 Okay, so let me start the class, okay, so let me start the class, today's class would be about modern physics and I have tried to include as much as I can your board syllabus and the objective of this class would be to give you enough exposure from the board perspective about the topic modern physics. So what I have included in this particular chapter is first I'm going to discuss cathode rays, production of cathode rays and cathode rays too. I will also discuss what are the properties of cathode rays, usage of cathode rays, then I have moved to x-rays. After I do x-rays, why it is important to understand cathode rays is that it is used in production of x-rays. Then properties of x-rays I have included and then I have gone to radioactivity. So I'll start from natural radioactivity and then I'll go to alpha, beta and gamma radiation, properties of alpha, beta and gamma radiation, then radioactive transformation, radioactive series. We will also be as half-life period and all these things are not there in your syllabus. I have not included that in this. Then I will move to isotopes, isobar, isotones, artificial transmutation and then I will discuss with you the nuclear reactions, so nuclear fission reaction and nuclear fusion reaction. So this particular class would be about the topics that I have just discussed with you. If you want me to cover any other topic apart from this, you can write that in the chat box. I would be covering that topic also. Also what happens is that nuclear fission and fusion reaction has been explained in your book, but you don't know what is binding energy and what is mass defect and what is atomic mass. So without using these things, it will be very difficult to explain the concept of nuclear fission fusion. So what I intend to do in this class is to not only do the core topic, but also take the peripheral topic so that your concept becomes clear. Now I'm not saying those peripheral topics would be important from the board perspective, but having said that, what happens is if you have a little bit of idea about those peripheral topics, it will be helpful for you to understand the topic well and then perhaps you can write the answer more beautifully so that you can fetch more marks. So this is what I'm going to do in next one and a half, two hours. This will be the last class of physics. We have covered almost all the classes and hence there are three more classes left out next week and that will be end of your revision syllabus. So let me start with the topic. So I'm starting with cathode rays. Now what are cathode rays? So cathode rays are produced due to electric discharge or discharge of electricity. What do I mean by discharge of electricity? So what happens is if you see that or if in normal air you start passing current at low voltage, there would be no conduction of electricity. If any live wire is kept near you in the air and if you are not touching it, you won't feel any electric shock. Why? Because conduction of electricity in the air at normal temperature is not possible. So how electricity is produced inside gases? So try to understand that if we have very high potential across a portion of gas, there would be discharge of electricity and this particular principle, this particular concept is used to produce cathode rays. So what happens is you look at here, I have taken a cathode ray tube in which if I discuss about cathode ray tube, this is my high voltage generator which is giving very high, this is creating very high potential difference. This is around, this creates very high potential difference. What are those? These two plates, these two plates, the positive side is called anode. It means the positive side of the battery, the plate connected to the positive side of the battery is called anode and the plate connected to the negative side of the battery is called cathode. Now you have seen here, this is what our vacuum pump, now why vacuum pump has been used here? You understand the first concept that I told it for cathode rays was that gases do not conduct electricity, no conduction of electricity at normal potential difference or at normal voltage. Now it has been seen that if I take gas and I start providing it or I start applying high voltage across it, so we find that there is some discharge of electricity, why does it happen? How does it happen? I will explain now. First you understand why low pressure, why air at low pressure has been taken here? So air has been taken as at low pressure over here because of the reason that if more number of air molecules would be there, hence if any particle is emitted from here, so it will collide with more number of particles here and if it collides with more number of particles here it will lose more energy before reaching this cathode tube. So if particle which is emitted from here loses more energy before reaching this cathode tube it will not be able to transfer its maximum energy to the particles which are available in this cathode tube. So try to understand what happens over here is that when, let me just remove it, that when I supply some power in electrical power to this particular cathode tube what happens? It's the electrons over here will gain energy and due to that there would be ejection of electron particles from here which will go and collide with this cathode tube. Now what will happen is due to that reason what will happen that, one second, this is, so due to that reason what will happen is that when electron collides with any other particles it will go and transfer its energy to the particles in the other plate and due to that transfer of energy there would be radiations which would be emitted from the other particles. So what happens in case of cathode rays is that we take this discharge tube, this is called discharge tube, this is 30 centimeter to 40 centimeter and nearly 2 to 3 centimeters in radius in in diameter. Now we have two plates, these plates are made of aluminum. Now once you give this particular cathode plate a lot of energy electrons would be emitted and will go and strike here at anode. The higher potential will work in this manner, the more the potential more would be the attractive energy for the electron. So electrons will go, I mean electrons will have more energy, more potential energy with them so that they can go and strike with the other plate. So that is why this higher potential is used so that higher potential difference is available. Due to higher potential difference more potential energy would be there in the electrons emitted. So that will translate into more kinetic energy of electron and electron will go and hit the other plate with more energy. The more the energy transferred by the electrons to particles with which it collides the more would be radiation emitted. So this is what the fundamental of cathode range emission. So I have already explained that as electrons are traveling in this area I keep the pressure low, I keep the pressure low because this electron will not collide with too many particles and hence will not lose much of its energy. Hence what happens is, hence what happens is that we are able to transfer maximum number of electrons with maximum energy possible. Now what happens? So what happens is when the pressure of the gas inside the tube is gradually reduced you will see that electric discharge passes through it. And when the pressure inside this tube would be nearly 0.01 mmHg this tube over here will become dark. So you will find that this tube is becoming dark. At what pressure when the air pressure is 