 So, optics is the first chapter of your NCRT textbook 2, fine. So what is optics, do you have an idea? It is a study of light. You study about the light and its phenomena, right? Why it is so important to study the light? What makes light so important? Because we can see using these wavelengths, right? So human eyes are blessed with an ability to see the object using certain band of frequencies, fine. These are visible spectrum, right? Now seeing is very important in physics because until you see anything, you will not be able to observe anything, right? And observation is an integral part of physics. In fact, the first chapter in class 11 when actual physics starts is about units and measurements, right? And when you measure anything, seeing is very important part, right? So it is important for us to understand that how seeing can get affected, right? So for example, if I see something using lens, it can get magnified, right? And there are several instruments like for example, telescope and microscope, which can enhance your, you know, visibility. So you will be able to observe something which you are not able to observe with only your eyes, right? So there are such instrument which will help you to observe things much beyond human capabilities, right? So if you want to build any such instrument, you should have a basic understanding of how light behaves, right? Whether it bends, how much it bends and all that, right? So ultimately what happens is that light from the object should reach the observer. That is the bottom line. If light from the object reaches the observer, then the image of that object gets formed. So anything which you see is a result of a basic process. Light from the object should reach the eyes, right? And that light from the object can come from the third source. For example, you know, these lights, when they strike the board, then they come to you. So you are able to see the board. If these lights are not there, lights from the object will not reach your eyes, fine? And at times, an object will have its own source. For example, your phone, television, they have their own source. They do not need any light to hit on it for you to see them, fine? But the bottom line remains the same. That light from the object should reach the observer for an image to get created. In fact, anything which you see is image only. When you see an object, an inverted image gets created in your retina and your brain, you know, corrects it and make it erect, fine? So you see the image of an object. You cannot, you know, directly see the object, fine? So that is what this, this is why optics is very important, right? Now when you see an object, for example, light ray is coming from this side. So where I will see the object, I will see that side, right? For example, if I draw a light ray like this. So I will feel that the object is along this line. Will I be able to locate where the object is? I may not, but I know the direction. I know from this direction light is coming. So object must be along this direction, right? But I am not able to locate where it is. In order to locate where the object is or where, from where the light is coming from, you need what? You need at least two rays. So if one ray is coming like this also, then you will feel that object must be along this line also, right? And there is only one point which is common, fine? So you need at least two lines to locate the image, right, getting it? So two lines, whenever you see, like whenever you see a ray diagram, you generally have two light rays and intersection of that will create the image, fine? So these are the few basic things before we even start the chapter, we should have an understanding of all this, right? So in order to create an image, there should be an object. There should be light coming from the object and the light should reach the observer, right? And between the object and observer, there can be mirror, there can be lens, there can be spherical surface, there can be so many other things, fine? So these lenses, mirror, prism, whatever you put between object and observer, they will deviate the path of the ray and when they deviate the path, something else will happen, fine? This is what this chapter is all about, fine? So entire optics can be divided broadly into ray or wave optics. Now why this happens? The thing is the study of light has been done centuries before, right? But then Newton was the first person to study it in a more structured way and he came up with lot of formulas and equations. Now the thing is Newton that point in time was also developing something called mechanics which you all know, right? And when he was developing mechanics, he was fascinated with the idea that everything behaves like a ray. Now everything behaves like a particle. So he was fascinated with the idea that everything behaves like a particle, right? So he thought that light is also a stream of particle which is travelling in a straight line, okay? And they are massless and no force can be applied on them and whenever they hit some obstruction, elastic collision will happen, right? So taking that notion, Newton has developed a set of theories which come under ray optics, fine? But at during the same time, there was another scientist, Huygens, who was developing this idea that light can be a wave also, fine? Now wave you might have learned in class 11, right? That longitudinal and transversive and all that, right? Similarly Huygens was of opinion that light is a wave. He was not very much influenced by Newton's idea. So he used to come up