Hey guys thanks for coming. So just to give you a quick introduction about myself why I work at Teotrip as the new designer. Previously I have actually worked with Burb. I also worked with a design company in Bombay called Pain Plain. I was working briefly with Yahoo sometime back that's how I actually moved to Backload. I'm briefly from Bombay. So yeah I mean that's about it about myself. I just wanted to get a sense for the people here right like in terms of I don't know what was sort of like the omniscient. So I just wanted to get a sense of so I can all the designers raise their hands. How many designers in the house? What about front-end engineers and these developers or other folks? So I mean guys you is clearly mobile at all. How many of you have smartphones? Pretty much everybody. And how many of you have iPhone guys? Android? Okay great.  I'll just give you a quick start off with just talking about what I'm covering today. So essentially the plan was something like this. I'll talk a little bit about the evolution of Teotrip mobile. Then we move into just talking about mobile as a medium today. Like how do we sort of how do we approach mobile as a medium? And based on that medium what are the design decisions we have to make while designing Teotrip mobile? So these are more like very high level sort of design decisions that we have to make. And dive deeper into the landscape in general like what are the different browsers? What are the constraints that we have to work with? What are the different screen sizes etc etc? And then yeah just some nuts and bolts in terms of you know what are the things that we actually used? Smaller things that actually may experience the way it is right now. And then we can probably you don't have some question answers. Okay so Teotrip mobile actually will launch Teotrip mobile in June 2010. And this is essentially what Teotrip mobile looked like back then. This is what Teotrip mobile looked like. We actually launched Teotrip mobile as a middle viable product. Essentially when we launched Teotrip it was mobile. It was just one way you could only search one way for one passenger. So if you notice you can't even actually add a passenger. We were sort of only looking at business travelers who are on the move and they want to book tickets for themselves. So it was more of like let's try this out and see how the market response was like. And as you can see the first cut was very very fair. After you know we just I mean like when we started seeing the response to Teotrip mobile and it was phenomenal I mean like we really was surprised at like sort of the traction that the mobile product started getting. So we started to actually invest a little more time trying to understand how we can make the product do better. And then we you know smartphones were kind of the penetration were getting better and better back then. And then we sort of said okay let's let's kind of design something for the touch phones. And what you see here actually the previous version right. Pretty much so this is where actually at this point we actually forked our product into our smartphone version and like a very basic version. So what is the screen chart that you saw on the previous slide. The basic version is not really changed that much. Apart from obviously we made some cosmetic changes and some of that. But our focus is going to be smartphones, phones that are movable in terms of of course I mean that means that we are also focusing on touch phones right.  So earlier it was just fly bookings. When we said okay why not you know let people access their clips on the booking industry that they've done. Let people actually consume content about you know the destination that they are traveling to. So we had we had tips we had small world integration. So if you booked a ticket for Bangalore you would actually be able to access information about Bangalore where to go what to see stuff like that. And I think the biggest thing that we did back then was actually launching Express checkout. I mean right from the beginning we felt that you know we want to make the mobile product really really simple and easy to use. So we actually cut down as many features as possible. We just kind of ruthlessly cut down functionality. We brought it out to the bare minimum and then we said okay what next right. How do we kind of make it even easier for people to actually run that on the mobile. Because in general people are you know it was new right. People were using they were snacking on you know Twitter and they were snacking on Facebook. But you know transaction was something that people are not really ready for. So we kind of took this plunge where we actually we introduced Express checkout. We had HTML5 you know phones were starting supporting HTML5. So we said why not kind of use local storage, store all the details on the phone itself which kind of. And then we had obviously a security layer where you couldn't really actually use the data without a clear grid password. So yeah I mean this was in September 2010 and this is what the product looks like. I mean in terms of the user experience like there being obviously I mean what I mean what the stuff that I actually talked about all this while it was like the major milestone right. In between there's been a lot of stuff we added trains and we you know independently improved the product a lot. But I think very recently you guys have sort of been following what's going on and we actually took Express account. And then we said hey you know what this is working very well for the mobile. We were actually surprised when we looked at the stats we saw that people are actually adopting Express checkout. So we said you know what this is going to work for the desktop side as well. So we actually launched it for we rebranded Express way Express checkout and we rebranded it as Express way. And no longer do we use HTML5 local storage. But the credit cards are stored with us and you can access it on the mobile as well as the desktop. So I think I think like in general like you know in my entire sort of experience of mobile. I've seen like you know the fact that there are so big constraints in the mobile. It really makes you think out of the box and I think we obviously related about this on a blog where we've actually the stuff that we built for the mobile we've actually brought it in other places like we just felt that this works so well. So design wise functionality wise stuff like that right. So yeah I mean that's pretty much where mobile stands right now. You can actually book trains, flights, you can look at your details and obviously you can access your credit card details for an easier transaction on your mobile. Talking about the medium right like mobile this like going back to where we started. We said okay like you know we've got to build something really great. We had a good desktop side we wanted to build a better mobile experience right. So what are we dealing with right the screen size obviously the form factor is a big driver in how you actually desire for it. The other main factor is the connection speed right like in India I mean I mean how many people are 3G.  So I mean there's no way out of it we have to design for low connection speed at least in this country. So that was a big driver in terms of the decision we needed to make. The interface itself right the mobile is so different you don't have a keyboard you don't have a mouse. The form factor is so different how does this actually you know change things. And obviously the you know one another factor that kind of drives the design is just the capability of the phone itself right. How much can it process. Moving on to the design right now based on what we had like this is what we are dealing with. How do we sort of like build the mesh experience right. It requires a lot of forces right like we need to kind of really look at I mean for me as a designer right. When I look I mean my benchmark was really the desktop side. Our desktop side is already very minimal. Now I could make it even more minimal so like that's really requires some amount of focus right. So I think it's just to sort of really understand what can users actually live with and what can they not live with right. I mean it's just like kind of really thinking very hard about those things. And you've got to be ruthless. You've got to just tell guys in your team boss this is not required. You've got to cut down stuff. You've got to cut down data. You just have to sort of keep it as minimal as possible because you know sooner or later things will actually start creeping in right. So when you actually go to market it has to be really really simple. So just to give you an example in terms of you know where we are on the desktop side and where we are on mobile. This is an interesting example. This is our desktop one way search and this is the same search on a mobile application. So as you can see a lot of stuff crunched into a small screen and it just jumped a lot of stuff. It just fell okay fine you know price and time is the only thing that we care about. Just don't worry about anything else right. We'll figure it out. So yeah we've cut corners as much as we can in terms of presenting data. And the other thing like we've been also very receptive to user sort of behavior right. So we saw that you know like our search patterns on mobile and our search patterns on desktop is pretty different. We realized that people are actually on the mobile actually searching for today and tomorrow right. So we actually changed our sort of search results to kind of allow users to actually quickly access previous and next day right. So what you see here is someone searched for a day and then you just got kind of you can easily sort of you know talking between a previous date or because people actually searching they have to go all the way back you know and just to enter into data because you know your your destination and source city doesn't really change it just the dates are changed. So we actually sort of made these design decisions to kind of incorporate what users were actually doing on the site. This is another example of our itinerary so we basically before users actually start booking to present the itinerary so that they can review what they are looking at. So this is what it looks like in the desktop and I mean this entire thing gets crunched into this much. So I mean it was an interesting challenge from a design perspective like you know because it involved sort of you know really think about how do we build the right visual hierarchy you know how do we you know how do we like sort of make all that information available you know at our arms length stuff like that. So I mean like my experience has been that you have to obsess. I mean you got to obsess like you see you got to think about it day in day out. Like I mean is what you see today is like not out of one or two iterations. There have been like 50 iterations in terms of we I mean you know it's like we've come from say you know 60% information to like 20% information and like it's just been a continuous iterative process. You got to obsess about details because I mean with retina display everything is of course I mean like it's like you are literally under and we could die. So I mean you know you have the technology that can make things beautiful. It doesn't take too much to kind of you know sweat the details and at least you know make sure that the people who actually have this technology have a good experience right. So we have we actually started supporting retina display the day it was actually launched in the other day. The iPhone 4 was launched in there we actually the next day we actually started supporting retina. Another thing that you got to obsess about is just like the page request sizes, page weights right. So just to give you an example I wanted to walk away. In fact we got this sort of small experiment done just before with the retina because we wanted to showcase this as case study. So you know I spoke about the poking of the basic size and you know the HTML5 size. Essentially what used to happen is users actually landed up. So one is how do we you know there is obviously this e-direction right. It will land up on www.theatrep.com now and they need to go to theatre mobile site based on if they are you know coming from a mobile device etc. So just all this sort of round trip right made like people landing up on the mobile site all the more slow.  