 So, the discussion today we are going to have why an optically active or chiral molecule rotate the plane porous light. That is the thing we want to understand. We know all of us that it happens and some of you have done this experiment over and over in your lifetime, take a sucrose solution, pass through the plane porous light and you see it actually changes the plane of the rotation. That is fine. The question is why? So, all these things actually begins with something called light matter interaction. So, before we go there, we are going to talk about light matter interaction and how an optical absorbance happens. So, what do we know by light matter interaction? So, this particular figure you have done a lot of time that there is an electron in the ground state, Gs stands for ground state, which has an energy of say E0. And then I shine light on that and this electron go to a higher excited state E1 and generally how we draw that there is a vacant electronic state and the electron basically go over there and that gives you a vacant ground state. Now over here, I am going to think a little bit, does this concept of a vacant orbiter does really valid? So, what do I mean by vacant orbiter? So, if I ask any one of you that what is the definition of an orbiter? So, what do you say? So, let me ask this question. So, Harjeet, what do you say that what is a definition of orbiter according to you? Orbital basically is an electron we can say. Okay, a little bit more if you want to discuss, electron and orbiter are not the same thing, right? Orbital is a space basically electron occupied. Yes, that is very good. You want to add anything? Yes, sir. So, where the probability of finding an electron is maximum? Okay, so both of you are giving the right answer. So, electron is there, it is moving in a finite space and you find out what is the probability of finding that electron and wherever I am going to find it out that I will say it is my orbiter. So, now if you don't even have an electron, can you have an orbiter? No, unless you have an electron you have no probability, the probability of finding an electron is going to be zero if there is no electron and that means there is no orbiter present. So, why do we still say vacant orbiter all the time? That is because it is very easy for us to see or visualize that the electron is sitting over here. Once it gets excited by this incoming light, the electron goes in this particular place and we achieve the excited state. But this is actually not how the electronic transition happens. Electron transition happens a little bit differently. We are going to achieve the same starting and the same final state but the actual interaction happens in a different way. So, let's take a look how this interaction actually happens. So, first of all, why this ground state system of the electron even interacts with this light? That is the first question. So, whenever we say there is a light it is nothing but electromagnetic radiation. And this electromagnetic radiation is going to interact with this particular electron and this electron belongs to a system or matter and this electromagnetic radiation in general I am telling light but it can be any kind of electromagnetic radiation starting from radio wave to gamma over. All this electronic transition or any particular transition is actually follow the same basic theory. So, that is what we are going to understand and because there is going to be an interaction between this electromagnetic radiation or light with this electron or matter that is why we say it is a light matter interaction. Now, the question is why this electromagnetic radiation interacts with the electron? So, for that again we have to go back to our favorite equation in quantum chemistry that is known as the Schrodinger equation. The equation is E psi equal to H psi. What is H? Again Hamiltonian. What is psi wave function? And what is E? It is the energy of that particular electron. The electron is moving on its own way. It is settling in a molecule with respect to the Hamiltonian or the surrounding which is given by this H. And we can say at that particular condition the ground state has energy of E0. That means that is the Hamiltonian. The wave function is defined by psi and I am writing it Gs so that we understand it is the ground state of that function and the energy is easy. That is how the system is stabilized. Now what happens? Here comes our electromagnetic radiation. And this electromagnetic radiation is going to have an electrical field. It is going to have a magnetic field. That is why it is known as the electromagnetic radiation. And this electrical and magnetic field is going to change the overall scenario. Because previously say the electron is moving around the nuclei so it is having it is just facing the interaction with the nuclei. It is facing the interaction with the other electrons present. Now once this electromagnetic wave comes in it changes the Hamiltonian. It changes the surrounding. So now I cannot define the system with the same age. It has to be a different Hamiltonian H dash. If H dash is changed my psi function will not be same. It is going to be different. And that is going to be the psi star Es. Stars generally put to define that it is an excited state. And Es I am still writing to define that it is actually an excited system. And obviously now this equation is going to have a different energy. So that is going to be the E1 psi star Es. So over here what is actually happening that when this electromagnetic radiation sets in it changes the Hamiltonian. It changes the Hamiltonian and this particular phenomena is known as perturbation. That thing most of you probably have heard or learned in your quantum chemistry class. This is known as the perturbation in very like simpler way. And due to this perturbation what is the effect? Your psi changes your E changes. And how can I define the change in this shine? So previously again I am drawing this figure again and again. E0 is your ground state. And once this H nu comes it changes the overall scenario. Now your system is such that it is actually stabilizing in a totally different system in such a way that what it is happening that it is actually giving a totally different energy state. And what is this particular change physically means? Physically it means that previously you have an electronic