0.01 mmHg you will find that this particular tube is becoming dark. And the wall of the tube opposite to the cathode initially I told electrons are emitted from anode that is not correct. Electrons are emitted from cathode because this is the negative potential has been given over here and that is why positive energy has been given over here. Why positive energy has been given over here? Because electrons are negatively charged. The more the positive potential here the more the force of attraction will be towards the anode. So what happens is the wall of the tube opposite to the cathode it means that the anode will start glowing and a green fluorescent you will find that a green fluorescence would be coming out in form of radiation from anode tube. This is what cathode ray is. So what did I say let me summarize cathode ray and cathode ray tube for you. So first let me summarize cathode ray tube so that you are very sure that what are the parts of cathode ray tube and then let me go to the working of cathode ray principles of cathode ray and properties and uses. So cathode ray is nothing but cathode ray tube is nothing but tube which is 30 to 40 centimeter long and 2 to 3 meter in diameter. We have a DC voltage over here which is of very high potential and we have the terminals of this voltage generators are connected with the plates. These plates are made of aluminum the positive side of the battery is connected to the plate that plate is called anode and the negative side is connected to another plate which is called cathode. Now what happens we apply a vacuum pump over here so don't forget to make this vacuum pump the vacuum pump is to lower the pressure why pressure is lowered I have already explained it I will repeat it also. So this is how construction of the cathode ray tube is now what is cathode ray what happens is the principle is that the air does not conduct electricity at ordinary pressure and but what happens when we apply very high voltage then you will see that there is discharge of electricity what is discharge of electricity conduction of electricity through gases is known as discharge of electricity or it is also known as electric discharge so when at certain potential the inside the gas will act as a medium through which there is a conduction of electricity. Now how that conduction of electricity happens is that from this negative side because of the very high voltage what will happen is the electrons will be emitted they will go strike over here and radiations will start coming but at very low pressure of 0.01 mm hg you will see that this particular tube becomes dark and you will see green fluorescent light coming out of that that becomes your cathode ray. So this is what I wanted to discuss for cathode rays now let me go to what are the properties of cathode rays and what are the uses of cathode rays so first let me go to the properties and then I will yeah so this is the property of cathode rays the first property is cathode ray travel in a straight line now these rays are deflected by electric field it is obvious because these rays are nothing but electrons so I told that the particles emitted in terms of radiation would be electrons whenever these electrons will pass through electricity some force would electric field some force would be applied on it and due to this application of field the cathode ray will get deflected if positive side is there it would be deflected towards it if negative side is there it would be deflected away from it because this is a bunch of electron traveling together now these ray travel from the surface of cathode the direction to these rays is independent from the position of anode now these are not important ones these ray are independent of nature of gas and electrodes placed in discharge tube this is very important so instead of air if you put any other gas it doesn't mean that the property of the cathode ray will change why it will not change because cathode ray is nothing but a bunch of electrons traveling together so property of electron does not change whether it comes from a cathode it comes from from an aluminium material to air or it comes from aluminium material to any other gas so that's why it has been given now cathode ray possesses very high kinetic energy due to its high speed so I already told that this high pressure high voltage difference is created so what happens is due to this creation of high voltage over here and very very less pressure this less pressure is kept only because the the particle which is going and striking should have maximum energy so that it can transfer maximum energy and the radiation has equally equally good amount of energy so this is what it has been telling that it is it has more kinetic energy with it now let me go to another property which says that these rays are also deflected by magnetic field now why these rays are deflected by magnetic field we have learned in electromagnetic induction that in magnetic field if there is an electron it will it will it will force some amount of if there is an external magnetic field so it will force some amount of f is equal to q v into b so q is here in terms of electron it is traveling so v is there so v is there so f would be q v into b I have already discussed that so due to this some force would be applied and you will find that you will find that these these cathode rays are deflected in magnetic field now next property is they can produce fluorescence so we have already seen how fluorescence is produced they can they can affect a photographic plate why they can affect a photographic plate because because of the property of fluorescence these rays has small ionizing power why they have small ionizing power because if they strike any particular atom because of its kinetic energy they will be able to they will be able to take out one or two electrons from its orbitals and they ionize it now next one is cathode rays travel with high high speed values from 138 to 110 of speed of light they are used to produce x-rays they have some penetrating power because of kinetic energy they are behave like waves and exhibits process of interference and diffraction so these are the things that you need to take care of nobody will ask you 15 different properties of cathode rays maximum you will be asked 3 4 or 5 properties so you should remember 3 4 5 properties from here try to write properties which are of significance don't write anything just for the sake of writing now let me go to another slide which is x-rays what is x-ray so try to understand x-rays were discovered in year 1895 and it was a German scientist named Roentgen he discovered it I'm sorry Roentgen was the scientist from Germany who discovered it how did he discover it first of all so he was studying the phenomenon of electric discharge discharge which we just discussed what is electric discharge electric discharge is processed by which there is conduction of electricity inside the gases now what happened is that there was a screen coated with barium salt which was placed near cathode retube and whenever cathode retube was working it means that whenever there was fluorescent lights coming out of that cathode retube so that particular screen was glowing why it happened because he observed that even if screen was coated with black paper or tube sorry the tube was covered with black paper still that particular screen screen was glowing so due to that what happened is he found that photographic plates wrapped in black paper when they were kept near the cathode retube they showed