So we actually optimised the flow in such a way that you know what is we are people used to actually land up on theatre mobile slash m. We actually removed m because there was an e-direction we landed we did some changes in the back end. If you type in theatrep.com they actually land up on the mobile site. So a lot of magic on the back end to sort of just make sure that our sort of you know the page goes in the time that I didn't like it to be right. So just to give you an idea right we had about 7-8 requests coming in from the home page right. Like he said I mean this is not happening approximately I think the home page on a gprs connection was taking upwards of 10 seconds or something like that. So we separated out the mobile page and we said okay fine let's look at this exclusively and see how do we cut down the amount of requests that come to this page. We basically brought it down to half the number of requests and then we also cut down the size of the actual page weight right. So we are at about 5 seconds per minute I think we are at about 5 seconds now. So it's almost twice as fast as what used to be. Initially we used to actually do like I mean I don't know how many people are familiar with my new page 5. So we were actually caching our home page because it was slash m but I think now because it's landing up on theatre.com it's a little complicated because we do allow users to actually access our desktop site to send in. They need to access more bug-cells because you know pretty much all phones actually support full running browser like full app applications. So I mean we've actually removed caching and manifest on the home page. We plan to sort of introduce it in other pages that people are familiar with. Okay so I mean that's sort of the design slash you know performance build that we'll take some time back. Just talking about the landscape right mobile landscape. I mean as designers we hope they're not like you know rejoicing over the fact that I use finally that and we just got ourselves into all this right. Millions of devices, different screen sizes, different models, you know you sort of now take care of all this stuff right. Another challenge that we do.  So you've got a sort of balance you know between touch and cursor based interactions. Browser capabilities right. You'll have browsers that are very very advanced and then you have browsers which are sort of you know very basic even today. So just talking about the browser right. Pretty much like this you know calling to like a more popular category and then there are new browsers that are emerging. Chrome for Android and Firefox are sort of you know just launched. So I mean we on and off we do get like feedback from our customers and things are breaking and sort of that. But yeah these are all challenges so I mean if you guys are designing for mobile. So you've got to take all this into consideration. So we have I mean resolutions, different resolutions that you've got. I mean even though you know the small screen sizes are constant you have various different resolutions within that sort of category. You have different orientations. So you still have blackberries around which is a landscape orientation. All the smart phones and all the touch phones have kind of moved into a landscape orientation. And you have different pixel densities to sort of worry about. Okay so the good news is that you know the majority of the browsers are actually getting powered by Perkit. So you have one group obviously I'm not saying that all the browsers are powered by Perkit. But most of them I would say 50% of now all the browsers that are kind of becoming popular are powered by Perkit which is the good news. You also you have Opera and the older blackberries, the newer blackberries are moving into Perkit which is a good thing. The other good news is like even though this transition is happening browsers are quickly catching up so you know that's a good thing. There's tremendous CSSP support. So talking about you know just the nuts and bolts right like see I mean we actually didn't do anything special in terms of I mean yes we spent a lot of time on sort of you know cutting down the clutter but there are things that are readily available. I mean we did have to sort of it's not rocket science it's not documented it's out there like none of this stuff. All the stuff that I'm going to talk about right now is like available and just I mean pretty much everyone is talking about it. So I think this I kind of only picked up things that we used and we saw benefit in. So I think like you know it's a good checklist this pretty much sort of covers you know 60-70% of the things that you would like to do from the book. So on the HTML front you know I think one major thing just talking about like you know the actual sort of execution right. One thing you need to know really sort of worry about is the viewport right. Now why do I mean what is a viewport right the viewport is essentially the space in which you know your content goes in the browser right. And that means completely because all browsers kind of have default sort of sizes that they enter. So you probably noticed right in the open of the website in your Safari browser it kind of constrains it in a specific way. Now that means Safari sets it at 986 right. So all your sort of content will actually so it kind of sort of assumes that most websites are approximately that size and will actually sort of go into that size. Essentially what it means is the physical sort of is the sticking of physical width of your website to 980 pixels and shaking it down. So looking at this image right essentially what it's doing is I mean the actual resolution of iPhone is 320 pixels and this image is also 320 pixels but it's assuming that this page is 980 pixels and that's why it's scaled down right. So that's why you need to actually configure your viewport. So talking about the viewport meta tag right you can actually so one is you need to sort of define the width that your content needs to render it right. So I mean like so the way you do it is basically you use there's a meta tag that you can actually put in and say ok fine this is you know this is the width that I want my content to render it and then you can have some like sort of more controls like what you wanted to actually scale at when it actually rendered. So normally I mean for mobile websites it's good to have it at like one because that's the actual size. One means actual size and then the other thing you can do you have more controls like when actually switch off or rather disable I mean disable the browser from users to scroll so I mean I would rather zoom. So they can't really pinch and zoom but we've done that on a site because for us that's important we don't want to play and we are sort of working in that space. So yeah I mean viewport is something which is like the bare essentials to start up in a mobile site. Some of the other things that sort of you need to take off you know for us we had sort of like numbers and stuff like that and also we had a lot of numbers in general right so we had booking IDs on our site we had flight timings and the browser essentially thought these are phone numbers and it used to actually link them right. So I mean there's a way to sort of switch off like the browser to actually automatically make it a link but you can sort of fall back to a more sort of robust way by actually giving it a link with the user. One of the other things that you need to really do is really making like you know texting code really really simple right so all this is all available iPhone has been supporting it for a really long time slowly all browsers are supporting it back then 6 is supporting it Android in its recent sort of upgrade has almost supporting all of these HTML5 attributes so you know you say input type is equal to email you get an email keyboard tell if you say tell it's a telephone number and there's also a number but actually there's a small problem with that actually iOS 5 actually considers it as a number and puts a comma so we had this problem on our we used to use type is equal to number on our credit card and it actually put commas which used to kind of look like a sort of you know it put commas in for months so we had to remove it and actually there's a fall back you can actually use pattern is equal to 0 to 9 and then you should see only those keys also I mean another good thing to do is just sort of switch off all auto corrects and sort of auto complete wherever you don't want it so you know your first name I mean if you are collecting information about the names email address you don't want the auto complete to come in the way of the customer right so all these small limit things actually sort of remove all the friction between the user and the product and it sort of becomes much easier to use the other thing you did is like you notice you know Safari and Android actually have a address bar that sticks to the bottom you can actually use a small script that kind of scrolls the address bar back into place so you have more sort of space for your partner so my specific stuff that you get to I guess I mean this is things that you probably you've done mobile so you've been doing it already you can add two types of icons that you can actually add your web page on the home screen which sort of almost feels like it's an app it's essentially a bookmark to your website there are two ways of doing it you can actually put a flat icon and then so obviously it's not very clear but there's a slight difference this was actually a flat icon which iOS automatically put that last sheet you can also customize your icon you have to use a specific tag and it actually customizes it you can open your website in a full screen mode where it actually looks like a web app so I mean you can customize certain things like the status bar and stuff like that on the CSS front I guess one of the biggest things that you guys need to sort of look at is media queries media queries is everything when it comes to like designing for mobile you can do a bunch of stuff right I mean you can sort of like target specific screen sizes you know have customizations to specific screen sizes, specific orientations pixel densities so I mean these are I mean I'll just put this up so that you guys I mean this may not be very useful but you will get reported to a computer how do you support Latina higher density or higher density so I mean just to look at one example we have you have a default like sprite this is essentially all the third card logos and then you have 1.5x so basically Android has I think three levels of three different pixel densities they have a 3 port 1x and 1.5x and they have a 2x IOS just has 2 1x I mean there's a 3 port pixel density and like HD version of it so yeah I mean you can sort of like actually deliver a much crisper image to like browser that actually supported some of the other handy tweaks that you can sort of do is well I mean what Safari does by default when you actually rotate your phone is actually adjust the screen size right also in the phone size so you can actually disable that if you want to I mean right now we've not done too many customization when we are in landscape mode but if you sort of disable this you can actually do a lot of customization in the sense that you can actually showcase more content in a landscape mode and stuff like that some other CSS tweaks you probably noticed there's a sort of a very steady transfer and create highlights that shows up when you actually click on something you can disable that you can prevent users from actually selecting tests next and I think that pretty much rounds it open more questions yeah any questions? yeah