with some experiment which Newton always used to counter that this thing can be explained by ray optics also. So the idea of light being a wave was, you know, buried. But then there was this guy, Young's, who gave an experiment, Young's double slit experiment, which Newton could not explain using ray optics. So it was a very interesting experiment. You could actually see in YouTube also, lot of talks are there on that experiment. It's called Young's double slit experiment, okay? That cannot be explained by ray optics. So that point in onwards, people have recognized that, you know, light can behave like a wave also because that can be fully explained by taking light as a wave. But that doesn't take away the fact that light can be a ray also, fine? Now the thing is, if light is a ray as well as wave, then how do you define light? Now what I'm telling you, this is not a valid question itself because suppose there is a person like you yourself, depending on one situation, you may be very sad or depending on other situation, you may be very happy, right? So I can't define that you are a sad person or you are a happy person, right? Depending on scenario, you change your behavior. Similarly here, depending on what scenario light faces, it may behave like a ray and depending on the other scenario, it may behave like a wave, fine? So these are just behavior of ray, just like humans have multiple behaviors, light has, you know, two behaviors only, ray or wave, okay? So we are going to learn today about this behavior where light is taken as a ray, okay? And that is why it is called ray optics, fine? So first write down the name of the chapter. As we have already came to an understanding that in order to form an image, you need two rays, right? And there'll be mirror and lenses and all that which will deviate the light here and there, right? So geometry takes precedence here. Geometry role is enhanced here. Like you'll be using lot of geometry here to form the image, fine? So this same chapter is also called geometrical optics, fine? So do not worry about the fact that people start talking about it as a geometrical optics also, fine? We will today discuss this ray optics and do not feel that, you know, a lot of mathematics is coming, so do not, so expect it. Expect a lot of mathematics, a lot of, you know, trigonometry in this chapter. In fact, this chapter is more of mathematics chapter than physics, fine? So just go by that mindset when you study this chapter, okay? So this chapter is divided into three parts. I am broadly talking about it. One part talk about reflection, okay? Other part is about refraction. And third part, like always, is optical instruments. Like I said, like always, is that in class 12th, you will see that every chapter towards the end talks about instruments. Electrostatics has Vendigraph generator. Current electricity has, you know, that Wheatstone bridge, Mitter bridge and all that. Moving charge and magnetism will have cyclotron, fine? Similarly, ray optics has optical instrument. In optical instrument, you will learn about two basic instruments. One is microscope and other one is telescope, fine? Microscope can be of two types, simple and compound. Even telescope can be of multiple types, but then we will not go into multiple types of telescope in your syllabus, okay? Refraction, when we study, we will take care of plane mirror, we will have a spherical mirror. So before we take a plane mirror and a spherical mirror, we will also talk about laws of reflection. What are the laws? Reflection should follow, fine? And when we talk about refraction, refraction is slightly broader topic. Here we will have surfaces, lenses and the third part is prism. What is a surface? Surface is a boundary between two mediums, okay? So surface, we will take care of plane surface and then we will take care as spherical surface. They are in your syllabus thin lenses only, okay? So you have convex and concave. Like here, spherical mirrors, you can name here concave and convex, fine? So prism, you have just one experiment to find refractive index, okay, fine? So this is the entire chapter, there may be one or two more topics, but broadly this is what this chapter is all about, okay? This is the like the bigger chapter of two, that is ray optics and ray optics, fine? So this will take around two, two and a half classes, right? Four hours each to finish this, all right? So I will take only that much to finish from my side, but you have to work a lot more than that. You have to practice a lot more questions, okay? And the best part about optics chapter is guys, this. It is not connected to any chapter. You don't need to be master of mechanics, you don't need to be master of electrostatic, nothing. You don't need to know anything. This chapter itself is complete by its own, fine? Not connected to anything. So there should not be an excuse that I did not do this chapter properly and that chapter properly, nothing, nothing will work, fine? So you can study it from scratch and master it. You don't need to know anything else, all right? Another good part about this chapter is that the questions cannot be very difficult from this topic. Of course, if you want to make extremely difficult question, nobody can stop you, but usually I am talking about there cannot be very difficult question from this chapter, fine? So you will not see much of a difference between J mains level question, CT level question or advanced level question, fine? So if you do this chapter properly, you are guaranteed to get marks from this chapter.