 So far we said that we are actually shining a plane porous light and the molecule is either allowing it to go slow or allowing it to go fast. But that is not the only thing it can happen in a molecule, right? What else can happen? The molecule can absorb, that is the factor D we are talking about, the molecule can absorb. And a chiral molecule, again as it sees the right hand and left hand circular porous light differently, it can absorb one of them different to the other. What will be the effect of that? So let's take a look into it. So over here I have a right hand circular porous light and I have a left hand circular porous light. And then I expose that to our sample and again I am drawing the RCP and LCP as different circles but they are actually on top of each other. And again for our better understanding I am drawing it separately. Now once it is going through that, my chiral sample, my chiral sample can absorb either of them and not only absorb but it will also differentiate the absorbance. It will absorb one of them more than the other. Why? Because it is again chiral molecule and this can detect the chiral difference and RCP and LCP are actually chirally different molecule. So it will detect the difference. What will be the effect? Now say effect is happening such that the RCP is remaining as it is or it is absorbing pretty less and it is moving like this with not much difference with respect to the amplitude it was having before. It hits the sample whereas the LCP absorb more compared to the other. So that is why this amplitude goes lower. Now what will be the resultant values? So over here I am not showing anything that it is moving differently. It can but we will come into that later. Now we believe that this refractive index of RCP and LCP is remaining same. Just assumption for this moment. We are only considering the absorbance difference. Over there what we are saying that the absorbance of the right hand circular poise light and left hand circular poise light are not set. And that is why we are seeing that the, you can see the left hand circular poise light is severely diminished. Because it is getting absorbed whereas right hand is not. So what will be the effect of that? So let us see. Previously before it hits the system what was the resultant? The resultant was that it was actually having a plane polarized light. Now what happens once these things come? So let us put these things together. This is the right hand and this is the left hand on top of it. And I am doing the resultant at the bottom. So it will be a larger because they are on top of each other. Then this is the right hand one and this is the left hand one. Previously because they are of the same amplitude the resultant remain on the same position. But now it is going to be not in the same position. It is a vector algebra you can think about. So it will be somewhere rotated towards the right hand circular poise light because it has the higher amplitude. So together it is going to be in the same direction. But its amplitude is going to be lower compared to this first case. Because over there both of them are in the same direction. The blue right hand and the red left hand circular poise light. But now they are in the opposite direction. So that is why it is going to be moved towards the right hand circular poise light because it has the higher amplitude. Then in this case they are exactly opposite. If it is same absorbance then it is going to be 0. But because they are different absorbance their amplitude are not same for the right hand and left hand circular poise light. They are going to be some resultant and it is going to be on the right hand circular poise light direction. And that is the minimum value you can have because they are exactly opposite to each other and trying to cancel each other out. Then comes this one which is exactly again very similar case to this where we are going to get the value over here. And the last one again they are on top of each other and they are going to show a value like that. So now this thing are happening at the same place although we are drawing in differently but they are happening in the same place. Now if I combine all these five things together what I am going to get? So let's try to do that. First it was like this then at the same point this then this one this then this one like this then this one like this. So all together if I draw it what I am getting? I am getting nothing but an ellipse. So instead of a plain polarized light what I am going to get if there is a different absorbance what I am going to get is an ellipse due to this different absorbance. So again why it is happening? There is no change in the speed of the light. So they are moving in the similar speed but their absorbances are different because one of them get absorbed more than the other. And what I am saying the resultant it is happening such a way as you have drawn over there. It is going to show an ellipse instead of a plain polarized light. So again I am showing you the video of it. So take a look into it and later on you can find out how it actually happens. So especially look at the bottom one over here. So you can see the left hand side figure is actually showing that before it hits the sample. So you can see the green and the blue color ones are actually have the same amplitude. So the resultant is actually the blue one is actually plain polarized light. But as soon as one of them is actually absorbed more than the other. Now the red and green are not of the same amplitude. Even they are moving in the same speed the resultant is actually not