some glow that that particular photographic plate was glowing whenever the cathode retube was working now what happened in between that screen and the cathode retube he put his hand and when he put his hand in between the cathode retube and the photographic plate he found the photographs of his bones so at that point of time he named it x-ray why he named it x-ray because we know that in mathematics anything which is unknown we call it x we take variable as x so that is why that kind of ray which he discovered at at at at in 1895 he named it x-ray now how do we do the production of x-ray so it has been already shown in the figure so x-rays are produced when fast-moving cathode rays are stopped by a block of metal called target so here is a block of metal and this is stopping fast-moving cathode rays what are cathode rays cathode rays are bunch of electrons at very high energy now what happens this particular tube where cathode ray is produced is known as coulis tube so what is the name of this tube if x-ray is asked in your examination don't forget to write coulis tube very very important from the exam perspective and it has been named after one scientist named coulis and it is called coulis tube please write it remember the spelling it so what happens inside this coulis tube is that it consists of highly evacuated why I already told you that we need very low pressure over here so that electric energy of the electrons emitted or cathode ray I mean energy of the electrons emitted from here is not less so what happens is we have you look at here here I have given very positive this particular electrically heated cathode is there and I am how it is electrically heated so you look at here this is the heater and I am giving some potential to it positive and negative some DC voltage has been applied over here high voltage and due to this electrical heating what happens is electrons come out of this surface in form of cathode rays these cathode rays fast-moving cathode goes and strike with this target and after striking it it passes its energy to to the material here and due to that there is some kind of radiation outside at certain at certain wavelength that radiation becomes x-ray now why there is water cooling over here you see water cooled anodes are here because there is so much of heating due to emission of radiation that this may melt if if if if it is not cooled down so that's why we have we are cooling it this is water cooled anode now you see that high voltage to accelerate the electrons has been given so very very high voltage is given to accelerate the electrons so this will be positively and negatively charged so that electrons are transferred at very very high speed now there is one more thing you see that this particular target is at certain angle I could have made this target like this why I am making it at certain angle so there is a reason behind it that it has been folded at a certain angle if you make it direct the x-ray will go and again strike over here so that is why so that the x-ray can be taken outside x-ray can be used outside this has been made at certain angle now what else is there in coulis tube so I have already explained you the coulis tube I have already explained you the working of the coulis tube now let me go to properties of x-ray so what are the different properties of x-ray just let wait for you wait for one minute so properties of x-ray so I told that x-rays are x-rays have certain wavelength that wavelength has been given as 10 angstrom so 2 0.01 angstrom so they belong to if if somebody is asking you properties of x-ray please write this point so electromagnetic radiation having wavelength between 10 angstrom to 0.01 angstrom now they travel through space in in in wave motion in free space they travel in straight line they travel with same speed of visible light because it is an electromagnetic radiation the speed would be I didn't tell cathode ray is electromagnetic radiation its speed was lesser so this is electromagnetic radiation as they travel through space they can produce an electric field at right angle to their path of propagation this is just explaining the first point that why they are electromagnetic radiation because they can produce an electric field at right angles to their path of propagation and magnetic field at right angle to the electric field now they are invisible to eye and cannot be heard or smelt they cannot be focused by lens they cannot be reflected refracted or deflected by magnetic or electric field as they do not possess any charge so you should understand that cathode ray is bunch of electron x-ray is not bunch of electron x-ray is a type of electromagnetic radiation are under certain wavelength they do not require any medium for propagation again matches with light and x-ray are pure energy no mass and they transfer energy from place to place in form of quanta so if you will understand quantum energy sometimes in in future you will understand what what quanta is so you should understand that today don't have any mass they transfer their energy from one place to another place in in term in in quantized bunch so this this is this is what the property of um x-ray is just give me a moment guys hello class there are class okay so now what i'll do is i'll move to another slide which is so there are few properties of x-rays left out so x-ray can penetrate liquid solids and gases the degree of penetration depends on quality of x-ray beam so what do i mean by quality and intensity and wavelength of x-ray beam you saw that x-ray is is is not one particular wavelength x-ray is combination or x-ray is a range of wavelength so if i need more penetration i will keep wavelength as small as possible if i need less penetration i keep the wavelength high now what i do is absorption of x-ray x-ray are absorbed by matter and absorption depends on atomic structure of the matter ionizing capability of course they'll have ionizing capability ionizing capability mainly depends on the energy of the particular radiation or particular uh particles going and strike striking some other object now x-ray has property of observation absorption scattering um all these things so there are too many properties which i have discussed with you you should remember five to six property at least remember seven properties and write those five of those properties nobody is going to ask more than five properties of x-ray now what are the uses of x-ray okay let me go to the previous slide and let me discuss with you usage of x-ray usage of x-ray is first one is medicine we all know for what egg fractures dislocation of bones presence of any foreign body certain disease like tuberculosis can be identified with x-ray in industry and engineering it is used industry and engineering also it is used for finding out if there is any fracture in machine parts or or or metal structure is if there is any shape defect or something like that now x-rays are also used to detect goods hidden in in wooden boxes and leather boxes so they are used in testing then radiotherapy is there so what happens without surgery if malignant tumor is there and we need to destroy it x-ray is used in that with with high penetrating power then x-ray is also used to study the arrangement of atoms and molecule in crystals and matters so if we have to study the arrangement of molecules atoms and molecules then we will have to then we will use x-ray in that particular case now after having discussed all these things let me move to radioactivity now what is radioactivity so try