so I was very curious to find out whether you are how you have done your Dubai question I thought whether or not you should be able to explain okay why? I'll tell you why just because you have been in mind and comes as you go to a very very simple I want to know where do you how much for an architecture is to change and how much of a video is there to make a video just to explain so the question is essentially how much of we write a video yeah where do you use video for this so we essentially it's a different site or it's a completely different front end in fact it's different I mean obviously the data is the same but like the front end and the back end is sort of different because the flows have changed a little bit on the mobile so it's completely different from that perspective media queries we've not used apart from like targeting specific pixel density so if you want to do retina then we've done written a media query to kind of support the printing inside and so forth but apart from that you've not done media query I think the media query comes in handy when you want to actually customize for a specific size so we say that hey you know what everyone gets this experience but guys who come who I mean any guy comes to my site who has a sort of browser that's less than 240 pixels like do this right so you kind of overhead style so we luckily we've kind of designed from grounds up so we've taken into consideration what the experience would be for a smaller screen so we just use it for pixel density to answer your question given the fact that the technology is good today and if you had a let's say if you were to decide as well to move from desktop to mobile today you think that media query is sufficient enough to kind of do the same motion or say it would take a different function or whatever it really depends actually I mean in the sense like we it depends on like how one is how heavy your current sort of desktop position is right like is it an interesting if it's too heavy then you probably need to sort of rewrite everything but like things like for example like like the products that we have like for example we have you know a blog you know then we would not you know put it in the effort to actually build something from scratch we just use media query and it works pretty well you know means switch off some content that will load up which will not really have which is sort of just you know and we switch it off but yeah I think those those are less risky so we can do that so yeah I mean like this question yeah so the question was what was it I mean what are the discussions about internally about like native versus mobile right I mean mobile right so we've been obviously like you know big believers in sort of being able to push more functionality to our customers as soon as we can native apps unfortunately you know don't give you that like that flexibility here we can push stuff in the middle of the day if things are not right so we've shied away from native apps obviously but we obviously have their advantages it's a good distribution channel it has a better user experience so we are spending time to kind of see you know there are ways to you know actually look at that as a channel as well in fact I mean like we are sort of you know we are working on some ideas right now yeah so driving consideration was how fast we can push to market here WebKit is essentially a engine that powers I mean it actually powers the rendering on the browser so it's a rendering engine for the browser yeah it is sort of open source I mean everyone is using WebKit you have Chrome using WebKit and Safari so the question is what kind of analytics tools are you using not using analytics tools right now because that's a distant sort of script that you need to run on the front end so we've kind of avoided that but we do look at our server logs to see you know how people are behaving so it's kind of right now it's not it's kind of rudimentary but it does what we need to do so yeah we look at conversion we see where the drop-offs are so if say for example we realize that you know people are actually not going beyond you know like say one step we actually experiment with some like obviously we'll have some inclaim what the problem could be we'll actually try it out for couple of days switch it you know just kind of make a small tweak see if people respond to it and that's normally how we actually do it you know AAP requires a much like requires a bigger infrastructure I mean like in the sense right now yeah we are obviously you know investing you know effort in like building a mobile product but still you know we don't have the resources that we actually like to have build that infrastructure for AAP tests and stuff like that so right now like we are not debating like that lets actually access our desktop site so let's talk about why you've chosen you know insurance policy so yeah you know like we kind of didn't want to have so like you know insurance and all that comes into view sorry what do you mean by insurance? you know we don't have so the question is whether why the insurance is not part of the mobile right yeah we excluded it because it kind of requires a decision making right and we want to kind of like I said cut down on any sort of issues yeah people don't have to make a decision right suddenly there's one more sort of dimension I think yeah we've not done it for a mobile site so the question is are we using are we not using the GSP work to actually for a mobile site we're not using GSP work because our time is I mean pretty I mean in terms of it's not it's not very dynamic we still need to kind of figure out what the browser can support right in the sense there's too much between browser so we want to we've kept it simple because you know very minimal jobs that we do we're not using it in fact a lot of the years that we're using on like which it's we're using is like we split that down expected only the things that we need so it's very very sort of it's a stripped down version of our original thing but we've not even done it are we experimenting or are we experimenting sorry are we experimenting or are we experimenting like I said like our desktop site has obviously taken you know as sort of evolved over four or five years right there are things that we don't have to actually you know we don't have to have customers pay for that in the sense in terms of bandwidth cost right where you can actually deliver the same experience you can deliver a better experience with something which is much more lighter and you know I mean it's just actually it's bandwidth it's just like it's an additional effort right you can always hide it at the end right no but that's dead weight right so if you have content that you're hiding the content is still getting delivered through the pipeline it actually goes to the client side and then you actually like sort of switch it off so like I said like for lighter sites it makes sense like our blog is like that I mean I mean we just wanted to use this concept we tried it out experimented our account, the new account to a certain extent is responsive like you would probably check it out on the tablet to see how it works yeah okay so you done thanks guys


Now it is time for me to go over relations between properties. Now what I am going to do is I have a set of slides which I am going to use. We have said earlier that second law of thermodynamics provides us three things, first possibility that we have seen. In limits on processes we have seen efficiency is less than or equal to something, delta s less than or equal to something. If you say less than delta s is greater than or equal to dq by t that means that for a given delta s there is going to be a limit on the amount of q that you can have. In fact when we combine first and second law we will be able to quantify this limit in some way the so called exergy analysis. The third thing the second law allows us to do is to derive property relations. And the first property relation for a simple compressible system or a fluid system at rest is we have already seen T s equals du plus pd and that is what we are going to do. In property relations we are going to consider relations between properties of a simple compressible or fluid system at rest and we will be assuming a unit mass of the system. So we work with specific properties and the mass of the system does not have to be explicitly included in the proceedings. The tools which we are going to use are calculus of exact differentials. We have used exact differential but only one property of that exact differential. Now here we will be using some more properties. We will be using calculus of partial derivatives along with calculus of exact differentials. Before we go to the property relations proper we define some energy functions. Energy functions are properties derived properties using the internal energy of a system. The internal energy of the system is the primary function, the energy itself and it is u the thermal internal energy. The second law defines s which is the second more important energy function. Then we have already seen the basic property relation between the two. Tds is du plus pdv. Now apart from basic property relation we will be also using the first law which is dq equals du plus dw. It is important for us to see that this is a property relation. Here pdv is only pdv nothing more. Everything here is property or a change in property. No interaction is involved in the basic property relation. Although you can say pdv represents the expansion work done. Let it represent but here the significance of every term is simply a property or a change in some property. But we will also be using the first law in which dq and dw are interactions. And here dw is the differential of the work including all its components. So do not jump to the conclusion that dw is pdv. It could be pdv plus some other components of work. And we will also be using the second law which is with transposition of T turns out to be Tds is greater than dq or Tds is greater than du plus dw. Here I have replaced dq by the right hand side of the first law expression. Now let us do some exercises. We first have the internal energy u and let us expand this the expression for first law. dq is du plus dw which is du plus pdv plus dw other than pdv. So this just tells us that dq at constant volume is du plus dw other where dw other means dw other than pdv. We are also familiar with the second energy function enthalpy. It is a derived function and we have defined enthalpy equal to u plus pv. Differentiating this we get dh equals du plus pdv plus vdp. If you use the first law dq is du plus pdv plus dw other substitute for du plus pdv from this expression and you will get dq is dh minus vdp plus dw other. From which we see that dq the heat transfer absorbed at constant pressure is dh plus dw other. So these two expressions one should be familiar with because common mistake is assumed that for a constant volume process dq equals du that is not true. It will be true only if dw other is 0. Same thing for a constant pressure process dqp will be dh only if dw other is 0. Now we are going to define two spatial energy functions. The first of them is the Helmholtz function given the symbol A. Kemi's quite often call this the Helmholtz free energy or simply free energy and use the symbol F. So you should be familiar with both the symbols A as well as F. A the Helmholtz function is defined as u minus ts. Notice that ts has the dimensions of energy. So we can combine u and ts. It is defined as u minus ts. If you take a differential you will get da equals du minus tds minus sdt and if you apply first law du is dq minus dw and if you now apply the second law you will get da plus dw plus sdt equal to dq minus tds which according to second law is less than or equal to 0. Keeping dw on the left hand side it tells us that dw will be less than or equal to minus da minus sdt. And now if you consider an isothermal process it tells us that work done by a system in an isothermal process will always be less than or equal to the decrease in its Helmholtz function. Now remember that the work done equals our traditional mechanical way of looking at things is work done is decrease in some potential. The potential energy goes down, the amount by which it goes down is the amount of work you can do. If everything is ideal, if there is friction or some other dissipative process the amount of work which you can extract will be less than the decrease in the potential energy. So using that idea we can consider the Helmholtz function a to be some sort of a potential and hence sometimes it is called the Helmholtz potential. A decrease in which represents the maximum work that can be obtained in an isothermal process. The next important energy function the final one which we derive define it g known as the Gibbs function.  