 So, so far the things we have covered let me just point it out at the following that molecular structure and molecular properties are very much connected so they actually depend on each other so there is a close connection between them and one of such molecular properties that we can follow to understand what is the structure of the molecule what is the orientation of the molecule is chirality and we are mostly interested in chirality because this chirality can be used as the biosignature this sign of life because most of the important factors that actually considered as the building block for cellular life such as amino acids and proteins carbohydrates and nucleic acids they are all known to form chiral molecules especially when they form their polymeric structure and over there we have specially looked into the D and L amino acids and we have also looked into the functionality of the effect side by side implications of D and L amino acids on life so over there one of our goal is to how to follow chirality and the way we can easily follow chirality is by following something called CD spectroscopy but before we go to CD spectroscopy the last class we have learned what is a light matter interaction and what is happening in the molecule level during a simple optical absorbance so up to this part we have studied and today we like to go into a little bit more details what happens in a chiral molecule especially the phenomena we have all know optical rotation why does it have so let go into the details so what you have known so far that if we really want to observe a optical absorbance for a chiral molecule one thing we actually need is called linear polarized light now the question is why do we need a linear polarized light so before going there we have to go back a little bit and talk about the light as an electromagnetic radiation itself so when we talk about light we say okay it is a magnetic radiation and in the last class we know that it actually has an electric field and it also has a magnetic field sitting perpendicular to each other now when you look into that especially the electrical field we are more interested in because that is the most strong feature of this electromagnetic radiation it does contain both magnetic and electrical field but electrical field is much more intense so when we draw this particular line over here what do you mean by that line so if I want to bring it over here and draw this line again what we mean that this is the direction of this electrical field and it is not a static it is actually having more of a wave nature so if I look into this particular electrical field from the side what it will show me is this an electrical field so that is what we are going to see from the side so that is the direction of the electrical field which is actually going to wave nature so it has a maxima and a minima and it is going through a particular line now what happens if we look this particular electrical field from the front so what happens if we put our eyes over there and try to look into it what is actually coming to my mind so how that will look like so let us look into it with more details on that so now say of this particular wave I am just drawing on portion of it so over here this is the maximum say this is point a so at this point a if I start again looking from this side what I will see is actually a line like that and this point over there signifies this middle line so that is what I am going to see at point a now say I come to point b so what I am going to see so when we are looking at point b what we are actually going to see that this peak would be much more smaller because the intensity is going down now say we move forward go to point c at this point what we are going to see is nothing because there is no electrical field present because right now it is changing the direction now say I come to point d so over there what I am going to see now it is moving towards the opposite direction similarly if I want to point e which is the maxima of this I will be able to see that and this intensity at this intensity is going to be the same because of this web nature then if we move point f it is again going to shrink down a bit maybe going to point g it is going to be zero again and then say if you start moving forward you're going to repeat again and say what to point i that is going to show with the maxima again so altogether what I am going to see is that this is going to start from very high amplitude then it slowly goes to zero then goes to the opposite direction again slows down comes to zero again and moving forward so altogether what I can say if I combine all these things together because now when I am looking from the front I am not going to see them in different points what I am going to see they are actually going to fit on top of each other and what I am going to see is the following that this particular thing is actually going like this it is more of a like it is a blinking thing is going to go so that is why we try to write that in this particular way it double headed error which actually shows that it is actually blinking and is blinking means that it is actually following this particular web nature and this is how the electrical field is actually oriented now if I go further and write down this line like this and say I am covering that with a circular motion so the circle actually showing that a particular area of the light that is actually we are actually seen so over there so over here what we are going to see that this particular line is actually showing that if I see the electrical field up front I am going to see this is actually blinking along to this particular axis this is the electrical field how it is coming up now when a light actually comes up it is not like it is going to show electrical field one particular direction it is possible to have this electrical field in any possible direction all possible direction you can think about and this is something known as unpolarized light so you can think about what I am actually saying that this electrical field I was talking about if I see from the side it is not only happening in this particular direction it is happening in this direction in this direction so all possible direction I am not drawing any more otherwise it will be very easy so it is possible that the electric field the wave is coming in all possible direction and that is how the unpolarized light looks like and that is how we actually present them because that is the electrical field and electrical field is the strongest feature of this electric radiation so that is what we are representing on that so that is how a unpolarized light look like now if I take this same unpolarized light and now say that I am passing it through a particular crystal and coming into that what is that particular crystal that this crystal has a particular property that it can absorb all the possible electrical fields in all direction except one and what will happen when it passes through it is the following that once it passes through that it actually shows electrical field in only one direction so this is a unpolarized light passing through a particular crystal and at the end I am seeing the electrical field only showing up in one particular direction so this will be called as plane polarized light and this particular crystal is known as polarizer so what is the role of this particular polarizer I am not going into the detail physics of it but if I think about it is like a comb that we use to brush our hair so this is such a way oriented that when it sees that your hair is coming from all particular direction but when you comb through that it has to follow the the trend of the comb how it actually wants to orient your hair similarly the unpolarized light coming from all direction but this polarizer have some grating or have some space that the light can move through only through that particular direction and by that it actually ensures that the light is passing through only one particular and this is very easily controlled because polarizer has molecular planes to control so this is how the plane polarized light is actually coming up so again it is the electrical field it is coming through only one particular direction now what happens is the following if you take this plane polarized light and now passes it through a chiral sample so it is a chiral molecule in a sample very similar that you do optical spectroscopy but instead of any particular unpolarized light now you are using a polarized light and what you are expecting to see is the following that this light actually passes through but when we are detecting it what we detect is the following that the light should be along this plane what we actually