 Hi, I'm Zor. Welcome to In user education. This lecture continues my few series of lectures actually about light. In this particular case we are talking about photochemistry, which basically is a combination of light and electromagnetic oscillations and chemistry and how this electromagnetic oscillation can affect the chemistry. This lecture is part of the course called Physics 14's presented at Unisor.com. I do suggest you to watch this lecture and every other lecture from the website rather than from any other source you might actually find something like YouTube because this is a course which means there is a menu, it's a hierarchical menu, this lecture is part of certain chapter, chapter is part of the part of the course and lectures are very much depend on each other. So there is also math 14's course on Unisor.com which I consider to be a prerequisite to this course. It's a solid math course for high school and maybe a little upper, maybe first year of college, but that's what you need to study physics seriously. Now the website is totally free, you don't even have to sign in if you don't want to. Now there is certain functionality of the website which depends on the sign in name, but again it's totally free, there are no advertisements, no financial strings attached, no monetization, so just study and gain some knowledge. Okay, now let's talk about photochemistry. First of all, we are talking about light and how light affects the chemical reaction. Well, let me start with a very dramatic statement. If not for photo chemistry, there would be no life on earth because everything which grows around us in the process of growing all these trees and grass and whatever we have, they absorb light and it actually affects the chemical reaction inside. It's a photosynthesis, it's cold. That's how molecules are built, that's how trees are growing, that's how wheat is growing, I mean everything. Now there is also effect on human bodies. I mean there were cases when people grew up without sun and obviously their development was significantly depleted. So sun is necessary and basically it confirms that photochemistry is extremely important for life on earth. So number one, I wanted to exemplify this particular photochemistry kind of relationship in something which is called photosynthesis, that's how everything grows. Now I didn't mention it before but this is one of the few lectures where we will have no formulas. Very rare occasion and it will probably be a very short lecture too. I'll just exemplify certain photo reaction, photochemistry reactions and again photosynthesis one of those. What else? What else is generation of vitamin G inside of our body when our skin is exposed to sun? It's a clear example of photochemistry. Now to tell the truth I don't think we understand this process. I mean we know what's happening but to properly explain this generation of vitamin D for example or photosynthesis I don't think we have a clear explanation of how it actually happens because if we did we might have been able to basically replicate it. We cannot. We cannot produce life generally speaking whatever we think about life whether it's a life kind of animated kind of life or planned or something we don't. We only can just use whatever we already have like we can take a branch of a tree and grow into a new tree but that's not what I'm talking about to understand how the process is made down to nitty gritty. It means we can replicate it completely and we cannot. So whoever created this life really did a very good job. Okay what else as an example? Okay we have used lots of plastic and we know that plastic degrades with the time. Why? This is example of photochemistry. Sunlight when it actually directed onto plastic. Gradually you see plastic contains very long molecules polymers whatever and under the sun there are certain chemical reaction which actually breaks a long molecule into smaller pieces. That's photochemistry and that's what actually happening with plastic when it degrades under the sun. You see something simpler than plastic something like metal for example it's a very short molecule couple of elements and that's it but plastics are very long molecules containing like carbon and nitrogen or something else. Now these long molecules are breaking under the sun. Now I promised not to have any formulas in this lecture. I will not have the real formulas but I will use certain symbolics to basically explain how the photochemical reaction can happen a little later when I will finish with all the examples I have almost finished. What else? Okay our eye we see things now what does it mean that we see things? Well electromagnetic oscillations light visible spectrum of life of light comes into the eye and it causes chemical reaction. Chemical reaction in turn excites the nerves inside the retina somewhere in the eye and these signals are going into brain. So that's basically how we see things and again right in the middle of this is a chemical reaction inside the eye which basically happens in response to light coming through our pupil and based on different picture whatever we see different parts of the retina are activated and based on different frequency of light the activation has different degrees because there is different energy carrying which is carried by the light so it actually causes different chemical reaction in different places of our retina. All these signals are going into brain and that's how we see things and the last which I have in my list I mean there are many many different photochemical examples solar cells again under the sun there is something happening in like probably it's a semiconductor whatever is underneath of the transparent covering and something happens it's a photochemical reaction and as a result it produces electricity converging basically the energy of sun which is carried by electromagnetic oscillations of course waves into certain other form of energy in this case electricity. Certain amount of energy is lost but certain amount of energy is still kind of used. Okay now I wanted to talk about different it's a schematic representation of these chemical reactions how many different types of chemical reaction are possible. So I will use components which participate in the chemical reaction just with letters a b c so one of the first things is a b plus plus light now I have already mentioned a couple of times that light carries energy now before like at the end of 19th century physicists were thinking about light energy as infinitely dividable and it's like flowing well in a way like waves on the sea so it's a wave theory of light and energy can be any basically it all depends on amplitude and frequency. In 1905 Einstein published an article which was actually considered to be true or at least true at that particular time and hours not until 1922 if I'm not mistaken like 17 years later people were very much inclined to the wave theory of light because it explains all these diffraction interference etc etc however photoelectricity effect which I have addressed in the previous lectures was not explained using pure wave theory because always specific frequency was able to kick electrons out of some metal plate so Einstein came up with certain theory that energy which is carried by oscillations electromagnetic oscillations cannot be infinitely divisible there is a minimum which depends on the frequency and the amount of energy is equal to Planck constant times frequency of light. Now Planck constant came from Max Planck was like 10 years before Einstein was researching how heated objects emit the electromagnetic oscillations like for instance you heat piece of metal and it's like red hot or even white hot so all these colors are basically emitting of electromagnetic oscillations it's light right so Planck was researching that and he actually came to this we call it right now quantization of energy means that energy can only be emitted or absorbed in certain chunks called quanta so it's a plural singular quantum so right now we are talking about this as a quantum of energy another word for this is photon which everybody heard so this is a photo this is basically light energy so what happens if something which contains two different elements consumes certain amount of energy well one of the reaction which can happen is this these two elements are separated so this is one type of photochemistry photochemical reaction so this chemical reaction a b to a plus b this is something like one molecule which contains two elements and these are two elements separately or a bigger molecule is separated into two smaller molecules like degradation of plastic under the sun so this is one type of photochemical reaction another type when you have certain molecule or element under the sun it just converts to another one another type of chemical reaction now the third one plus b plus h f which means you have two separate elements and energy of light it can convert to you combine these two elements and maybe there is something left over in some way now these are most formulas don't take me wrong the i promise this lecture not to have any formulas these are most formulas i'm just trying to graphically explain what i was just talking about chemical reactions with all kinds of different chemical reactions are possible based on this another a plus b c plus h f so there is a region regrouping a b plus c so b being connected to c now it's connected to a under the influence of light and another one i have two separate elements i added some energy and as a result i still have two separate elements but one would be positively and another would be negatively charged so basically there is a separation of electrons regrouping of electrons from one to another for that regrouping we need some energy and that's the energy so all these are regrouping of atoms and electrons between different elements or molecules and it's all done based on energy supplied by light okay this is basically a very short lecture again no formulas these are not formulas and i was just trying to explain how light actually affects the chemical reaction that's it probably next lecture we'll have formula i will try to explain basically a little bit more in more details how exactly these electrons are jumping from one place to another under the influence of light and how they can emit light if they're jumping into proper place all right that's it for today i suggested to read nap notes for this lecture basically it's exactly the same which i was just talking about my big deal but notes are on unison.com you go to physics 14 course choose the part which is called waves and then if you open the next page whatever comes up as a menu you will have photochemistry as a topic it's a very short topic so just you know familiarize yourself with it that's it for today thank you very much and good luck