to understand first let before radioactivity let me ask ask you what is atomic number so atomic number is number of protons present in nucleus of an element so whatever is the number of protons present in a nucleus of an element that will become atomic number now what is the mass number mass number is sum of protons and neutrons inside the nucleus of an element so my atomic number is number of protons and mass number is number of sorry let me write here number of protons plus number of neutrons now why electron is not taken over here electron is not taken over here because the mass of electron is very very low as compared to mass of proton and neutron so we only add mass of proton and neutron and not electron so these are these are the two things which these are the two terminologies which I would be using in in in further slides now let me go to something called natural radioactivity now the phenomenon of radioactivity was first discovered by Bacchurin scientist named A.H. Bacchurin and then he did not do the complete study the further studies were completed by or made by madame Marie Curie and and professor Paris Curie so these are the two three important personalities in study of radioactivity now what is radioactivity radioactivity is there are few unstable nucleases not all the nucleus if you look at the periodic table elements above atomic number greater than 82 will have unstable nucleus what do I mean by unstable nucleus unstable nucleus means they will self disintegrate and you will find spontaneous emission of radiation what do I mean by self disintegration self disintegration means one particle breaking down in two or more than two different particles or one elements breaking down in more two or more than two different elements and why does it happen because the binding force is not that great in in in atomic numbers more than 82 due to that unavailability of high energy binding force of the nucleus nucleus tend to disintegrate in in two or more more than two different parts and this disintegration is known as radioactivity and what and disintegration happens how disintegration happens in in in in in the form of radiation so you will find one new element and you will also find some kind of spontaneous emission coming out in the form of radiation from that parent nucleus so how was it studied so in 1896 I told cathode retube in in 1995 in 1896 there was a friend scientist when he was studying fluorescence so that was the time when a lot of things were going on cathode retube and and and and resulting fluorescence and all those things henry bachrell was studying radioactivity I mean studying fluorescence and he found radioactivity what he found was that a photographic plate when when when get fogged by get get get fogged by uranium so what happens is substances like uranium and radium emit some rays and why that fogging happened because the radiation which was coming out of uranium and and and radium so the the discovery was that whenever I keep a photographic plate in near and or like uranium or radium you find that there is some kind of glow coming out of that photographic plate why there would be a glow coming out of photographic plate when it is kept near to certain element there has to be something in the element there has to be some radiation coming out of that element so this particular property was found and then curies started working on it and they found that this is because the at elements like uranium and radium emit radiations and these radiations are due to self disintegration of the nucleus so this is what is fundamental this is what is fundamental of this is what is fundamental for natural radioactivity now I told that disintegration happens in form of in in in form of one element and then you have what do I say then you have radiations now let me move to how how radiations are emitted so try to understand what is alpha e beta what is alpha particle beta particle and gamma particle so look at here whenever an article will get it disintegrated so look at here suppose this is my particle z suppose atomic number is I'm writing here atomic number is just give me a moment atomic number is represented by atomic number is represented by z p is particle any particle a is atomic mass now what is alpha particle so remember alpha particle is a alpha particle is nothing but helium so what is helium helium the mass number is four and the atomic number is two so this is mass number mass number is denoted by a so whenever there is a self disintegration of any particular particle p which has a atomic number z and mass number a and if alpha radiation is coming out what do I mean by alpha radiation alpha mean radiation means radiation of helium particles radiations in the form of helium particles so that those helium particles will come out these helium particles are known as alpha particles so what effect it creates on you look at here let me explain it to you you look at here if there is z p a over here and there is an alpha particle coming out alpha particle is denoted by two h e four so what effect it creates the atomic number will decrease by why because two of its protons have gone to two electrons and two protons have gone to helium so now this will become z minus two and atomic sorry mass number will decrease by four so this will become some other element q and mass number would be a minus four because already four mass number two protons and two neutrons has gone in form of helium particle so this is what alpha emission is so alpha whenever there would be an alpha emission you should know that alpha emission creates a deficiency of two atomic number and our deficiency of four mass number why because the helium particle which is alpha particle has atomic number as two and mass number as four so I have explained you alpha particles now let me go to let me give you one example of alpha alpha radiation so alpha radiation is look at here suppose I take uranium the mass the atomic number of uranium is 92 the mass number is 238 when an alpha particle will be coming out so I'm writing here to achieve four the particle which is created after the disintegration of alpha particle is known as thorium so 90 thorium 234 so see here the mass the atomic number decreased by two and the mass number decreased by four this is how it is now let me go to beta emission now what is beta emission or beta particle so what happens is whenever there is excess neutron available in the nucleus it will make make the nucleus very very unstable and due to that what will happen is neutron will keep on changing itself in the proton by emitting a electron so what happens beta particle one thing that we need to remember is one thing that that that should be known for beta particle is why beta emission happens excess of neutrons excess of neutrons make it make the nucleus unstable unstable nucleus how nucleus tend to get stability nucleus will tend to get stability by changing neutron so neutron changing neutron to proton and how neutron changes to proton by emission of electrons so it means that if one electron is leaving electron I said emission of electron does not affect the mass number but emission of electron will affect the atomic number why it will affect the atomic number because in any particular element number of protons would be equal to number of electrons so what happens is if one electron is going out and neutron is changing into proton so if one neutron is changing into one proton it means that number of protons are increasing so one neutron changes to one proton the