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Module 1: The Main Source of Clean Energy

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Radiation Coming From Stars

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This is in joules in of energy going from 10 power minus 33 joules to 10 power minus 12 joules right. And an electron volt is a joule and the if you want it in an electron volt is 1.6 into 10 to power minus 19 joules. So, one electron volt is 1.6 into 10 to the power minus 19 joules okay. So, it’s the charge of an electron 1.6 into 10 to the power minus 19 coulombs. So, a coulomb volt is a joule. So, 1.6 into 10 to the power minus 19 coulombs is the charge of an electron into 1 volt. So, that combination gives you 1.6 into 10 power minus 19 joules. So, if you use that constant, you can convert this axis that you see here in which is in joules to this axis which you see here which is an electron volt. Using this relationship that one electron volt is 1.6 into 10 power minus 19 joules. So, with these kinds of relationships, you essentially have the same spectrum here. So, in the middle is the spectrum here, the spectrum that you see out here that is the spectrum that is their common across all of this axis, you simply have the same spectrum being explained to you as a range of frequencies or as a range of wavelengths as a range of energies in joules and as a range of energies in electron volts. So, this is what you see in this plot that you have got in front of you. Arrange of values across different you know variables that you can look at to explain this spectrum. As I said a tiny fraction of it is the visible spectrum, and so, you can see that you know even though we see a rich range of colours you know violet, indigo, the entire VIBGYOR range of colours going all the way up to red from violet to red. In the grand scheme of all the, you know electromagnetic radiation that is out there that’s a very small fraction of the spectrum. So, incidentally, I mean we all give out as I mentioned in our discussion about blackbody radiation, at any temperature bodies are giving out radiation right. So, even we as human beings or any animal that is out there is also giving out radiation corresponding to its body temperature; for example, we are all giving out infrared radiation alright. So, that’s why if you look at the military applications they have these things called infrared goggles. So, they wear infrared goggles in the night, and that is what they also refer to as night time vision goggles. So, in that, they have sensors which are sensitive to infrared radiation. So, in the night if there is some person that is walking, just because of the heat of their body they are giving out infrared radiation and there are sensors on this glass, which are sensitive to that infrared radiation they convert that to some visible colour, that your eye can then see. And that is how we were able to see that there is a person in front of you even though otherwise there is no light in that area it's pitch black you are still able to see this person standing in front of you at some distance. So, therefore, we all give out radiation. So, this blackbody radiation is not you know an esoteric concept, it’s not some theoretical concept, that you know you wonder why you are learning. It’s a real thing, were all giving out that radiation, the radiation that is coming off of us corresponds to blackbody radiation coming from stars corresponds to blackbody radiation, and interestingly radiation of the background of the universe people have found out the temperature of the background of the universe which means what? It is the temperature of the space, that is there all around us in the universe right that we presently think of as the space that is there between the stars that are where there between the galaxies etcetera. That space has some temperature associated with it, it is not at 0 Kelvin it turns out that it is around 2 to 3 degrees, Kelvin. Somewhere in that temperature range is what the background temperature of the universe is, which means that in under natural circumstances this is due to radiation that is there in the space, due to the formation of the universe from the time of the formation of the universe. So, the theory is that the universe has been cooling since then and some background radiation is still there, that gives you this background temperature. And it means that nowhere in the universe you can naturally get a temperature less than this. This is the minimum temperature that is there, at any place in the universe. So, naturally, you will never see a temperature lower than this. If you ever see a temperature lower than this at any location, it means it has been artificially created, it means some intelligent life created it. So, this is the way you know this is a fascinating concept; it does not seem like much it’s just radiation; it’s a spectrum. But when you understand the significance of the spectrum you understand the features of the spectrum, and then you understand you know what would happen if you went to one extreme of the spectrum to the other extreme of the spectrum, what does the spectrum represent, and what can you infer from what the spectrum reference represents, then you find all these interesting concepts. You can learn about the universe, you can learn about stars, you can learn about the space between all the stars and galaxies, and you also know can have you have the possibility of learning about intelligent life if you ever discover something colder than this right. So in fact, in the labs these days it is possible to reach temperatures lower than this 2.75 degree Kelvin or something else, much you can go much lower than that in labs. But the point is those are temperatures that are not naturally present anywhere in the universe. So, that is a phenomenal accomplishment, imagine that in your lab you have a small setup which does something, and you can say with confidence that this what you are just now seeing in this lab does not exist naturally anywhere in the universe okay. Okay so, that is not even though you have never been to the rest of the universe. So, you can say that with confidence simply because of this understanding of the background temperature of the universe, which has got directly to do with the fact that background spectrum is consistent with blackbody radiation, and they can tell what is the temperature of the background, simply based on this blackbody radiation behaviour okay. So, this is the spectrum, you can see here as I said this is the visible part of the spectrum. So, just for you to get an idea of you to know what values you are looking at here if I extend this up here. So, you are looking at less than 10 electron volts right. So, this is 10 electron volts, this point here is 10 electron volts, this point here is 0.1 electron volts right. So, if you look at this you are somewhere below 10 electron volts is the energy corresponding to this, and in terms of wavelength, if you just go down here, you are looking at a value that is around there. So, it is a little higher than 10 power minus 7 in meters. So, it’s more like 10 power minus 6 or in that range between 10 power minus 7, 10 power minus 6 and. That’s what it is. It is less than a micron 0.4 to 0.7 microns is what you are looking at as the wavelength of visible light. So, the wavelength of visible right is somewhere in the 0.4 