in a plane. The resultant is now moving in an elliptical motion. So that is why it is actually happening over there. It is an ellipse out of it. Again in your free time take a look into that and try to figure it out if it is really making sense. If not I will again go through that later. So so far what you have learned that it might also happen that the right hand and the left hand circular polarized light can actually absorbed in different way starting from this to So this is the right hand circular polarized light and this is the left hand circular polarized light and we have our sample over here. My chiral sample. So previously what was actually a plain polarized light after it has different absorbences what I end up with is nothing but an ellipse. And over here this ellipse I am saying it is actually a direct reflection of the fact that this chiral molecule can detect the difference between again the RCP and LCP and over here they are absorbing it differently. Previously it was for the optical rotation they are actually letting one of them move faster than the other because it is interacting one of them and slowing it down. Over here it is much more direct it is absorbing one more than the other. Over here it is absorbing the LCP more than the RCP and that is why the LCP light actually lowers down in amplitude and that is why it ends up in ellipse. Now an ellipse over here this particular angle say it is shy we can say the tan of the shy value is nothing but minor axis divided by major axis. It is taken from the trigonometric relations what we have learned in the Pugh standard. So not going to details of that but I am just writing it and minor axis over here is this one and this minor axis is this one and the major axis is this one. So we can write it in a way the minor axis is nothing but A L minus A R the difference between the absorbences of left hand and right hand. Whereas major axis is actually when they are actually in the same phase. So A L plus A R just going back over there this is the major axis I am saying because when they are on top of each other so A L plus A R and this is the minor axis. So this is A L minus A R and this is A L plus A R. So over here that I can write and in generally although I drawn that ellipse in such a way but in reality the difference of A L and A R is so small that this actual angle of shy happens to be in the milli degrees. So you can say the shy value it is going to be very small. So in that way we can say the tan shy value is going to be very similar to shy and if you do some more math I am not going into the details. You can write the shy value as the following 2.303 A L minus A R 180 by 4 pi whereas these values are given in degrees. So A L minus A R is actually very small value and it directly defines what will be my formation of the look of the ellipse or this elliptical behavior how much it will look like. This 2.303 you can imagine there is some log change from the natural log to the log is 10 and this again 180 by 4 pi those values are radiant humidity changes. So anyway so those are not important things. What is the important thing is that this ellipticity that how much is be the elliptical character is actually directly dependent on the difference between the absorbance of left hand and right hand circular horizontal. Very similar to the optical rotation which is dependent on the difference of the refractive indices and over here the ellipticity value is dependent on the absorbance of the left hand and right hand circular horizontal. So go a little bit further so we can say shy is dependent on A L minus A R and which can be written as delta A that means the difference between the absorbance of the left hand and right hand circular horizontal. Now we know absorbance is written as epsilon into C L the bare numbers log. Now if I want to write A L and A R values which of these 3 parameters written over there epsilon C L will be showing the difference for the left hand and right hand circular horizontal out of these 3 parameters. Anyone? Epsilon. Yes thank you Pooja that is exactly correct epsilon value. Epsilon. Yes that is true because concentration has nothing chiral in it. Path length has nothing chiral in it. Whereas we have discussed that epsilon value is something to do with the molecular property which is known as the molar extinction coefficient and has something to do with the oscillation and this oscillation is a molecular property. So that will be the best bet for us that it is going to show some difference for the left hand and right hand circularly porous light. So I am going to write it as A L C L and this is epsilon R C. So I can write over here this will be a difference of delta epsilon. C and L is a constant so I am moving it out. So it will be a function of delta epsilon and as epsilon value is again a molecular property so from there we can directly connect it to this electricity. So it is really showing that a molecular property is controlling how much of the electricity I am going to find out and this value can be also given as this molar electricity and by the way this phi value is defined as electricity. Similar to the optical rotation is given by phi and this is written as 100 into psi is the value we have measured C into L. So you are going to get a molar electricity and this molar electricity value will be directly connected with delta epsilon and this equation it values is 3300 into delta epsilon. So whatever the value you are measuring from the ellipticity you can convert it to a difference in the molar extension coefficient and that will be an independent property