Differentiating it you will get an expansion in terms of du, dv, dp, ds and dt. Now apply first law du plus pdv plus dw other will be dq so du plus pdv will be dq minus dw other and then keep dq and tds on one side and you will get dg plus dw other minus vdp plus sdt which is equal to dq minus ts less than or equal to 0. This less than or equal to 0 is a consequence of the second law. Keeping dw other on one side now you will get dw other other means other than expansion work less than or equal to minus dg plus vdp minus sdt and hence if I consider a process which is constant pressure, constant temperature. Now remember that if you have a chemical reaction taking place electrochemical or even an ordinary reaction it is possible for us to change the state while keeping the pressure and temperature constant because the composition will change and if the state changes you can always do some work and hence we can show that dw the work done other than expansion work in a constant pressure, constant temperature or isobaric cum isothermal process will be less than or equal to the decrease in the Gibbs function in that process during that process. Compare this with the expression for the Helmholtz function and we can now say that the Gibbs function is a function similar to the Helmholtz function in that it can be considered as a potential the decrease in which represents the maximum work other than expansion. Notice this dw other here other than expansion that can be produced that can be obtained in an isobaric cum isothermal process and since most of our chemical reactions are isobaric cum isothermal processes they take place at ambient pressure, ambient temperature or can be made to take place at ambient pressure, ambient temperature. The Gibbs function is a is an important idea and that is the basis function on which much of physical chemistry and chemical engineering is based. Chemical engineers are very very comfortable with the Gibbs function just the way we mechanical thermal engineers are comfortable with Enthalpy. Now this was the introduction to the four functions u, h, a and g. Now we look at the properties of these four energy functions and we will use the specific property version everything will be considered per unit mass of the system and we will use the property relation and we will be using the following identities from calculus of partial differentials. We will say that if z is a smooth function of x and y smooth means the first and second derivatives are defined and if the differential of dz m dx plus n dy is an exact differential if z is a function then dz will be an exact differential if it is expressible in form if it is expressed in the form m into dx plus n into dy then m must be partial of z with respect to x at constant y and n should be partial of z with respect to y at constant x. Not only that equating the cross derivatives the second partial derivative of z first with respect to x and then with respect to y will equal the second partial derivative of z first with respect to y and then with respect to x. And hence the partial of m with respect to y at constant x will equal the partial of of n with respect to x at constant y. These are the two relations which we are going to use innumerable times henceforth for our derivations. Now the next four slides would apply these to the four functions u, h, a and g. First let us look at the internal energy u. We have the property relation a transpose property relation. The basic property relation is Tds minus pdv. So sorry basic property relation is Tds equals du plus pdv. So here we have du equals Tds minus pdv. So notice that because we have a ds and dv on the right hand side it is natural for us to consider u to be a function of s and v. We can say that s or v are the most natural independent variables to consider u to be a function of. Now when you do that from our laws of partial derivatives we know that T must be partial of u with respect to s at constant v and p must be partial of u with respect to v at constant s. In fundamental thermodynamics these two relations are known as the thermodynamic definition of temperature and thermodynamic definition of pressure or thermodynamic temperature and thermodynamic pressure. For the simple reason that consider this triad of properties u, s and v. We realize that the moment we define any thermodynamic system we are able to define its volume. First law and second law of thermodynamics are able to help us define two properties of state. First law the internal energy u and second law the entropy s. So u, s and v are considered the basic thermodynamic triad of properties for any systems. Based on this now we know that temperature has to be the variation of u with entropy at constant volume and pressure has to be the variation of u with volume at constant entropy. And using this relation particularly the pressure relation we can define the thermodynamic pressure for a system which is not a fluid at all. For example we can define it for a system purely containing radiation. If we know what is the energy of that radiation field is what its volume is and what its entropy is then we can determine its pressure simply by obtaining this partial derivative of u with respect to v at constant s with a negative sign. And now look at the next thing. If we use the cross derivative relation we get how temperature varies with specific volume at constant s and how pressure varies with specific entropy at constant volume. An esoteric relation which we could not have imagined being able to derive using the first principles without going through this algebra of partial derivatives. And we will see more such interesting relations. These are very useful relations in thermodynamics. Note this relation which is in a box here. Now let us come to enthalpy. It is defined as u plus pv. So if you take its differential and expand it and use the property relation for Tds you will get dh equals Tds plus vdp. So now we consider enthalpy to be a function of entropy and pressure because we have ds and dp here. And we immediately get temperature to be partial derivative of enthalpy with entropy at constant pressure and volume to be partial derivative of enthalpy with pressure at constant entropy. Look at the funny relations that we have. And the cross derivative relation gives us partial of temperature with pressure at constant entropy and partial of volume with respect to entropy at constant pressure. Note this expression in the box. Now let us come to Helmholtz function. It is defined as u minus Ts. So expand it, expand du as Tds minus pdv. Combine terms and you will get dA equals minus sdt minus pdv. Consider A as a function of T and v because there is a dt here and a dv here. And you will get entropy to be negative of partial of A with respect to T at constant v and pressure is negative of partial of A with respect to volume at constant T. And look at what the cross derivative tells us. It tells us that variation of entropy with volume at constant T equals the variation of pressure with temperature at constant v. Now this expression is very important because look at what it tells us. On the right hand side we have just the data which we get from the equation of state p, v and T relationship between p, v and T. But the left hand side tells us how does entropy vary with volume at constant T. So this is a very important relation which leads the variation of entropy with respect to volume to purely pvT data. Finally we come to the Gibbs function which is defined as h minus Ts. Take the differential of dg, use the expansion of dh and use the basic property relation and you will get dg equals minus sdt plus vdp. And that means it is proper for us to consider dg as a function of temperature sorry g as a function of temperature and pressure because these are the two differentials remaining on the right hand side. When you do that we realize that entropy is the negative of variation of g with temperature at constant p and volume equals the variation of g with respect to pressure at constant T. And the cross derivative gives us a relation partial of entropy with pressure at constant T is negative of the partial of volume with temperature at constant p. And again this is important because the variation of entropy with pressure at constant temperature is provided to us in terms of only pvT information which is available simply from the equation of state. Now these four blocked relations from the four slides I have put together here and these are known as Maxwell's relations. And even their reciprocal for example take this relation partial of T with respect to p at constant s is partial of v with respect to s at constant p. And if you take its reciprocal we will get partial of p with respect to T at constant s equal to partial of s with respect to v at constant p. So these four relations and their reciprocals are known as the Maxwell's relations and these are perhaps the most celebrated relations in thermodynamics. So these are very useful property relations. The third and fourth ones relate entropy variation to purely pvT or equation of state data. They help us reduce the requirement of Cp and Cv data for mapping the state space entropy variation is available at least in part using only pvT data. And one of the important tasks in the study and application of thermodynamics and one of the hardest job for a student of thermodynamics any student of thermodynamics including us is how to remember them without having any funny material inside your trouser pockets. And we now find out a trick to do that a physical realization and then some mathematical tools. We consider a reversible cycle. Remember the area under a pv diagram area under a quasi static process and area under the same process on the T s diagram that we have realized we have discussed represent a reversible cycle on a pv diagram and also on a T s diagram you will see two pictures like this. On a pv diagram there is a reversible cycle the same reversible cycle sketched on a T s diagram looks slightly different but it is another closed loop. Let the area of the cycle in the pv plane be a pv. Let the area of the cycle in the T s plane be a T s are the two areas equal. If all of you were to have a clicker or the all of you were to have the akash with a clicker application at this stage I would have rested for a few minutes and asked you to answer this question yes or no. It is difficult for me to obtain such a feedback but I suggest all of you write down in your notebook yes or no because soon I am going to give you the answer to this question and proceed from there. I hope you have written that now I will relieve that the answer is yes the areas are equal and why are they equal because we have a reversible cycle. So area under a curve represents heat transfer on the T s diagram area under the corresponding curve on the pv diagram represents the area represents the work done pdv work done and hence for a reversible cycle the work done would only be pv the heat transfer will then be equal to the work done pv diagram and hence the two areas are equal. Mathematically the area of the loop in the pv diagram is the loop integral of dp dv and area in the loop on the T s diagram is the area integral dT ds and this should be true for any reversible cycle of a simple compressible system at rest and now go to our