to 0.7 microns or 400 to 700 nanometers, and its energy is around less than 10 electron volts is what you are looking at. (Refer Slide Time: 37:31) And more specifically you will see that we will look at those 2 values in just a moment, but what I wanted to also highlight is the fact that the solar radiation, the radiation that’s coming from the sun which has this you know the range of the spectrum, but the visible spectrum is what we tend to see the most. Has a set of features for it's you know spectral distribution, which matches that of a black body of temperature roughly about 5500 degrees C that is the temperature of the surface of the sun. So, what you see as the surface of the sun has a temperature of about 5500 degrees C and so, corresponding to that you have the blackbody radiation that I am just showing you here this curve that you see here and so, the radiation from the sun that reaches the outer reaches of the atmosphere. So, if you come to the earth’s atmosphere and then you cross that earth’s atmosphere, let’s say you are more than I say 12 kilometres above the surface of the something like 20 kilometres above the surface of the earth. So, there was about 80-90 per cent of the atmosphere is now below, you are very little of the atmosphere is above you. At that point, the radiation that’s reaching the earth has the spectrum that you are seeing on your screen okay. So, so that is the spectrum you see and that is blackbody radiation spectrum corresponding to a temperature of that body sitting at a temperature of 5500 degrees centigrade. So, that’s. So, therefore, this is a very beautiful aspect of science than physics that people have discovered, that you can say so much about the sun by just looking at the spectrum right. So, now that is at the outer part of our atmosphere. From there the radiation has to penetrate through the atmosphere and reach the surface of the earth. Now as it penetrates through, you have various gases in the atmosphere you have water, water vapour, in the form of you, no water vapour, moisture, clouds and everything that is there, we have oxygen and we have CO2 and so, the range of gases that are present, these are some prominent gases that are present. So, these will absorb radiation. So, they have some natural frequencies at which they absorb radiation, corresponding to various vibrational frequencies of the molecules that are present. So, they absorb this radiation. So, what reaches the surface of the earth is a slightly modified version of the curve that you see on your screen, because this is what reaches the upper surface of the atmosphere, and as it comes through at specific wavelengths you have absorption. So, at specific wavelengths you will see a drop in intensity, this is just a schematic that I am showing you here. So, corresponding to this you will have some drops in frequency and these correspond to I am sorry a drop in intensity. So, these correspond to absorption that is occurring at those frequencies, due to the gases that are present. Due to the various gases that are present in the atmosphere, you see absorption occurring at specific frequencies. So, when you are at the surface of the earth, at those frequencies you don’t have enough intensity. Those frequencies have been taken away by the absorption phenomena that are occurring between the surface of the earth and the upper surface of our atmosphere okay. So, that is how this happens, and we also find that interestingly chlorophyll which is there in our plants has an absorption spectrum which sort of has you know some features in this region. So, it can able to absorb very well in this region of frequency. So, chlorophyll absorbs very well, absorbs very well in this frequency range and so, it’s something that has evolved in nature in the plants, in the leaves of our plants, that has a very good absorption spectrum in the range of the visible light. And since it has evolved that way it can do a great job of capturing the sunlight that is coming in, and carrying out the photosynthesis process right. So, that is how the plants are interacting with the sunlight that is coming in. Because they have this pigment in them which has an absorption in occurring in the right set of frequencies, which correspond very nicely with the spectrum coming from the sun okay. So, you can see, therefore, a fair bit of the solar spectrum is in the visible range, but you have quite a bit in the infrared and the ultraviolet range also so in fact, you will see later that a fair bit is available in the infrared. So, if you only capture the visible spectrum, we are not doing enough justice to the solar spectrum, a significant part is available in the infrared and the chlorophyll that exists naturally in plants is doing a good job of capturing this sunlight because of the absorption spectrum and how it relates to the spectrum coming from the sun okay. So, that’s the reason why it was of interest and that’s the reason why I have spent this you know some time in this class, looking at the solar spectrum looking at electromagnetic radiation in general and solar spectrum in particular. Because it tells us a lot about what is happening in our nature, what is possible from our perspective when we try to make devices that are trying to take advantage of the solar spectrum? (Refer Slide Time: 42:28) So in fact, as I just pointed out when we looked at that spectrum our visible spectrum ranges from 400 nanometers to 700 nanometers, which is what you saw in that plot that I just showed you, and the corresponding band gaps are about 3.8 3.1 to 1.8 electron volts. So, these are some you know parameters that we should be aware of. When we utilize these parameters I will again draw your attention to these values. So, that there is continuity and we did make that discussion, but this is the sort of range of values that we are looking at for the spectrum and so when we try to capture it, we have to be aware that our ability to capture should be good in this range, and that’s the point that we need to bear in mind. (Refer Slide Time: 43:08) So, in conclusion, we have seen looking at the solar energy spectrum, and we see that it is consistent with blackbody radiation kind of behaviour; we have looked at the electromagnetic spectrum in general. The solar spectrum, in particular, we also see that the visible spectrum, it’s a very small fraction of the electromagnetic spectrum. So, you have very large electromagnetic spectrum, a wide range of things are happening in that electromagnetic spectrum, and we utilize various parts of this electromagnetic spectrum fairly routinely, for various you know activities that we do have, for various technologies that we use we are using this electromagnetic spectrum. The visible spectrum is a very small fraction of this overall electromagnetic spectrum. In fact, in the entire diagram that I drew is a very thin region was the visible spectrum. There is so much in other parts of the spectrum, and we also found that you know it so happens that the chlorophyll that is there in the plants is ideally suited for absorbing the visible spectrum and therefore, does a great job of capturing solar energy doing photosynthesis and creating you know the food that we can utilize okay. So, with these comments, I would like to conclude this class, where we have looked at the electromagnetic spectrum in general, and the solar energy spectrum in particular. Thank you.