sense of light. He did explain great now How can We explain the different colors for instance Well the explanation which Newton has suggested was that these particles have different sizes now every size Agitates our Cells in in the written in different ways and that's why we perceive it as different colors Well, that's a plausible explanation All right, so the colors are explained also There is a very simple explanation of Reflection for instance if you have the beam of light here and this is the mirror it reflects So basically the light particle behaves like elastic little ball. So it's elastic Reflection nothing wrong with that seems to be working fine Okay now There is another very important aspect which Newton observed That's a refraction. So if you have for instance air and water And there is a light here Then It changes direction and yes, that was obviously observed many times How to explain this Well the suggestion which Newton actually made was that different medium in this case air and water for instance They are basically slowing down these Particles of light in different ways So he suggested that the change of the speed of of these Corpuscles these particles of light Is the reason why the angle is changing when this particular event occurs So that's a refraction If with a reflection the model of elastic ball really works quite well With the refraction It's not exactly as clear at least to me That different speed can explain the change of the angle, but that was his explanation All right What else well Another thing which he basically observed was that the white color is the combination of different Elementary colors like red green blue, etc. Now the way how to observe it was to To Send the light through a prism right so we know if this is the prism And if this is the white light Then it goes to different colors Something with blue Something with red green Etc So how can that be explained well this is basically a a refraction So the the light goes in different ways different Particles since they have different masses different sizes They are Refracted differently at on at each Boundary between the air and the and the glass for instance So that was his explanation which might work actually Now I would like to devote certain Amount of time in this lecture to explain why this model is really insufficient And there are certain known at that time or almost at that time Things about light which the theory cannot really explain Which basically makes this theory Kind of insufficient All right, so right now I will talk about certain things which we observe with the light which just cannot be explained using this particular uh Corpuscular theory of light. Okay, here they are I will mention three of them and there are others actually Now the first thing is It's called Defraction if you have a very small Hole and you put the light Then what you will see on this screen Well, if you will look from this side, this is your screen And this is how light goes What's interesting is you will see concentric circles with changing from Bright to dark, bright to dark, bright to dark, etc. It's called defraction well Obviously corpuscular theory cannot explain this particular Case of property of light So the light somehow By just going through this particular hole. It's a small hole supposed to be then It goes somehow in different ways And again the dark Are areas are changing Uh with the light one So this is the view from here This is the light And this is how the hole experiment Now this is called defraction And it looks like something is happening on the H of this hole and the smaller the hole The more Visible these concentric circles are Now in the notes for this lecture and by the way, I forgot to mention it But every lecture on unizor.com has textual notes and certain references. So Among the Notes the notes for this particular lecture Contains a picture how this thing really Uh looks like when there is a red light And the red light actually when it's going through this small hole produces this picture of bright red and and dark concentric circles. So this cannot be explained by the Newtonian corpuscular theory And that was one of the reasons for well developing some other things which we will talk about in next lecture Okay, the next experiment which again defies the corpuscular theory Is interference now interference is if you have two holes And you have light here What's interesting is that we will have certain waves again dark and light Areas on the screen which you have here Again in my notes, I have a picture how it looks in reality. I'm just explaining this but You will see there. Obviously if I had certain equipment here, I would be able to present it to you I don't have any equipment. It's purely theoretical course, but picture always Works, so I present the picture over there And I'm sure you can find Many many different examples on on youtube etc. How these Interference pictures are developed So this is another Property of light which cannot be explained By corpuscular theory. I mean if the corpuscular theory was right then all you would see here Was on the screen One bright spot and another bright spot and everything else would be dark around it, right? But in reality It's very very complicated Interference pictures so to speak Now the next and the last Experiment is the following Let's say you have a straight Ray of light Now let's position here a crystal of Thermal line It's some kind of a crystal. It's green If a green color, etc. Well the light goes there It becomes green actually, but then it goes no problems Now let's put another Thermal and thermal line crystal on on on its way. So we have two of them Well, if you're lucky, we will see the light goes through this again But now let's turn this So we will turn it around this axis which coincides with the With the light Well, we will see that this particular and this is a screen So we will see that light which is projected onto this screen Will actually change from from relatively bright To darker and darker and at some at some moment it disappears again And then as we continue rotating It will start getting brighter and brighter again. So the one rotation actually will result in changing from Bright to dark and then to bright again How can that be explained? In corpuscular theory, it's completely impossible The analogy Of this I can say the following If you have a slot And then you have a rope here, which you are rotating in all the different ways But it after it goes to a slot It will go only along the slot up and down up and down. So your circular rotation If there is no friction obviously After it goes through the slot all these circular rotations Turn to a flat Which corresponds to a slot, right? So it's called polarization of light. So it looks like this thing has a slot or somehow all the whatever Different Oscillation of light now i'm talking about oscillation of light, which means It's an introduction to the wave theory, which will be next lecture. So all the different movements within these Within this ray of light are somehow Going through something which is similar to this slot and that's why only these vertical, let's say Movements of these particles Are going through But then if this has a slot in exactly the same position it goes through, but if it will change it 90 degree obviously It will not go through a different slot right And there is no light here So it looks like we are talking about waves And that's exactly what can be an explanation of the Diffraction, which I was talking before Interference and in this case Polarization of light now this wave theory would be the subject of my next lecture And meanwhile, I just wanted to show certain Examples of certain properties of light which Most of I think they were known at the time of newton or at least at the end his career And and people realize that crepuscular theory Is not exactly a good model of what's happening With the light and again i'm talking about modeling Nobody's talking about how it is in reality. Nobody knows this But we are building models newton built his model of light as a crepuscular theory And it turns out to be insufficient So the next lecture will be about the next step And there will be the next step and the next step, etc so I just suggested to go to the unizord.com Read the explanation Notes actually which accompany this particular lecture. So you go to physics for teens Energy and there is an energy of light and this is a lecture about light light as corpuscles So thank you very much and good luck