coordinate geometry this is where the coordinate geometry is needed. Remember that if we transform xy coordinate to some uv coordinates then the area element dx dy gets transformed to area element du dv by a local scale which is known as the Jacobian and if the areas are always equal that means the Jacobian of the transformation is 1. I am not going to spend any detail on discussing the properties of Jacobian and how it is defined. I recommend that you look up your elementary calculus and coordinate geometry book but the symbol for a Jacobian in terms of the Jacobian symbol this means that the Jacobian of T s with respect to pv is 1 and so is the Jacobian of pv with respect to T s. Now we will use the properties of Jacobian to derive many of our relations. These properties of Jacobian are important Jacobian is a combination of partial derivatives which behaves like an ordinary derivative that is the special thing about it. So the first relation is Jacobian of uv with respect to xy when you multiply it by the Jacobian of xy with respect to uv that should be 1 this comes from the area transformation if you transform it one way and back transform it you should get back the same area. The product rule of ordinary derivative is also applicable you have derivative of a with respect to b multiplied by derivative of b with respect to c is derivative of a with respect to c. So Jacobian of pq with respect to rs multiplied by Jacobian of rs with respect to Tu should be Jacobian just cancel this partial of rs and partial of rs and you get Jacobian of pq with Jacobian of Tu with respect to Tu. But the real important relations for us are these you know the partial derivative of u with respect to x at constant y can always be written down in this form it is shown it can be shown to be equal to partial of u or Jacobian of u and y with respect to x and y. Notice that what is the left hand column here u and x are the numerator and denominator functions in the derivative and what is the right hand column the common variable y is the variable which is maintained constant. You will find this in your calculus book and you can check this out for yourself using the definition of Jacobian. U and x need not be in the first column the common variable the constant variable y needs to be common in some column either second column or the first column. So you can also write it like this but if you have the common variable on a diagonal either the principal diagonal or the secondary diagonal then you have to have a negative sign. So this is an important relation that partial of u with respect to x at constant y can be written down in terms of Jacobian in four different ways select one which you find convenient and we will use these and other properties of the Jacobian to manage the Maxwell's relations. I will give you an example here first suppose we want to find out the equivalent of partial of t with respect to v at constant s. First we have to determine whether this is a candidate for Maxwell's relations going back we will notice that the four Maxwell's relations have something in common if you go to this slide you will notice what is common if you spend a few minutes with this you will find that the pair T s and the pair p v occurs in a combination where either the numerator and the constant function or the denominator and the constant function is either T s or p v notice there is a T s here there is a p v here similarly there is a T s here and there is a p v here in some order similarly there is a T s here and there is a p v here similarly here there is a T s and a p v. So this tells us whether a given partial derivative is a candidate for Maxwell's relation or not. Now given partial of temperature with respect to volume we notice that there is a T s pair the constant function with either the numerator or denominator should form the pair T s or p v. Now it is T s so we can go ahead with our scheme this will be a candidate in the Maxwell's relations. So you first write this as a Jacobian partial of T s with respect to v s just put S as the common variable in the numerator and denominator. Then because T s is in the numerator multiply this by 1 which is partial of p v multiplied by T s by thermodynamic this Jacobian is 1. Now we use the product characteristics of product property of Jacobian there is a partial of T s here there is a partial of T s here cancel out we end up with partial of p v with respect to v s. And now we notice that a diagonal is common v so this must equal negative of the partial derivative of p with respect to s at constant volume. I recommend that we spend some time in deriving these because you do not have to really go through this detail when we do exercises I will show you a quick symbolic short form by which you can do this. You need not write all these things down although for the first few attempts you should do this. Now after this what we are going to do is derive I will leave it to you as an exercise to derive all these things all the four partial derivatives all the eight partial derivatives involved in Maxwell's relations and the reciprocal and the next thing is going to be exercises which we will do after the lunch break. There are still some seven minutes to lunch break so I have a choice I am just going to go to 1, 2, 4, 1 NRI institute Bhopal Madhya Pradesh. Over to you sir. Sir one question that arises in my brain is we have centigrade scale of temperature we have ideal gas scale and we have thermodynamic scale of temperature. All three scales have independent different origin centigrade scale is based on melting point of and boiling point of H2O whereas ideal gas scale is based on the fact that at constant volume temperature is a function of pressure whereas our thermodynamic temperature scale is based on reversible is based on reversible Carnot cycle. Now the question that arises in my brain is though their origin is independent why or how this coincidence has occurred that the unit the magnitude of unit of temperature is same on temperature scale same on centigrade scale as well as Kelvin scale as well as thermodynamic scale. Over to you sir. Okay good question I would go the other way round I will put the Celsius scale at the end purely from a thermodynamic point of view the only scale which we define is the thermodynamic Kelvin scale. Actually just because the traditional way of thermodynamics is teaching about ideal gas first and then coming to second. If I were to teach thermodynamics in a very pure way I would not talk about any temperature scale till I come to the end of the second law and define the thermodynamic Kelvin scale of temperature. Then I will ask ourselves the question a reversible engine cannot be implemented. So what is the best we can do to implement and measure the Kelvin scale of temperature the thermodynamic Kelvin scale of temperature. Then we would discover an ideal gas and we would show that the efficiency of the ideal gas Carnot cycle reversibly implemented is given by 1 minus T2 by T1 on the thermodynamic Kelvin scale by definition and using the ideal gas equation of state we will show that the efficiency of a ideal gas Carnot cycle will be 1 minus PV product at the lower temperature divided by PV product at the higher temperature and that way I would be able to show that the thermodynamic Kelvin scale can be implemented in practice using an ideal gas by implementing this demonstrated relationship that T2 by T1 or T1 by T, let us say T2 by T1 on the thermodynamic Kelvin scale it shown to be equal to P2 V2 for an ideal gas working a Carnot cycle where 2 is the state representing the lower temperature divided by P1 V1 where 1 is the state of the higher temperature. In this case I do not even have to define the ideal gas Kelvin scale. The ideal gas Kelvin scale had to be defined because we had to work some exercises simple exercises using first law before coming to second law. So this explains that although we have for our convenience defined them to be separately in the end we have shown that the thermodynamic Kelvin scale and the ideal gas Kelvin scale are the same. That is the first question I think that answers the second part of your question that is ideal gas Kelvin and thermodynamic Kelvin. Now when the first part of the question I agree that the Celsius scale or sometimes called the centigrade scale was defined using the ice point and steam point and till the early part of the 20th century that was so. But later on when you know there is an international union on pure and applied physics IUPAP and they decided that look we should not have these two different scales Celsius and Kelvin. So at that time it was decided that the Celsius scale should be got rid of and today Celsius scale is defined as temperature on the Celsius scale is temperature on the Kelvin scale minus 273.15. So today there is no separate definition of the Celsius scale. The Celsius scale is today defined in terms of the Kelvin scale. Now the question arises what is that funny number 273.16. The funny number 273.16 has been derived it is extracted using experimental data excellent experimental data the PV product of an ideal gas and the ratios of these between the ice point the steam point and the triple point of water. It was discovered that the ratio of the PV product of an ideal gas at the normal boiling point of water to the PV product of the same ideal gas at the ice point turns out to be almost exactly 373.15 divided by 273.15 and based on this it was decided to use the standard value of 273.16 Kelvin for the triple point of water because if you define it like that the ice point turns out to be almost exactly 0 degrees C but not exactly 0 degrees C. The steam point turns out to be almost exactly 100 degrees C but not exactly 100 degrees C just to keep the numerical difference in the temperatures of water at its ice point and at its steam point to be 100 units which was the so in the earlier Celsius scale. We wanted to keep that on the Kelvin scale so that all the earlier data based on Celsius scale can be converted to the Kelvin scale just by adding the number 273.15. It was just a matter of convenience. It was open to people to keep the define the Kelvin temperature of the triple point of water to be 1.0 exactly or 100.0 or 1000.0 or any other arbitrary number. All that would have happened was the distinction between Celsius or the relation between Celsius and Kelvin on the temperature scale would not have been as simple as Kelvin minus 273.15 each Celsius. That is all. The idea that Celsius is defined in terms of ice point and steam point is now history. That is the historical definition of the Celsius scale. Today's definition of Celsius scale is in terms of the Kelvin scale. I hope that satisfies you. Over to you. One more question sir. One day one of my students gave an explanation to me that I can create a reversible Carnot engine which can violate the second law and the simple thing that he told that I will make T lower equal to absolute 0 and definitely when T lower becomes equal to absolute 0 then all the heat that the engine takes from the high temperature reservoir is getting converted into water. So what answer I should give to my students over to you sir. There is an inherent assumption in the argument of the student and that assumption is that he will create a system whose temperature is 0 Kelvin. And now we will say that look you are assuming here that you are creating a system with 0 Kelvin and you can simply use the argument that you cannot have such a system or you cannot create such a system because if you create such a system you can work an engine reversible to T engine between say the ambient temperature and 0 Kelvin the efficiency of that will be 1. And the Kelvin Planck statement says that you cannot have a 2 T heat engine the efficiency of which is 1. Actually this argument is an argument to the fact that you cannot create or you cannot have a system whose temperature is 0 Kelvin. Use his argument against him because he is assuming that the Kelvin Planck statement is violated and hence it says second law can be violated. Thank you over and out.