started with but if it is chiral what it is going to do it is going to move the plane it is going to rotate it and this particular phenomena is known as optical rotation so where the plane polarized light was and where it is now there is a difference between them and this difference can be given by an angle say it is 5 so that is it is known as the optical rotation now the question is we all know so far all these facts but the question is why it is happening and that we are going to look into details so so far we have discussed that this chiral molecule can show this optical rotation and if you have two different enantiomers so say this is enantiomer A and there is another one enantiomer B they are going to show similar rotation but exactly opposite direction it is not exactly circular so over there previously the plane polarized light was like this but once it is rotated one is this way the other will be exactly the other way if all the other factors remains same that means the concentration temperature everything remains same this angle is going to be similar magnitude but opposite in direction so say one of them I am showing it as positive one of them showing it as negative so that is what we also know the question is that why it is happening why a chiral molecule can induce optical rotation that is a lies in the simple fact that we have already learned only a chiral environment can detect chiral molecule so that means this plane polarized light have to have something chiral in nature which is actually detected by this chiral molecule what is present in this plane polarized light that can be chiral so let us take a little bit so what has been found that when we are talking about a plane polarized light that can be seen as a superposition of two different circularly polarized light but we mean by circularly polarized light so so far I was talking in such a way that the electrical field direction is actually only blinking in this particular position over here at a particular plane but in reality what is found that it is actually not a simple plane polarized light but it can be given as a circularly polarized light so what do I mean by circularly polarized light circularly polarized light means that there is electrical field like this but instead of staying in the same plane they can actually rotate and when they rotate they can rotate in two different directions either in the right hand side or in the left hand side so what do I mean is the following so once we have it so this particular line over here is actually going to rotate like this in the right hand direction on the other hand this one is going to rotate on the left hand direction so these are the two different circularly polarized light present which actually generates this plane polarized light and over here you can see this light is rotating on the left hand side so this is called the left hand circularly polarized light sometimes the hand is taken out so it is known as LCP and this one on the other hand is known as the right hand circularly polarized light or known as RCP and this superposition of RCP and LCP actually going to provide you a linearly polarized light and how that actually works so let's actually draw this at different level and see how it is actually going to work so what we are actually saying at this moment that the plane polarized light you are seeing it is nothing but actually a superposition or combination of two different lights which are actually rotate in nature actually rotating not just blinking in a particular plane and one of them rotate on the right hand side one of them in the left hand side so how they are actually going to give me a system the plane polarized light so what I am going to do over here going to draw different versions of right hand circularly polarized light and left hand circularly polarized light and at the bottom I am going to draw what is the resultant will look like so I am drawing the right hand circularly polarized light in blue left hand one in red so this blue one is moving like this and the left hand one is also moving and they started from the same phase so they are exactly opposite to each other and after this half rotation they arrive the same place but rotating through the opposite direction so they are it is written on the left hand side where this is written by the right hand side so this is the RCP and this is the LCP now what I am wanting to do to see what is the resultant will look like so I am going to take the resultant in this five point A B C D E and see how it looks like so the first point the blue and the red one will be exactly on top of each other red one this is the blue one will be exactly on top of each other so the resultant will be like this in the same direction now the next one what will happen this is in this direction this is in this direction now previously there are the same directions so the resultant will be the maximum but now it is a very simple fifth addition you can think about what will happen the resultant will be somewhere in middle but it will be lower in intensity compared to the previous because now they are in the opposite direction there are different directions whereas in point C they are exactly opposite to each other so the resultant will be zero they will be canceling each other out and then they go to the next position but both of them are in downward motion so the resultant will be also coming on the top and in the last one they are exactly in the same direction so this is how it is going to look so take a look into that how that they are rotating in different direction doesn't matter but the resultant looks like stays in the same plane all the time and it is nothing but together it is the same blinking motion the plane follows so that is why we are saying that the left hand and right hand circularly polarized light their superposition is giving me a plane polarized light now some of you probably hearing this for the first time and might take some time to digest it so there is a very nice website where we actually can go through there and this is the website cddemo.azialab.org where you can find a video of all these things showing up and I am going to show you a few examples how to do that the first one we are going to look into how it looks like when two circularly polarized light is moving and their resultant is a plane polarized light over here so let's take a look into over there so over here you can see there are look into the first on the right hand side figure the simple one so the green one is actually rotating left hand side motion the red one is rotating on the right hand side and the blue line is the resultant you can see in the same phase they are moving on the opposite direction and the resultant is nothing but a plane polarized light and on the left hand side they are actually showing how a circularly polarized light actually looks like so it is actually moving forward while it is taking a circular motion so it looks like more of a like a spring a helical structure so the green one is moving on the left hand side red one is moving on the right hand side and as a resultant what we are getting this plane polarized light if I look from the side it will look like it is creating this wave the blue color wave staying in the same plane so that is why we can say that when we actually superimpose two different circularly polarized light it is actually going to create this plane polarized light so later you guys can take a look into it and find out a better idea on this so any questions up to this part why we can explain a plane polarized light by superposition of two circularly polarized light because that is how it is actually behaving okay so now and over there I'm going to draw that how it actually looks like if I look from the side so from the side the right hand circularly polarized light will look like this moving on the right hand rotation so it is kind of rotating right hand side and you are moving forward at the same time so it is going to provide a helical structure and over there for the LCP it is exactly going to be the opposite so it is also moving forward while making a left hand turn all the time so that is how this RCP and LCP actually looks but when you look actually from the front side it is going to show me this kind of simple rotations and this is what it looks like from the side view and this one are actually the front views okay so that is how this left hand and right hand circularly polarized light is actually being used now you can see over here this right hand and left hand circularly polarized light are actually exactly opposite in nature and not only that they are actually tidal because you cannot really superimpose this RCP over the LCP