atomic number goes by one two two neutrons changes to two protons it means that addition of two extra protons atomic number goes by two so how I can show you this is that suppose I take an example of thorium atom and thorium is going through beta disintegration so how thorium will go through beta disintegration so I will write 90 thorium 234 this is my atomic number this is my mass number and if a beta emission is happening it means that one electron is coming out or I write electron as beta particle minus one beta zero I can write the because one neutron has changed to one proton so this increases by 191 and it becomes what it becomes protest it becomes protactinium so you have pa here which is protactinium and you should always remember that emission of electron does not affect the mass number emission of electron will never affect mass number I am repeating it time and again I told alpha particle will affect both atomic number and mass number atomic number reduces by two mass number reduces by four the beta will only affect mass number it will increase mass number why it will increase mass number because neutrons are changing into protons as neutrons are changing into protons there is an increase in number of protons atomic number is total number of protons hence with increase in number of protons increase in atomic number is observed so this is the phenomenon of beta emission now let me go to gamma emission now what is gamma emission so look at here it has been written that gamma rays are a high frequency form of electromagnetic radiation they travel at a speed of light so gamma emission is not in the form of particle any particle is not emitted try to understand in alpha emission helium particle got emitted in beta emission neutrons sorry electron got emitted which is also called beta particle in gamma radiation particle is not emitted when I say in any particular radiation particle is not emitted it means that in the absence of emission of particle the atomic number and the mass number of any particular element will not change why because it is not emitting any particle it is only emitting radiations now there is one thing to consider in this particular concept is that if radiations are getting emitted in case of emission of radiation the energy conserved inside the parent nucleus will come down why it will come down because the emission of radiation will take away some portion of energy from the parent nucleus and it will take that energy away and go out of that parent article so you should understand that whenever there is a gamma decay or gamma emission there is no change in atomic number no change in mass number there is only change in the energy level of in the energy level of the element now let me move to another topic which is properties of alpha beta gamma particles okay so properties of alpha beta and gamma particles are let me just remove this so I have made some made a table where where I have denoted some properties of alpha beta and gamma particle so what are the particles alpha alpha rays I already told they are nucleus of helium beta rays I told they are fast moving electrons and gamma rays are electromagnetic radiations if you don't write it and write any other if you don't write these three things as as first difference in alpha beta and gamma rays and you write any other thing your one mass would be deducted because when you are writing difference you need to know that what is the composition of all three three rays so this is the first thing which is required the second thing is alpha ray because this is nucleus nucleus will always carry the positive charge so alpha ray alpha ray will carry positive charge beta ray is emission of electron electron is negative so that is why this will carry negative charge gamma rays only radiation there is no particle involved so there would be no charge now they carry as they are positively charged they will be deflected towards a node negative side there will negative things would be deflected towards positive side and gamma rays are not charged so that there would be no deflection in electric field now what is the velocity in case of alpha ray the velocity would be one one-tenth this is not electromagnetic radiation in case of beta rays and gamma rays beta because of presence of electron and gamma rays because of electromagnetic radiation they travel with the speed of light let me move to another property of alpha beta and gamma radiation alpha radiations have highest energy hence they have ionizing power of gases then beta radiation it is less and and gamma rays has the least now let me go to the other property alpha radiation will have lesser penetrating power beta will have much greater and gamma radiation due to its non-material nature and very high speed the penetrating power would be more mass mass of this thing alpha radiation mass of beta you don't need to remember masses i have given it for your reference if you want to remember you can refer to this slide you can write it down i'm giving you 30 seconds of time but remembering masses is not necessary at this particular place now there are three four more properties that i would like to discuss and those are first is stopping substance so what does stop it so alpha rays are stopped by thin paper humans skin beta ray about one mm lead or about three mm of aluminium plate is required because the penetration is more for beta ray as compared to alpha ray and gamma ray you need 30 centimetre of iron or few meters of concrete because of the high energy possessed by it effect on photographic plate i told you that all three of them will affect photographic plates because of the radiations produced due to radioactivity due to radioactivity the last one which i want to discuss is biological damage alpha ray can cause damage to human being it is harmful beta ray is also harmful to human being gamma rays are useful to kill cancer cells so generally what happens is gamma rays passes through human body and they destroy human cells so if they are destroying healthy human cells that would be damaging to the body but its uses can also be brought into application by killing malignant human cells which are the basic region for cancer so if treatment of cancer is concerned the malignant cells of human body is treated by gamma rays now let me go to another slide which is so i hope you understood the properties of alpha beta and gamma rays i have tried to discuss properties i have already discussed this slide alpha beta and gamma radiation decay with you so radioactive transformation i have already discussed how alpha beta and gamma radiations happened after this let me go to something very important called a radioactive series and what is radioactive series so you see here i'm starting with thorium series so what do i mean by thorium series you look at here and don't look from from from this side from from bottom to top look from top to bottom that what is happening so you look at here so in in in thorium series what will happen you will have 234 thorium 90 here perhaps this i have not included from here you can see an alpha decay so here you have two alpha decays after alpha and gamma decay it converts to other particle then it it converts to another particle and you see here how different particles are and and what these lines are suggesting you try to understand this is my mass number and this is my atomic number so try to understand that after decay atomic number and mass number reduces that is why you see this bottom down approach not uh sorry bottom uh yeah