 Now the next thing is write down optical isomerism. We can't do much today, we don't have that much time but we'll start see the basics like what is optical activity and optical isomerism. Heading right down optical isomerism. Optical isomerism is actually the behavior of a compound, the behavior of a compound of a compound towards towards the plain polarized light. It is called PPL in short, plain polarized light. So all those compounds, compounds having, having similar physical and physical and chemical properties differs only in the behavior, behavior towards plain polarized light called optical isomers. This phenomenon is, is optical isomerism. Done? Copy? Yeah, guys. Okay. So what happens in this? Suppose you have a compound given and you need to find out the optical behavior of this compound. Okay. Any organic compound we have optical behavior means optical activity, you need to find out what you will do. You will take plain polarized light, plain polarized light are the light which is along the same plane like this and you allow this plain polarized light to pass through this organic compound. The light which comes out, if it rotates in any direction, either clockwise or anti-clockwise, if there's rotation, no matter in what direction, if we observe this rotation, then the compound is said to be optically active compound. Yes, I'll repeat. First of all, what is PPL and what happens in this? We'll discuss all these things, but this is the, you know, this is the working we need to do. Is it clear the working? Yes, suppose I'll give you the plain polarized light, then what you need to do to find out whether the compound is optically active or not? With this PPL, you know that, correct? Understood? So obviously, this method is not going to help because it is a laboratory method we have. Right? We perform all these experiments in a lab, right? So examine, you won't get, this concept won't help you finding what compound, given compound is optically active or not. So you must have some alternate way, which is more important actually. So what is the alternate way? We'll discuss that obviously in the next class, not today, but today we'll understand what is this PPL and what happens over here, right? See, first of all, the, you know, the whole idea is based on whether the plain polarized light rotates or not. Correct? Yes or no? Right? If there is no rotation, then the compound is said to be optically inactive. Okay? Why we need plain polarized light? Because if you take a light source, this light source radiates lights in all direction, isn't it? Any light source we have, it radiates lights in all direction. See, if we allow this light directly to pass through this organic compound, right? Since it is there in all direction, then how do we get to know that the light has been rotated or not? Are you getting me? What I'm trying to tell you? Since it is plain polarized, since it is polarized along a single plane. So if any rotation we observe, if we find that this light is not along the plane it was earlier, we can say there is some rotation in the light and hence the compound is optically active. But when the light is in all direction, then we cannot say this, whether the rotation has been placed or not, right? Yes, the concept you got it? Yes. So that is why we are using PPL here, plain polarized light. In order to find out that yes, the rotation has been done and hence the compound is optically active or not. Now what is PPL then? And how do we get it? So actually what happens? This entire thing is done in a device called polarimeter. Need may I have a question? Like the experiment in which device it takes place? The device name is polarimeter. Polarimeter, a device that just may we have all the arrangement, right? And there's a place in which we can, you know, insert the compound which we need to find out and then we can allow the light to pass through from one end. And in that polarimeter, we have a prism, this is called Nicole prism. So actually what happens, I'll tell you what happens within this polarimeter. We have an inlet where the light will strike, okay? So suppose it's the light source we have, which obviously radiates lights in all direction, but this won't help us because we need to find out whether there is any rotation or not. So we need to convert this into along one single plane. So for this purpose, this is the source of light. So in polarimeter, there's a Nicole prism placed between the, you know, object organic compound and the light source. Before going into going to the organic compound, it has to pass through a Nicole prism, right? This is the Nicole prism. Now in this Nicole prism, what happens? We have ultrafine slits, Nicole prism. It contains ultrafine slits, right? So what happens in this? You must have a, a, a, a, like something like this. Suppose it's the window we have. And in this window, there are, you know, metal bar is placed like this. We use to have this, isn't it? Yes. Have you seen this kind of window? Yeah? Jail, right? Jail? Why you got this example? Yes. Jail kind of thing. Correct. So similar thing we have here in Nicole prism also, but that is very