Merry Christmas, Freedom Family! It is the end of the year of Hindsight 2020. Hindsight is 2020? I'm so happy for the year to be over and the brand new 2021 is about to begin, and I hope you had a good year as much as it could be, and are also looking forward bright-eyed and bushy-tailed to the shiny new 2021 that'll be here very, very soon. Keep safe, have a happy holidays, and until the next year, I am George, and you've been watching... Oh, you're dreaming but love here. Click that I to partner with Freedom and join the Freedom Family so we can all grow together. You get many perks like position music, you also get epidemic sound, a lot of other access to royalty-free videos, sponsorships, and many things to help you grow. Just click the links down there in the description below to get involved in our community, our forums, our Discord chat servers, meet our graphics team, meet our community team, all of that on Discord and the forums. What are you waiting for? Get started. And we will grow together as a family because this is the Freedom Family. You are part of it, we are all part of it, and we're all growing together.


Good morning John. Aliens. I love aliens. And there's lots of reasons to do, especially in fiction. Adventure and danger and the unknown and self-reflection and mystery and revelation. But the thing I'm talking about here is real aliens, which is a very different thing. And I should start out by saying that I think that life elsewhere in the universe is a physical inevitability. Even if the galaxy were not inconceivably huge, which it is, the chemical system that is life obeys the laws of kinetics and thermodynamics. And while it is possibly the coolest, most interesting thing, it's not magical. Intelligent life does not seem inevitable in the same way. It just seems extraordinarily likely. In statistics there's something called the mediocrity principle. Basically if you have a sample size of one, chances are that that is from the most common data set. And to some extent we have pulled one solar system out of the hat and it has intelligent life on it.  Now there are some reasons to think that like our version of intelligent life as it exists right now on this planet is not super common, but zero reasons to think it's unique. But that's all preface. The thing I want to talk about is why do people like me, who have an understanding of how discovery works, how science works, seem to never think that extremely unusual things are caused by aliens. And that's relevant right now because there are these unclassified videos and there's a congressional report coming out about them, and we do not know what they are. But there are going to be, and have been many times, things that we really do not understand. This has happened a lot in the history of our species and it's often the first step on the path to discovery. We don't know exactly what's going on, we make guesses, we test them. That's how we end up knowing more things and being able to do more things. That's how we ended up with COVID vaccines and smartphones and in-depth knowledge of the Big Bang. But also when you are inside this world you know that there are a lot of big mysteries right now that are totally unexplained. We don't know what dark matter is, but it is most of the stuff. But also we don't know why anything exists at all, like why there's matter in the universe, that's a mystery, we don't know how life started on Earth. Also like we don't know why we sleep, we don't know why we yawn. But here's the thing, almost every mystery could be answered with the word aliens. And that makes it a certain kind of bad explanation. Now we want to understand things that we don't understand, that's totally normal. And one way to do that is to find an explanation that will explain everything. There's a fairly new version of this, which is that our universe is a simulation and whenever you see something that doesn't make sense, that's a glitch in the simulation. Now to me there are two big problems with explanations that can explain anything. First, every time we've ever had something go from mystery to not a mystery anymore, it has not been explained by something that could explain everything. We've got some biggies that have explained a lot, like natural selection or quantum mechanics, but they haven't explained everything. Like for example those two things don't do a great job of explaining each other. And the second thing, this is really important, I don't think it's not aliens. I think we don't know what it is. It's really important to be able to say that we don't know what something is without trying to immediately explain it. Whenever we do that, whenever we encounter something that's completely unexplained and we immediately try to explain it with some idea from our head, it's not based on data and observation and critical thinking, it's based on our biases and on our confusion and on defaulting to something that could explain anything. Things to know about these unidentified aerial phenomena, they are moving against the wind. They are detected by multiple sensors, including human eyes. They are moving in ways that known aircraft do not move. Are they aircraft? They might be. They might not be. The wrong thing to do is to dismiss these things as a joke just because like everybody's saying aliens. In science and in life you have to follow the interesting and this stuff's really interesting. It's weird. And honestly I'm a little worried that like the legitimate interestingness of this gets overwhelmed by the conspiracy theories and the just wild flagrant guessing about sexy things that could explain lots of other things. But most importantly, preferring a conclusion is the exact thing that scientists are taught to avoid. Investigating potential explanations? Absolutely. Figuring out new ways to gather more data 100%. But focusing on explanations that could kind of explain anything had so far not helped us explain anything. John, I'll see you on Tuesday.


We are nearing the end of our discussion of amplifiers, so the next couple of modules will first talk about the different designs of amplifiers that are there. And of course this discussion will be incomplete because really there are many, many designs if we try to discuss all of those it will become very boring. So I will give you an idea of sum just to understand that several techniques can be used and then in the next module we are going to briefly talk about the amplifier that we have. So right now we are discussing the different designs and materials that can be used to make oscillators and amplifiers. And as a recap we have discussed this already, we have discussed two kinds of amplifiers, one is a multi-pass amplifier in which the seed goes through the gain media multiple times and this is used for very short pulses no extra EOM or anything is used, so the pulses can remain short but the power that you get, the energy that you get per pulse, the extent of amplification is not as much as one can get using a regenerative amplifier. And in the last couple of modules we have discussed in as much detail as we could without getting into too much of instrumentation, the timing events that are there during regenerative amplification. And this is the one design that we have talked about, the design where the gain medium is in a cavity and then the seed is switched in to the cavity by using a lambda by 4 plate, it is kept in the cavity by a combination of the same lambda by 4 plate and the focal cell and then after the required number of round trips it is switched outside the cavity, out of the cavity by the second focal cell, so design of a regenerative amplifier where we have two focal cells but this is not the only possible design, there can be more. This is, once again now see in this discussion of today we are going to refer to optics journals very frequently optics letters and all, so this is an example of a regenerative amplifier design from 1993, that was the time when all this was being done, even now it is being done but we will see what is being done today. But whatever design we use now even that sort of got defined this 20, 25 years ago, so here you see here this paper where Moro is there, Corn is also there, the interesting thing about this paper, I mean there are many interesting things, one thing I would like to draw your attention to is that this is a collaboration between academia and industry. So most of the authors including Square to whom several designs are ascribed are from University of Michigan Ann Arbor but then there is one author who is from Clark MXR Incorporated and that is a company that actually makes and markets lasers and we will briefly talk about what kind of lasers Clark MXR is focusing on right now. But here this is an early report of an amplifier, repetition rate is still 1 kilo Hertz and what they had been able to do was they had taken 40 femtosecond pulses as seed, I would like to give you the feel of the numbers as well, this 40 femtosecond pulses were stretched to 370 to picosecond and then the specification of the grating is also given and it was amplified to 0.7 millijoule energy, what is the energy we get out of our amplifier? 4 millijoule, so this is not really as much as what we get today and after compression you get 0.35 millijoule, 55 femtosecond transform limited pulses, so it is not even 0.7 millijoule, 0.7 millijoule is what you get out of the region before it goes into the amplifier. There is always some loss at every stage during stretching during well during amplification of course it gets amplified but maybe not to these extent that you would expect in the ideal case and during compression also some loss is there. So you see before and after amplification 0.7 millijoule to 0.35 millijoule, half the power is actually so before and after compression, after compression half the power is gone and if you look at the full width half max of the seed and that of the output 45 femtosecond pulses were put in as seed and you get 55 femtosecond pulses, so not only have you lost energy per pulse a little bit after compression. You have in the entire process you have not been able to get back the entire, you have not been able to get back to the pulse width that was there in the seed. So I just like to draw your attention to this because when we do discussion without doing experiments many times we are geared to think of the ideal case scenario but ideal case scenario is very difficult to achieve there is always some loss or the other. The challenge of course is to minimize the loss and get to near perfect near ideal situations. So let me show you the design and here I would like you to I have not animated the whole thing not because it would take a long time but because now that we have seen all these pulses passing and the polarization getting flipped and all that I think we should now be able to work out what happens at every stage. So I will start I will get you started on how it works but then let us see if we can do the rest of the path together to start with see what is there I hope everybody can read what is there. So first of all you have this Arganion laser in 1993 Arganion laser was the state of the art DPS's lasers were just about coming mostly people use Arganion laser and even now there are people who work in imaging and all sometimes they prefer Arganion laser because it has very sharp lines but then it is very difficult to maintain it goes bad you have to change the tube which is very very expensive and then when you have changed it what