environment that is how the electron distribution was. That means that is how the probability was to finding that orbital. So that is psi gs. Once it is interact with this H nu the electronic distribution changes. And that is a totally different electronic distribution. Yes. And over there what is happening? The electron distribution actually changes. And that is why I go to a different state. So over here the same state, the same electron after interacting with the electromagnetic wave changes its orientation in the three dimensional system. And that is why the probability changes. That is why we say it is actually going to have a totally different orbital. And what actually is happening there? It is a change in the overall electronic distribution. The electron is not moving to a vacant orbital. What is happening? The electron is redistributing itself such a wave that it is attaining a totally new system. And that is what is happening during an optical absorbance. Through the interaction of the electromagnetic wave my electronic distribution changes from one to the other. And it changes because this electronic electronic interaction is giving a perturbation in the Hamiltonian. So anybody has any question because that is very important. That is how an electronic transition and electronic transition happens. No matter what it is an optical absorbance, vibrational spectroscopy, rotational spectroscopy the basic system remains the same. It is actually change in the distribution. If it is actually an absorption it is happening to the change the psi is depending the electronic state. If it is a vibrational state it is actually IR. If it is a rotational state it is happening in the microwave level a rotational state change. But the basic thing is same. Through this electromagnetic radiation be it in the radio wave to the gamma ray it is actually changing the Hamiltonian and it is redistributing the electronic distribution. Sir, if the distribution changes so does it change the shape of the orbital? Yes. So does the ground state and excited state orbital look same? No sir, no sir. So that is what is actually happening. It's shape changes, it's symmetry changes because it is redistributing itself. Ok. Yes sir. Ok. So now we go back again and look into the electronic distribution a little bit in more details. So when we say it is an electromagnetic wave it is actually having two different segments. One is the electrical segment in electrical field and the other one is a magnetic field and from the childhood we know this electrical field and magnetic field are perpendicular to each other. Now say I am talking about optical absorbance. So can anyone guess which of this field electrical or magnetic which actually triggers the change in the electronic distribution? It is the electrical field or magnetic field or both of them are actually present. Anyone? Which of them triggers the optical absorbance or change in the electronic distribution? Electrical field or magnetic field? Anyone? It is the magnetic field I guess. Ok. Good magnetic field. So actually it is opposite. The electrical field actually triggers that. Why? Because although we learn this, it is true. There is an electrical field and there is a magnetic field perpendicular to that. So let me draw that together. That will be a better idea. And there is a magnetic field there. But it never says that electrical and magnetic field they have the same amplitude or same intensity. They are perpendicular to each other. The angle between them is 90 degrees. That is true. And I draw that in a kind of three-dimensional orientation. However, the electrical field and magnetic field intensity or amplitude is not same. The electrical field amplitude is way too strong. Ok. And magnetic field amplitude is weaker. And now you can think about that I have an electron moving in. When an electron moves what are the different fields it creates? It creates an electron field. It creates a magnetic field perpendicular to that electric field. So it has both electronic and magnetic field. And my electromagnetic radiation also have both this field. But which of the interaction is going to affect more? It is the electrical field versus electrical field because the amplitude is much stronger. So for an example, if I want to draw the electrical field if the electrical field is that strong say it is the electrical field and say I am drawing a magnetic field it is going to be very weaker. So that is what is going to affect the overall interaction. Because you are going to change through some perturbation. The perturbation will be much more stronger when you change it through the electrical field. Yes, Lukash. Sir, electric field creates a charge. If it is charged, the electric field will create. If there is no charge in the electromagnetic field then there is no charge in it. Very good question. For asking this that is one of the most ultimate questions we have. Where does this electrical field from an electromagnetic radiation on light is bringing the electrical charge? So probably it is better to answer with the respect of the photons. All of you know the stump photon, right? So these photons are such quantum mechanical I shouldn't say particle but quantum mechanical system that it actually moves around with an electrical and magnetic field. Now if you want to find out what is the actual cause of this electrical and magnetic field it is better to understand through the respect of the electron. So it is a very good question. So let me take a short route on the different way and try to answer where this electrical field and magnetic field comes from. Sir, it means that the photon is already a charged particle? It is not charged. It has an electrical field. You don't always need to be charged for creating an electrical field. You can have an electrical field but it doesn't always mean that you have to have a charged system. The question is where this electrical field is coming from. Now say I have an electron and if I ask any one of you what is the charge of an electron and each of you will say there is one unit of negative charge and obviously that you can explain that with respect to ESU or EMU unit that you can do and that will come with a particular number. I am not over it too much about the actual value. My main concern is what is the charge? We say the charge is a negative minus