bottom down approach it goes sorry top top down approach it goes from top to bottom now try to understand over here that look at this section and if you look at it very properly this is only beta and gamma radiation so what i have told in beta and gamma radiations is in gamma radiation atomic number and mass number will never change in beta radiation atomic number will increase by plus one so see after one beta radiation atomic number is increasing by plus one again here and again here and then what happens no so so so two two uh increased by two and then then there is again and if you look at mass number mass number remains same here why it remains same here is that there is no change in mass number because of beta and gamma radiation after that what happens you look at here again alpha radiation beta 2 beta radiation again alpha radiation this will keep on happening till the time the nucleus becomes stable so this is the thing so this is what radioactive series is now let me go to another important topic which is isotope isobar and all these things so look at here the first one is isotopes what do i mean by isotopes so isotopes are those atoms which has same atomic number but different mass numbers so i have given over here two three examples if you look at it i've written that these are the elements having same atomic number but different mass number i have already explained this they have the same atomic number because number of protons inside their nucleus remains same the difference in their mass number is due to difference in the number of electrons so you should understand that if mass number is proton plus neutron if proton is same and mass number increases it can only happen when neutron number of neutrons are increasing since there are neutral isotopes of our elements having same number of electrons which have which make them which make them to possess identical chemical properties so what it is trying to say is that number of protons and number of electrons are same hence they both will they all will be having same chemical properties so you look at here i have taken few isotopes are of hydrogen so one h1 one h2 one h3 it says that the atomic numbers remain same the mass number remains different because of the change in number of neutrons now let me go to isobars what do i mean by isobars so i here i have written one thing very specifically which says that isotopes are chemically same as physically different but converse is true in isobars that is isobars are the elements which are chemically different and physically same what do i mean by that so i need you need to understand that isobars are the atoms which are having same atomic numbers but different atomic same atomic mass but different atomic numbers so what do i mean by different atomic number by different atomic number i mean that so same atomic mass atomic mass is number of neutrons plus number of protons so if atomic mass is same and atomic number is different it means that number of protons are different number of electrons are equal to number of protons hence number of electrons will also be different as number of protons and number of electrons or number of ions inside the atoms are different. Hence, they will have different chemical property. Why physical property would be same because atomic number is same. Hence, what happens is the number of particles inside the nucleus is same. So in that condition the physical property remains same. Now heavy nucleus have stable isobars and they occur in pairs. So if I give you an example of isobars, so you look at here 6C13, carbon and 67N13 or let me take potassium and calcium, so 19K40 and 20CA40. So you should understand one more thing also this is related to chemistry that the property or the element is identified by its atomic number not the mass number. So that is the thing. Now what are isotones? Isotones are the elements having the same number of neutrons. So what is happening isotope same number of protons, isobars different number of protons, isotones same number of neutrons. How do you remember it? Always remember in topes P comes wherever P comes what happens you have same number of protons and you look at here isotone here you have N wherever N comes you have same number of neutrons and what is this bars here A comes so your atomic mass is same. So you remember here there is no A here there is no A here A comes only in isobar so where is A is coming that that means atomic mass is same where P is coming proton is same where N is coming neutron is same. So that's how you remember these things. So this is your this is the concept of isotopes isobar and isotones. Now let me go to something called artificial transmutation. So what is artificial transmutation? Try to understand if there is an instability in nucleus that will lead to spontaneous radioactive disintegration as I discussed in case of natural radioactivity. But what happens is there was a scientist named Rutherford and what did he do? Rutherford in 1999 he succeeded in producing nucleus of lighter elements which also exhibits property of radioactivity. What do I mean by this and why this is important? So when I was discussing natural radioactivity I discussed with you that the nucleus of heavy elements as elements having atomic numbers more than 82 has unstable nucleus. I didn't say at that point of time that any particular nucleus having less than any particular element having less than atomic number 82 will show this kind of property. But I was very specific in saying that heavy nucleus will show some kind of disintegration but Rutherford proved something which is very opposite. He shows that even lighter elements can show property of radioactivity. So what is that that property or that particular procedure by which this happens is known as artificial transmutation. So what I mean by artificial transmutation is the process of transformation of one element into another by artificial disintegration by means of bombarding it with high energy particles. So there are too many different things which I am using in this particular definition. Let me break this definition into different parts. First of all what is happening? So I have suppose let me give you an example. The example is this. Example is I have nitrogen particles. What I do is I want to disintegrate this nitrogen particle. How do I disintegrate this nitrogen particle? For disintegration I will bombard this nitrogen particle with very high energy radiations, very high energy particles. So I take high energy alpha particle which go and bombard this new nitrogen. Due to that what happens is it converts into oxygen and hydrogen. So what is happening in artificial transmutation? If you look at nitrogen, nitrogen is very inert, very stable kind of element. But what am I doing? I am disintegrating it into oxygen. How does it happen when this small particle, when lighter element like nitrogen is getting disintegrated into oxygen with the help of bombardment of helium particles onto it? So this is the process of converting lighter elements or disintegrating lighter elements into some other element. This is called artificial transmutation. Now artificial transmutation is possible by alpha particle. It is possible by protons. It is possible by beta particles. So try to understand that artificial transmutation is a process in which lighter elements start showing radioactivity by the virtue of it getting bombarded by high energy particles. Now I move to the another segment of radioactivity which is nuclear reactions. So what I have