that you are measuring all over the galaxy because it is a delta epsilon value so it is not going to be dependent that much on the temperature or wavelength or anything. So that is what we actually measured and over here what we found if I go back again that from a linearly polarized light from a linearly polarized light from a chiral sample what we actually ended up is actually an ellipse and this value is known as the psi and this is actually generally tends to be very close to zero. So this is known as the ellipticity and this ellipticity is very similar to the optical rotation it is a direct effect or result from a physical phenomenon. What is the physical phenomena that my right hand and left hand circularly light is actually absorb different amplitude. So this is RCP this is RCP this is LCP and over here we found that this AL and AR not same and that is why we are ending up to this ellipticity. So what is the phenomena that is resulting in this ellipticity very similar to the optical rotation where we said it is a circular birefringence over here we call them circular because it is a different in circularly polarized light di-chroism di means again different. Chroism term means color that means two different color why two different color because it can absorb different because absorbance spectra is actually give you the colors so circularly polarized light can absorb differently so that reason is known as circular di-chroism and effect is ellipticity. So over here if I want to combine those things together so what you can get is the two factors over here first thing is that right hand circularly polarized light and left hand circularly polarized light are there but they are moving in different motions so NL not equal to NR and at the end what is the effect you are going to see is the following is that it remains as a plain polarized light but the original angle actually changes and you see an optical rotation so over here optical rotation is the effect and cause as we said circular bi-refringence is the cause similarly what might also happen that this right hand circularly polarized light and left hand circularly polarized light is moving in the same speed but absorbance is a different and as a result what happens what was previously a plain polarized light now it becomes an ellipse so we call them ellipticity. So ellipticity again the effect and the reason behind that is circular di-chroism and that is what it is actually happening in the real light. Now we actually talk about them differently is it possible that the circular di-chroism and circular bi-refringence can happen at the same time yes it can happen especially in the case of circular di-chroism when an absorbance is happening so circular bi-refringence can also happen and at that time how it will probably look like so let's take a look back to that so over there that is how it actually looks like so this is the light on the left hand side before it hits the sample so it is a two circularly polarized light same amplitude same speed you have a plain polarized light once it get not only absorb but also create some difference in the refractive indices you say the plane of the polarization changes so you can see the ellipse is not in the same plane as the plain polarized light it also has been rotated over here and not only that you can also see it actually creates an ellipse so that is what happens in the real light it shows both circular bi-refringence and circular di-chroism so that is what is actually going to happen if I have both the things coming together so it is the RCP and say this is the LCP and what is happening before after it hits the chiral sample so over here it not only rotate faster RCP but it also get absorbed the LCP so that is why what it is actually going to happen what you are going to see is actually the following and ellipse which is going to be rotated on the right hand side why right hand side because I am drawn in such a way that the RCP is moving faster than the LCP so the plane will move on the right hand side from the beginning so this angle will be the optical rotation and not only that because their absorbance is also difference so on this particular system this is the minor axis this is the major axis so this angle will be the elliptical so from there you can actually measure both optical rotation and the ellipticity so that what actually happens to get optical rotation by and ellipticity so that is what is one of the main reasons why a chiral molecule actually rotates the plane porous right and today we learn about the circularly porous life and we also learn it is not only can rotate but also show an ellipse by creating difference in the absorbance now two questions first one is much more simpler one like which one of them we should use experimentally the circular dichroism or circular bi-reference to monitor an enantiomer or enantiomeric axis or a chiral molecule and the second more important philosophical or fundamental question remains like I understand that it can create the difference and it can detect the right hand and the left hand circularly porous life but why what is there in a chiral molecule that it can separate this right hand and left hand circularly porous life so we have some clarification in our mind that the light itself is chiral so that is why we can create the difference but exactly what is happening in the molecule how the molecular structure is connected such a way that it can interact with the light and see or detect the right hand and left hand circularly porous life differently those will be answering in the next class