do you do with that old tube it takes up space in the lab it is not easy to throw away a laser tube it will burst hazardous. So anyway that Arganion laser was used to pump the Ticep air oscillator output of the oscillator is 30 to 40 femtosecond 3 nanojoule can you read this numbers 3 nanojoule okay then it comes here and I have not really read whether the polarization was vertical or horizontal but demonstrate the phenomena I have drawn it to be vertical okay vertical polarization what is next in line thin film polarizer TFP what would happen when the vertical lipolarized light goes through the thin film polarizer no it is a polarizer it is not a polarization rotator so it will go through provided it has a component that is allowed by the polarizer it will not go through if there is no component. So here the thin film polarizer is set so that it can pass vertically polarized light so this is an important thing to understand and remember polarizers sort of act as gates they do not rotate the polarization right do not get confused between a polarizer and a Pockel cell and today we are going to talk about something else as well okay after TFP there is a lambda by 2 plate lambda by 2 plate would rotate the polarization by 90 degrees after that we have something called a Faraday rotator well the name sort of at least the second word is self-explanatory it will rotate what will it rotate it will rotate the polarization of the laser okay and what is a Faraday rotator what is the difference between Faraday rotator and Pockel cell they use two different effects Pockel cell uses Pockel's effect and Faraday rotator obviously utilizes Faraday effect Pockel cell Faraday are names of very eminent scientists so we have already discussed Pockel cells where you apply an electric field and that causes rotation of polarization in a Faraday rotator you apply a magnetic field you still apply an electric field which causes a magnetic field and it is a magnetic field that causes the rotation right now we do not need to get into the integrity of whether they are interchangeable or whether you want to use this or you want to use that right now let us just take it like that there is some magnetic field as long as the power is on like it is now it is going to rotate the polarization and in this case by 90 degrees so what happens when the vertically polarized light reaches the thin film polarizer which is set to vertical polarization it will pass through then when it goes to lambda by 2 then what happens it will be rotated by 90 degrees so vertical polarization becomes horizontal polarization then when it goes to the Faraday rotator it is turned back to vertical polarization why we are doing this we will understand in a while then what happens is it goes through the thin film polarizer so see the design is such that you use the same path until a point and then there is a branching from the second thin film polarizer that is there the light beam has a choice it can either go straight and enter the stretcher or it can get reflected and go into the compressor okay that will be determined by the polarization this thin film polarizer is once again set to vertical okay so vertical polarization will go through and if it goes through then it goes to the stretcher and gets stretched however if this was horizontally polarized it would be reflected by this thin film polarizer and would go into the compressor that is what happens after amplification okay for now go straight and I have not drawn its path all the way to the stretcher but you can understand it goes in this is a mirror from the mirror it goes to the grating first grating gets dispersed goes to the concave mirror which sort of focuses it on to a second grating plane mirror then retraces its path comes back here and then goes back all the way okay so here there is an alignment where it is retracing its path and this alignment is actually more difficult than what we have discussed for the amplifier earlier because now you have two parameters not only do you have to take care of the beam going in one direction the beam coming back should actually go through the same path of course people working with lasers for them it is not so unusual because when you have a laser you have two mirrors and the gain medium to and fro beams have to have the same path so I do not know whether that was the reason why it was designed this way but that this was what it is in most early amplifiers and maybe even some modern ones because one thing it does is that it can use the same pieces of optics multiple times for doing different things okay so it comes back and when it comes back it still retains a vertical polarization so it will go through yeah and when it goes through oops sorry that was too fast when it goes through what will happen thin-fin polarization polarizer let us see through Faraday rotator is now off so Faraday rotator was on at the beginning right right after the beam passes through it is turned off so now it is not going to rotate the polarization anymore so it does not rotate the polarization anymore what will happen at lambda by 2 plate it comes as a vertically polarized light and turns by the polarization is turned by 90 degrees it becomes horizontally polarized now what will happen when the beam retraces its path to this thin-film polarizer will it get through no because that polarizer as we discussed earlier is set to vertical right so now it will come to the thin-film polarizer it will be reflected and after this I have not drawn the path let us see without the aid of those arrows and donuts if you can figure out what is going on in the rest of the cavity okay so this is how the beam is stretched and injected into the cavity and we have to remember the polarization now what is the polarization vertical horizontal horizontal right so horizontally polarized light goes from thin-film polarizer first plane mirror second plane mirror thin-film polarizer again and then it has to be reflected so if it is to be reflected what is the polarization that this TFP allows to go through vertical understood so horizontal polarization is going to be reflected into the Pockel cell okay Pockel cell is on so I will coming back it will become vertically polarized so now it will pass through the thin-film polarizer and then it hits this curved mirror which focuses it on to the gain medium through the gain medium on to the high reflector here and this entire arrangement is pumped by a Q-switch NDA laser as we have discussed yesterday 100 200 nanosecond pulwitthop maximum big pulses understood so vertical polarization all the way now it does whatever number of trips it has to do and you see there are only 3 mirrors so after the required number of trips then this Pockel cell is turned on again so vertical goes in turns by 45 degrees while coming back turns by 45 degrees again so now when it reaches this thin-film polarizer once again it is horizontal so it will not get through so horizontally polarized beam comes back to this thin-film polarizer okay what will happen does this thin-film polarizer allow the horizontally polarized light to go through or does it reflect this is what we need to remember okay go back to the it will reflect right remember we started with vertical polarization and that light went through so horizontally polarized light will be reflected turns by 90 degrees okay now do we want the faraday rotator on or off because now what is the next step it should go in this direction is it not it should go in this direction into the compressor so should we turn the faraday rotator on or off or should it remain off should we turn it on should it remain off remember what happened the first time when the faraday rotator was on it became vertical once again and passed through this thin-film polarizer went to stretcher do you want that to happen no so you do not switch on the faraday rotator at this stage so then what will happen it will still remain horizontally polarized hit this thin-film polarizer get reflected into the compressor gets compressed 55 femtosecond 350 microjoule pulses come out at the rate of 1 kilohertz what is it that determines that the repetition rate is 1 kilohertz where did the 1 kilohertz come from yes yes this Q switch 10 diag laser is operated at 1 kilohertz so see the way it works is that you need a timing circuit a timing circuit is some electronic board that takes input from the yag laser from the oscillator and can drive all these focal cell and your faraday rotator ok so it is very important to do a precise time that is where electronics comes in big time so of course nowadays up to this stage nobody builds an amplifier to very honest because amplifiers with as short pulse as you want are available in the market but you might have to fix one you have an amplifier in your lab in many places and do not think it is any better abroad service engineers always take a lot of time to come does not matter which country you are in unless maybe you are in Saudi Arabia or some such place so sometimes you might be required to fix a laser when you are a postdoc or when you have your own lab sometime in some labs the old homemade oscillators and amplifiers are still working so if you happen to land in one such lab in a lab like that you might have to use it so it is better that you know ok so this is the design of a regenerative amplifier with a single EOM but then when I say single EOM I am cheating a little bit because I am a sort of not telling you that there is also a faraday rotator with that great next one more design and we will go a little bit quicker now I wanted to show this primarily because all this time we are talking about people who are laser specialists or engineers and all this laser is made it was built in the lab of a true blue physical chemist Robin Oxtaser and just look at the parameters 18 femtosecond shorter pulse 18 femtosecond pulses from a self-prolucidized laser were amplified to 60 micro joule micro amplification is not so much right of energy at 4.9 kilohertz so the laser they used to pump the region had a reputation rate of 4.9 kilohertz with chirp pulse amplification in a tie sapphire regenerative system after compression 30 to 35 femtosecond near transform limited pulses are obtained. So once again you see you start with 18 femtosecond seed you end up with 30 35 femtosecond amplified pulses so that is why whenever you get an amplifier you want the oscillator associated with it to have as short pulse width as possible okay right. And this is the design I will not discuss it in more detail but I encourage you to try and work out the path of the beam in this one yourself let this be a homework okay. And I just like to draw your attention to this fact that here they are using a prism pair to add some more of compression to so that the smallest pulse can be obtained and sometimes even this is not enough you might have to use extra cavity prism pairs okay. Now let us move to another kind of system all this time we have been talking about titanium sapphire and titanium sapphire alone so might have given the idea that there is nothing in the world other than tie sapphire with which your ultrafast lasers are made that is not the case. Another material that is used is called alexandrite alexandrite is chromium dope VAL204 the range of tunability range of emission is 700 to 830 nanometer. In fact I think in 1979 or so the first tunable laser sorry state was made using alexandrite so it is been there in the market for a long time but why is it that tie sapphire is so