taken before nuclear reactions? Let me teach you what is atomic mass and what is mass defect and all these things. So what is atomic mass? The mass from atom is measured in unit called atomic mass unit. So any atom, what is the unit of measuring mass of that atom that is called atomic mass unit and what is atomic mass unit? Atomic mass unit is constant. So one atomic mass unit is equal to one-twelfth of the mass of neutral carbon atom. So whatever is the mass of neutral carbon atom and neutral carbon atom is denoted by 6C12. So I take mass of 6C12. I multiply this with one by 12 that is one atomic mass unit AU sorry U. So atomic mass unit is denoted by small letter U and one U is equal to one by 12 mass of 6C12. This is it. Now let me teach you mass defect. So what is mass defect? So mass defect is difference between the sum of individual masses of constituents in a nucleus and the sum of nucleus itself. So what happens is I teach you Einstein mass energy equivalence. You will find that always if you take any particular atom and suppose I take a helium atom so 2 HE4 so I have two protons plus two neutrons plus two electrons. So if I add masses of these individual constituents and then I find out atomic mass the actual mass of this helium atom you will find that the actual mass of this helium atom is lesser than the summation of mass of individual constituents which in this case is two protons two neutrons and two electrons. So this difference of mass or this difference of atomic mass and mass of individual constituents is known as mass defect. So I hope you understand mass defect. Now what do I mean by mass defect? So there is something called Einstein mass energy equivalence. What is Einstein's mass energy equivalence? So what Einstein said was that mass and energy are not independent. They are mutually not exclusive. They are mutually convertible. It means that anybody with changes its energy content undergoes a change in mass. So what he in very lamell language if I see that if mass of a particular body is changing there would be a change in energy. So this is what the fundamental of Einstein was and what I want to say from here is I don't want to get into get deep into Einstein mass energy equivalence. I just wanted to let you know that as there is some mass defect in the atom this mass effect due to this mass defect it means that due to this loss of mass while joining these individual constituents will lead to creation of some kind of energy. So this is what the energy stored when the atoms are created. Now with this particular information I go to teaching you nuclear reactions. So I have taken two kinds of nuclear reactions. First I have taken nuclear fission. So what is nuclear fission? So we have already discussed that natural radioactivity is present in uranium. So uranium has a different isotopes. So you will have uranium 235, uranium 238 and uranium is because of its huge size uranium is a very very unstable element and because it is very unstable it emits powerful radiations. So what happens is in this particular process of self disintegration what will happen? Uranium atom will keep on disintegrating and they will keep on disintegrating till the time they reach up to a stage where they convert themselves into stable lead. So uranium disintegration happens till the time they are converted to lead but during this disintegration I am seeing that there would be emission of powerful radiations. Now try to understand. So any uranium isotope will release energy of 1 microelectron volt to 5 microelectron volts and this particular energy is able to because it is possible to break unstable nucleus into two or more smaller ones. So in that process what happens is because these radiations are available and due to emission of high energy this creation of greater energy, this process of creation of greater energy due to disintegration of radioactive substances whether by itself or by certain other certain other particles which are helping us to create the situation of disintegration, this creation of nuclear energy, this creation of radiation is known as nuclear fission reaction. So nuclear fission is a process in which heavy nucleus of a radioactive substance like uranium, thorium all these things are split into lighter nucleus. Now most of the time it will not start automatically what we do is we bombard low energy particles like neutrons and in that particular process what will happen there would be a reaction due to that reaction what will happen is there would be disintegration and due to disintegration there would be release of energy. So with the release of energy so what happens is I have written over here so this uranium atom I am bombarding it with neutron after getting bombarded into neutron what happens it converts into barium and and krypton and three more neutrons are available. Now these neutrons can be used for breaking down these you know that this will not break down now so breaking down these things these these particles. So you see here what happens is if more and more uranium particles are there I strike one of the uranium atoms with neutron it converts into barium and krypton the neutrons so so generated will go and strike with other uranium particles these other uranium particles will get in disintegrated into barium and krypton again more neutrons would be released they will again strike with uranium so this chain reaction will keep on going so see this is first generation this second generation this is third generation so this is how this chain reaction goes on so nuclear fission reaction is a kind of chain reactions now try to understand the salient features of fission reaction what happens in a fission reaction. So in a fission reaction if uranium particle I have already explained it I am just revising it in in certain heading so uranium particle is bombarded with slow moving low energy thermal neutrons in this always remember that fast moving neutrons cannot produce fission then the other feature of this process is the product of fission reaction would always be lighter nucleus one lighter nucleus one heavy nucleus here the heavy nucleus is barium and lighter nucleus is krypton and generally you will see the presence of two to three neutrons over here now the atomic mass of the lighter nucleus can be any way between 85 to 104 always remember that this lighter nucleus would be in range of atomic mass would be range of 85 to 104 and for this heavier nucleus it would be in the range of 130 to 149 on an average so I am saying two to three on an average let's take two and a half neutrons are produced in one nuclear fission reaction now there are three kinds of fission reaction one is a spontaneous second one is prompt and third one is delayed reaction so what happens in spontaneous fission reaction is fission takes place continuously and spontaneously in in in certain substances without any bombardment so what what is important is in a spontaneous fission reaction bombardment of neutrons is not required in a prompt reaction fission will take soon take place as soon as you bombard heavy nucleus with neutrons while in case delayed fission neutrons enters heavy nucleus then they cause instability after instability is caused caused then the fission would be caused so these are the three types I will write here for your reference spontaneous no neutron needed prompt neutron as soon as ASAP with neutron and you have delayed so in case of delayed what will happen fast instability will be caused and then fission will happen so this is what it is now