fine, ultrafines slit we are calling it as. So when the light strikes at this Nicole prism, all those lights which are parallel to the slits passes through. Other lights will collide and reflect back. Correct? Understood? Right? So for example, suppose we have the slit, like this, we have the slit here in this Nicole prism. Just to make you understand, I'm just drawing it thicker like this. Suppose we have a Nicole prism. This is the slit we have. So do light ray I am considering here. One is suppose this one, which is parallel to the slit that we have in the prism. And other one is suppose, which is not parallel any direction you can take. I am taking in this direction. So this light will collide here and reflect back the horizontal one. But only those lights, which are parallel to the slit like this, the we can say this is the purple one will come out like this. And finally, on the other side of the Nicole prism, we get light, which is parallel to each other. Like this, isn't it? Did you understand? So in this, what happens, all those lights which are parallel to this slit, that will only, you know, passes through the Nicole prism. So obviously, on the other side, we get what? We get the light, which are polarized along a plane, actually. So this we call it as PPL. Is it clear? Yes or no? You can type in. Clear? Now this light, which is plain polarized, we allow this to pass through the organic compound, which we need to find out whether it is optically active or not. So this is the organic compound passes through this. And the other side, if we observe some deflection in the light, then it is said to be optically active. Otherwise, it is optically inactive. This happens under in a device called polarimeter. I hope this is clear to all of you. Yes, you can type in why, if you understood, and if you do not. If you want me to repeat, please let me know. Respond quickly, guys, all of you. Right. So this is one thing. Obviously, this method is, you know, it's a concept that you should know. In the exam, if they ask you, tell me whether this compound is optically active or not. Obviously, this method won't help you because you don't have polarimeter, you don't have all these things. You cannot perform the experiment in the examination hall. So we have the alternate way by which we can find out whether the given compound is optically active or not. But few experimental facts we have here that you should know. What is that? First thing, if the light rotates clockwise, if the light rotates clockwise, the compound is said to be dextrorotatory. Dextrorotatory. In short, we also write it as D or simply positive sign. Positive sign means dextrorotatory clockwise rotation. If the light rotates anti-clockwise, anti-clockwise, it is levorotatory, levorotatory, L, L or negative. So this D and L is the experimental thing. We cannot find out this theoretically. Suppose if you ask me, this compound CS3CHOHD, this compound, whether it is dextro or levo, I can't answer this. To answer this, whether it is dextro or levo, we have to perform this experiment. Theoretically, we can answer whether this compound is optically active or not. But this we can't answer. This is the experiment. If you want to find out dextro or levo, you have only two options. One is take the help of the book in which this data is given. Other one is take this object, take this molecule, go to the lab, perform the experiment, look at this, what deviation you have, clockwise or anti-clockwise, then only you can say this. So dextro and levo, in general, they won't ask in the exam. If they ask, there will be some information so that we can find out what is happening actually. But theoretically, we cannot find out whether the given compound is dextro rotatory or levo rotatory. Is it clear to all of you? Yes. Yeah, done. So this is one part of it. Like the theoretical thing, the experiment, what we'll do in the lab. This is one part of it. Like I said, we must have some theoretical way by which we can find out the optical behavior of a compound, whether it is optically active or inactive. Theoretical way we have. But before moving into the theoretical way, we must understand some terms. Sometimes we must understand, let it be. I can't start this now, otherwise I have to again start it from here. See, guys, fine. The terms we have just right down the heading, we'll start with next class, chiral center. What is chiral center? And what all other terms we have, we'll start from this next class. We don't have time, otherwise I have to repeat again from here. So we won't proceed from this now. You can copy this down, write down this heading and just let it be. We'll start from here in the next class. So next class we'll finish this chapter. I'll share the assignment also that you can solve. And then we'll start the last chapter of grade 11. Obviously, P block, we have P block you must go through on your own because I'll finish in one class. Okay, quickly we'll go. Okay, so we'll see this next class chiral center and all. I will share the assignment. You can attempt that. Okay, geometrical isonism. Yeah. Thank you so much, guys. Take care. Bye-bye.