one unit of charge and if I say does this electron also has any spin? What will be the answer? Does the electron have any spin? What is the spin of an electron? It is half. Plus or minus half we can define when we actually put a magnetic field around it and with respect to that if it orients towards or against it with respect to that I can say it is a plus or minus of directionality wise but in reality if it is just a spin it has a half spin. Where does this minus one charge or spin half comes from the electron and that is a question it actually take the sleep of many scientists especially in the early 1900 because around 1896 we find out we should say we discovered electron that there is something called electron and by JJ Thompson and he found out that it has a particular charge and we defined it as minus one charge. Later on when other physicist comes on around that time they found proton, they found neutron and they always found out that each of them has a particular charge, has a particular spin. Now the question is what is the origin of this electronic spin, electronic charge or the neutron or the proton? What is actually coming from? These are actually coming from. So then people actually have different theories for that. This is happening for that reason or those reasons and later on people figure it out that whenever we study still cluster or even early bachelors that we said that electron, proton, neutron that is the latest or the smallest thing you can find in an atom because those are the subatomic particles you cannot break down further. But later on people find out no it is not probably true, you can break them further down. Obviously not that straight forward but it can be break down and you find out that this electron, protons and neutrons they are made out of a very similar looking unit and that is known as quark. And there are six different quarks present but I should say we have found so far and these six different quarks can combine in different combinations. It is like a combination and the most stable three combinations give you electron, protons and neutrons and each of these quarks has a particular spin has a particular charge and when they combine together they give you this particular charge and this particular spin. So this electronic spin, the electronic charge it is actually a very natural phenomena coming to it. So how should we think about it? For an example if I say I have a mass of that X kilogram. So if you say why I have this particular mass I have this particular different system in my body, blood, then bones, other cellular system and all those things combine together I have this mass. So then the question comes why do you have the mass in bones or cells or in blood? You go down and down and down and at the end you find you have some atoms and each of the atom has some mass. So it is a natural phenomenon. It has this mass and that is why you have this mass. Similarly the quarks combine in such a way that it actually comes out with its original mass, charge, spin and all the basic properties and that actually represented in its combined form in the form of electron, protons, neutrons and these are not the only ones you can have neutrinos, you can have neons and all those things and probably all of you heard this story about this experiment in the CERN where they actually created a system where they actually collided these particles, these very simple particles to each other and try to find out when they collide if they are going to their original forms of quarks or something like that and then the fire finds out that yes probably you can see a few glimpses of that and from there we actually confirmed from the theory and the experiment that these things actually exist and the presence of this quark are the principal reason why do you have a charge and spin for an electron. Similarly when you talk about a photon, that photon is also made out of this particular original quarks at a particular combination which actually doesn't have any overall charge or overall spin but the quarks are there so that means in their heart they have some electrical field and that electrical field is the main reason for this electromagnetic regulation. Does it give you a little bit of idea Lokesh why it is happening? Yes sir Thank you sir. Sir basically photons have energy but it has no mass sir how is it possible sir? So it is actually combined with such particular quarks which actually mass is actually pretty low so sometimes if I cannot define we don't have any system to measure its mass so we say that it is almost close to zero mass for an example when we compare between electron and proton can we measure a mass of an electron? Yes we can measure how much difference between the mass of an electron and proton is such that an electron is almost close to 1836 times lower than that but if I say that electrons say it is one millionth time lower mass compared to proton so compared to proton what it would look like that it is almost a mass less quantity so mass is such that it doesn't really affect any phenomena combined to mass so that means this system doesn't have what kind of parameter I should say for an example a physical momentum so for an example if a light hits you you don't feel the light hitting you but you can get the heat or some sensation over there but a light cannot just push you around because it has a so low mass almost close to zero but in a similar system a photon with a photon can you shift a very low mass system with a photon? Yes you can so where this particular momentum is coming it is coming there because it have to compare with such a system with a very low mass so although we say very in an overview that it is a mass less system but does it mean this mass is actually zero unit or it is a so much of a low mass that I cannot actually measure with our current scale so that is the question so I don't want to go into further in the discussion of this particle physics so for that you have to take a class of particle physics or look into some books or some literatures on particle physics those are very interesting actually and put as a question like where everything actually starts from and it is coming from Big Bang and all those things you can go and look into that but it is a little bit out of the scope of this class so I am actually going back to our original discussion