there is one more thing which which I would like to discuss which is liquid drop model now what do I mean by liquid drop model so I have already discussed this chain reaction and all those things what do I mean by liquid drop model is that any heavy nucleus can be treated as liquid drop and and and we will assume that the positive charge are uniformly distributed so why this liquid drop models is studied so there were scientists who discovered fission was there were two scientists Otto Hahn and and Stasman so this they discovered this nuclear fission reaction in 1939 and there were two more scientists Neil Bohr and Neil Bohr three scientists Neil Bohr, Frankel and Wheeler they suggested I mean whenever there is certain physical activity physical phenomenon happening there has to be certain model or there has to be certain theory so that we can explain that particular theory so these three scientists Bohr, Frankel and Wheeler suggested liquid drop model to explain the nuclear fission and and and with the with the help of nuclear drop model what they did that they tried to explain various stages of nuclear fission so they assume that the heavy nucleus is in form of liquid drop in which the positive particles are spread uniformly what happened after that the next step that they explained is they said that there would be surface tension because they assume that this is the liquid drop and they said that the surface of the liquid drop would act as stressed rubber membrane and they said that the forces acting between nucleons inside the nucleus are different from those acting between nucleons close to surface so what I mean to say that if I take liquid there are two kind of forces which are applicable adhesive force and cohesive force which I already explained to you so I they are saying that the adhesive force and the cohesive force in this particular case is they both are different now what happens as the adhesive force and cohesive forces are different when the bombardment of neutrons happens on the nucleus neutrons get absorbed by this drop like nucleus and this disturbs I mean why this will get absorbed because there is difference of forces and due to difference of forces internal forces it will absorb this nucleus so that the forces can be balanced but it does not balance it is it instead of balancing those forces it starts setting up oscillations and due to those oscillations there would be huge instability that would be caused in the parent nucleus and it will get separated into two parts so this is what the liquid drop model is and I just explained you though it it may or may not be useful at this particular junction so it took some two three minutes of time I didn't feel like leaving liquid drop model so I have explained it for you now let me go to another slide and this is the last last slide which is nuclear fusion and nuclear fusion is exactly opposite of nuclear fission so nuclear fission was all about disintegration of heavy nucleus nuclear fusion is nuclear fusion is joining two light particles together and in that particular process huge amount of energy is generated so look at here I let me explain that for you nuclear fusion is the joining of two nucleus to form a heavier nuclear the reaction is followed by either release or absorption of energy so fusion of nucleus with lower mass than iron release energy while fusion of nucleus heavier than iron generally absorb energy the phenomenon is also known as iron peak with this opposite occurs the opposite occurs with nuclear fission so as I have already explained that in nuclear fusion there is combination of or joining of two lighter nucleases to form a heavier nucleus now there are two cases for lighter nucleus as compared to iron if they are combining together energy would be released if atoms heavier than iron are joining together then the energy would be absorbed now the other points related to nuclear fusion are that the power of energy in a fusion reaction is what drives the energy that is released from sun and a lot of stars in the universe so nuclear fusion is applied to nuclear weapons especially a hydrogen bomb a nuclear fusion is so first thing that we need to understand that what happens in sun so in sun nuclear thus energy of the sun or the heat of the sun is generated by nuclear fusion combining of hydrogen atoms into helium atoms it has been written written below and due to that the energy is produced the energy is in the form of light and we all get light nuclear fusion is also used in nuclear bombs and we also um specifically in hydrogen bomb so the the phenomenon is almost the same as it has been written over there when uncontrolled this process can provide almost unlimited source of energy and uncontrolled chain reaction provides the basis of for a hydrogen bond since most commonly hydrogen is fused so generally what happens is in in nuclear fusion the most common fuel which is used is hydrogen it is also a combination of deuterium atoms to form helium atoms so you you look at here a hydrogen atom is deuterium atom atoms are there and they are combining and they are they are giving helium atoms over here the i have i have written over here and this this particular thing is used in thermonuclear processes so this is what nuclear fusion is all about so this is what i wanted to teach you in terms of modern physics uh now before i leave you there are two three things i want to be very specific with right answers in in in in very elaborate manner don't be major with the words used in your answer copies write it very elaborately always remember that i'm not saying right rubbish thing but the more you write the more relevant things you write the more marks you get the second thing that i want to tell you is that board exam is just a process in your academic career so don't get too much tensed about board examination i'm not saying you should not be serious you should be absolutely serious about your board examination but at the very same time you should not be tensed about it a little bit of anxiety about examination is always good because it forces you to to prepare for the example to be concentrated on the example and it helps you in in better preparation for the examination having said that the stress level the high amount of stress level will reduce your productivity of learning and in that thing you will tend to forget most of the things that you are learning at this point of time hence it is always advisable to reduce your stress level before your board examination it's i know it is easier said that done i have been in that situation some 15 16 years back but having said that my advice would be to relax a bit and and and enjoy this process of giving board examination rather than taking stress and and and and somehow not getting the kind of marks that you deserve which i don't want the next thing that i would like to tell you is that if you have any doubt related to physics i have already come we have already completed the syllabus if you have any doubt related to physics you can get in touch with us either me or any anyone you can get in touch with me and and and i'll try to help you out with all the doubts possible so thank you so much guys for attending this class wish you all the best for your examination from the center academy family and i hope you all will pass with with very very good marks with with very very good percentage and and will bring laurels to to you yourself your family to us everyone so thank you so much for joining the class i wish you all the best thank you so much