popular alexandrite is not so popular the answer is sort of there on this slide just see the bottom line this paper we are discussing by major and co-workers was published in optics express which year which year oh the year is not even there I think 2012 if I am not much mistaken the DOI is given 2012 even in 2012 read the first line generation of 170 femtosecond pulses at 755 nanometer from a car lens mode rock alexandrite laser was demonstrated output power is 780 milliwatt more or less like what we have in our sun actually less than that and to the best of our knowledge these are the shortest pulses that have been produced from a mode locked alexandrite laser to date. So even to go down to 170 femtosecond which is not a big deal with tie sapphire at all that has been achieved only 3 4 years ago so that is why tie sapphire is much more popular but alexandrite is also used for several reasons and I want to discuss alexandrite especially because I want to talk about an interesting architecture but before we go there this is your well since this paper was published in this millennium the figures are in color okay. So you see even these people like what hockstressers group did in 1993 they have used intercavity prism pair in fact in the photograph this is the alexandrite rod and here you can see this prism can't you you cannot see the other one but you can see one now what do you get this is the spectrum of the pulse what is the full width half maximum 3.6 nanometer actually very narrow it is sort of like our diode lasers and full width half maximum of the pulse how much is it 170 femtosecond and that is the best one can do using an alexandrite laser but think of it this would be an interesting or useful laser if you want to do TCSPs spectral width is narrow the problem with using TICIFI laser for excitation or doing microscopy is that it is a broadband laser many times if your laser is spectrally very narrow you can and if it is tunable then you can excite molecules in different environments with TICIFI that I mean shorter the pulse that advantage is compromised to a greater extent because spectral spectrum is very wide here the spectrum is narrow so that is actually an advantage disadvantages you cannot go less than 170 femtosecond okay so this is the alexandrite laser and then I want to show you something interesting here then what we will discuss next couple of minutes is actually a little bit of digression in the sense that it is not ultrafast laser it is a CW laser but I still wanted to talk about it because it is an interesting design and I think that this could perhaps be one of the important designs for future where one people might be able to make ultrafast lasers using that design and you will see why that design is fascinating so here this paper is from last year and this is actually a conference proceeding proceedings of SPI SPI has this photonics west conference in San Francisco every year in the month of February there are several smaller conferences in it so this one I think was part of LAAC which is a laser conference so there LED pumped alexandrite lasers were demonstrated so see until now we have been talking about DPSS pumped lasers right if you can use LED the cost as well as operational issues go down by orders of magnitude right and now LEDs are so common place these are LEDs the light we have in this room okay everybody is familiar with LED there everywhere so what they had done is they had taken so this is the LED right here I have forgotten the size I think it is 1 centimeter by 1 centimeter they had taken LEDs that gave out emission which peaked at 450 nanometer that emission was concentrated by using something called a luminescent concentrator I will discuss how it works and these concentrators are very much topic of contemporary research only yesterday we had visit by an inorganic chemist who talked a little bit about how he is trying to make concentrators will come to that now this concentrator material that is used is cerium yag and emission of cerium yag is at 550 nanometer emission of LED is peaked at 450 nanometer and it it overlaps very strongly with the absorption spectrum of cerium yag which has a maximum at 460 or something somewhere like that then emission of the cerium yag is at you see very broad emission 550 or so and this emission spectrum has a very strong overlap with the absorption spectrum of alexandrite so the idea is use an LED excite cerium yag it will emit somehow concentrate that emission and deliver it to alexandrite now alexandrite can emit okay so we will take a break now and we will come back and we will start our discussion from how aluminescent concentrator works


The study reports high-resolution CREOEM structures of the yeast serene palmitoiltrants, SPT, in complex with TSC3, OM1, and SAC1, revealing tight interactions between seramide and OM1 that inhibit SPT activity and suggest co-regulation of sphingolipid biogenesis and sterile metabolism within the spots complex.


I'm so happy to be here because the fact that we're able to sit with people from China, from India and Spain talking about how film is shot and produced and distributed in their regions I think will definitely help with other countries emerging markets like Nigeria that are trying to emulate places that we probably don't understand the market and what led to them doing business the way they do and let's learn from them and get Nigeria better, Nigeria movies better. If we look at Bollywood or Hollywood films you'll find that sometimes the marketing budget they have for the PR angle for example is always very significant. Buy and Laugh Nigerian movies focus more on how much money they want to spend on shooting the film. A lot of the times they focus more on the money for the technical side, the equipment and then they don't pay that much attention to things like okay which actors are we putting in. Sometimes they put actors in it that are just known because they're maybe legends in the game but they might necessarily be perfect for the role and then they don't publicize it well enough and so I'll give you an example there's a movie called 50 which they did a lot of publicity for and so much so it meant that when Netflix came around to wanting to buy rights to put it on Netflix they actually paid them more than they paid any other Nigerian movie on that platform so far. PR. A lot of people will tell you that piracy is a huge issue in Nigeria and it is but I'll take that back and say that for me I think that it's more about the story that we're telling the stories we're telling why we're telling it and who we're telling it for. Once we can figure that out and sort it out we'll do a lot better with us Nigerian films.


And so now, turning to the question I get asked a lot when I do different meeting interviews or from my own clients, is what are the signs you look for that would get this all clear that the downturn we're in is over and that we can return to being bullish again on the market? And what I tell them is that we look for market thrust. What a thrust is, is this big explosion of buying activity. This really strong advance in prices of individual equities. We see a lot of stocks hit, for example, 20 day highs. Here's looking at the percent of the S&P 500 that's at a 20 day high. And we have at least half of the market hit a 20 day high all at the same time. That's showing us that there's a lot of buying interest in the market. And you see, here's a green line plot at each time that occurs. And it typically happens after we've had a major decline. Very rarely do we see half the market hit a new, essentially one month high at the same time when the bears or when sellers are in, in a lot of control of the exchange or of the price action. Looking again, turning back to that six month data. So we're using the same data just from different viewpoints. Again, looking at six month highs and six month lows. Here, we're getting looking at when we have 20% less than 20% of stocks making six month net highs. And when we get back to 80% tells us that we have a really strong stock market. And you can see this happened at the bottom after the dot com period at the bottom in 2009, 2016 at the start of 2019 and then in about April of 2020. These types of market improvements happen after bear markets occur. A lot of people try to time and they wanna get the absolute low and you can try to catch the proverbial falling knife. Instead, if we look for market improvement, we don't initially care that there's 20% of the market that's making new lows. We wanna see when the market's making improvements. And we have a net, a lot of stocks that are making six month highs. That tells us when the market potentially is at as put in a final low and buyers have resumed control. There's a lot of very popular very commonly discussed breadth-rust. Probably the most well-known one was invented by Martin Zweig and it's called the Zweig breadth-rust. And it happens very rarely in the stock market. What Zweig was looking for was for the 10 day average of advanced declines to go from 0.4 to over 0.615 within a two week time period. You can see going back to 2008, this hasn't happened very often. The last time this happened was actually just a few days after the market low in 2018 and actually hasn't happened since then to have that go from under 0.4 to over 0.6 within that short period of time. It's pretty difficult to do. But what it does happen has been a pretty good signal that the market has reversed higher and that buyers are back in control of the price action. Another popular one was developed by Ned Davis Research. And what they were looking for was 10 to one updates. Meaning if you take the number of stocks that are advancing divided by the number of stocks declining, they want at least 10 advances for every one decliner. And they wanna see that happen two times without the reverse happening. Meaning without having 10 decliners to every one advance. And when you had two days that are 10 to one within a two week time period, again showed the market was pretty strong. This has happened just three times since 2008. Happened right after the 2018 decline after COVID crash. And this one actually has happened within our current market. You can see the green line, most recently on the far right side happened in July where we had a really strong about 15 to one. And then we had a really strong day of actually over 40 to one advances to the decliners as the market had put in its low advance for several weeks and these really, really strong updates. Tons that buyers potentially were back in control. And so something that I focus on is looking at a lot of different gauges of breadth. I tracked about 11 or 12 different ways of measuring these different thrusts and I created a composite. And so from my research, I wanna see not just one or two of these thrusts occur. I wanna see three, ideally four of these thrusts occur. When we start seeing a lot of these really strong bullish days in the breadth data really gives us that confidence that the market is potentially put in a low. Again, nothing works perfectly and breadth thrusts aren't the end all be all crystal ball to the market but these are great signs that tell us when the market that there maybe have been a shift in the market from sellers to buyers. And you can see the green lines here is every time we've had, I believe it's four breadth thrusts from the collection of 11 or 12 that I track. And typically the market has had really strong performance after those periods of time.