 So, today we are going to start a section which is very very important right it is probably as important as you can say electrostats or entire magnetism and usually it gets ignored and the reason for ignoring anything is what easy it is easy first thing is it is easy you take it for granted you like ok so do it at the end but that end never comes so at the end when you try to do it quickly you will not be able to do it properly fine but then what happens if you do it like this you will lose out on some of the easy questions which you can solve at any level mains or advance at any level you will be able to solve modern physics question very quickly fine because the concepts are not so heavy or involved fine so we will not do that mistake we will not wait till the very end to finish this chapter fine we will start early this section modern physics fine so modern physics let us why it is called modern physics is because the classical physics ok because the classical physics which is developed by Newton, Galileo and various other scientists Faraday and whatever like Nikola Tesla and everyone they whatever concept they have developed though they are not sufficient to explain what is going on at some level like for example if you study what is going on inside the atom if you study the behaviour of electron you will not be able to fully understand what is going on if you try to understand it based on the laws which are developed by Newton, Galileo and other people fine so there need to be a different set of rules or laws which should be developed to understand what is going on so you may think that science is a very structured sorry physics is a very structured science but it is not for like day to day things or things which you can easily observe there is a classical physics to deal with it for objects that move with the speed of life there is a relativity you have to take into consideration fine for objects which are very very small you have to use a different physics called quantum physics to study that fine so physics is a very unstructured science fine similarly here also in modern physics this is a set of situations where you cannot understand what is going on using classical physics so that is why a different set of rules are developed and this is what is called modern physics although the name is modern physics but I think more than 100 years are done when quantum physics was first introduced fine so modern physics we will be learning few of the things which are beyond our common observations fine so this entire section modern physics is divided into three chapters in our syllabus right now the first chapter is dual nature of radiation and matter in this chapter you will learn wave nature of particle and particle nature of wave see for a wave we have till now a basic understanding that wave is nothing but it is a disturbance it is an energy but first time we will be learning wave as if it is a particle particle wave or two different kinds of concepts but now we will be studying wave as if it is a particle and not only that we will be studying particle the mass as a wave in this chapter till now we have learned in class 11 you have this law of motion work by energy kinematics there where we have learned extensively about how a particle behaves fine but we never thought of can a particle be a wave that is what we are going to do next is items see in this chapter we are I mean similar chapter might be there in chemistry or zyatomic structure fine the chapter the content is very very similar just that here we are more into physical aspects of it in items chapter we are first talking about the structure of it okay without getting into the hybridization part of it which is there in the chemistry all right then we are introducing Bohr's model to understand to understand the behavior of electron in a hydrogen or hydrogen like it fine Bohr's model is required because the model put forward by Newton was not sufficient to understand what is going on fine so we will be discussing this but when you get into detail of this chapter at the end of the day this chapter is all about electrons fine we will be talking about electrons excited state ground state energy excited state energy how much wavelength it emits and things like that so our focus is electron although the name of the chapter is items fine the last section or the last chapter from in this is nuclei in nuclei chapter our focus is nucleus fine nucleus the reactions it undergo fine there can be spontaneous reactions like in chemical reactions also we have dealt with spontaneous reactions right isn't it what are different between chemical reaction and this reaction electrons here nucleus takes part in the reaction that is why we call it nuclear reactions fine the spontaneous reaction is usually a decay which we call radioactive decay fine so there is a section which talks about radioactive decode and then there is a section which is non-spontaneous reaction these are fission and fusion fine so this is a broad outline of entire model physics fine more or less you know 15 percent 10 to 15 percent should be the weightage of this chapter on entire model physics all right and another thing is that you can combine modern physics with classical physics you can like okay fine electron comes out and then you have applied a magnetic field and then it moves in a circle you know and you know you can imagine so many other things since it can get combined easily with mechanics so more than 15 percent weightage can also be assigned to it and good thing is very easy all right but not so easy that you can just ignore it till the very end so we'll take today this chapter dual nature of radiation and I'll just I mean the chapter name is big I just write dual nature okay it is understood fine so now this in this chapter of course there will not be a systematic study because people never assume that there will be a particle nature of wave or wave nature of particle fine they they were never of so they were never of this like it was incidental they never thought that you know they have to study particle nature of wave or wave nature of particle so they were conducting several experiments and out of accident it came out that wave can behave like a particle fine and since the chapter itself is based on several experiments and incidents so we'll talk about what all those incidents that have led to the conclusion that there is a dual nature of particle as well as wave fine so the first experiment was from herbs see initially you will not be able to appreciate the dual nature of radiation fine but then slowly and slowly you will gain that gain this understanding that you will not be able to explain the behavior whatever is getting observed can be explained by cannot be explained by Newton's laws of motion fine so let's let's see what are those experiments right so our first study is actually the particle nature of wave fine so we are studying particle nature of wave fine so what Hertz has observed is that if you take a metal plate if you take a metal plate what is this metal plate consist of free electrons what does it mean free electrons free electrons are like free to move from one atom to other metal sorry one atom to another atom so there will be like a sea of electron that are just floating here and there fine but then they are not coming out of metal okay what Hertz has observed is that if you fall a radiation on the metal surface okay then few electrons will be able to come out okay and you can probably capture it if you put a positive positively charged plate above it so this is like the beginning of understanding towards the particle nature of the wave fine so Hertz you might be using this unit of Hertz with the frequency of the wave so he was doing several experiments of the wave right so he just bombarded few radiations on the metal and some charged particles came out okay so this is the understanding that electron can come out of metal but why they are able to come out of metal or first first you answer why they are they were not coming out of metal initially when suppose you have a metal piece like this why electrons are not coming out correct you can say that I mean see the thing is if electron comes out there will be a localized positively charged developed here okay so this electron if positive charge developed will get attracted towards the positive charge and then get again utilized fine so it is not a favorable situation for electron to come out of metal similarly for example you yourself you are bounded in the earth you will not be able to leave the earth if you jump you will again fall on the earth itself because earth attracts you until and unless you get sufficient kinetic energy or you have the escape velocity then you will be able to leave the earth similarly electrons electrons are bounded but if you give sufficient energy to electron then electron can escape from the metal getting it okay so what electron wants energy fine and when you bombard a radiation on the metal surface this is one of the ways for electrons to gain the energy and then they are able to come out can you think of any other way to give energy to the electron for them to come out heat fine when you heat the surface few electrons will get energy and they will be able to come out and the emission that kind of emission is called thermionic emission thermionic emission happens when you provide energy in the form of heat fine this is called photoelectric emission the example which we have taken is called photoelectric emission when you give energy in the form of radiation is there any other way to give the energy any other way you can create a huge electric field fine so if it is due to the electric field we call it field emission these are the three kinds of emission once Hertz had discovered that electron can come out then they started finding the ways in which electron can come out so roughly three ways you can give the energy to the electron and electron will come out okay but the study of this will lead to the conclusion that wave can have particle energy when you study this in greater detail photoelectric emission why because this is where the wave is getting used and you will not be able to explain how the electron is able to come out if you assume that wave behaves like a wave okay will come that come to that why it is like this but right now you understand right why we are studying photoelectric emission because this is where there is an interaction between a wave and the metal and also this is the easiest way out of these three to take the electron out this is very efficient way to give the energy to the electron so that is why our focus in this chapter is photoelectric emission and Hertz has conducted first experiment and concluded that if the radiation falls on the metal surface electrons will come out okay once Hertz has concluded this then several other scientists they have started to you know to study it in a systematic manner fine