 So, Einstein has given this equation write down h times mu minus phi naught is equal to k max write down then we will discuss. This is for one electron for a particular electron what will happen electron will absorb a photon which has h into mu energy and suppose it needs phi naught of energy to come out of metal surface. So, if h into mu which it has absorbed is more than what energy it requires to come out which is phi naught then the difference between them will become the kinetic energy of that electron are you getting it fine. So, if I mean in a metal surface there will be different kinds of electron those will be directly on the surface they will be able to come out the most easily those slightly below they will be able to come out with slightly greater difficulty fine. So, this thing will keep on varying for different kinds of electron fine. The minimum value of this the minimum value or the minimum threshold value for any electron to come out is called the work function of the metal. The minimum threshold energy required for the electron to come out the minimum energy write down the minimum energy for the electron to come out from the metal surface is called the work function of that metal surface getting it. So, usually in the equation phi naught is referred as work function fine. So, this is the maximum kinetic energy of the electron that are coming out what about minimum kinetic energy how much will be minimum will be what just phi naught 0 you need phi naught amount of energy for it to come out you get phi naught unit phi naught it takes phi naught and expended to come out it has now 0 energy fine. So, minimum energy when the electron comes out from the surface is 0 maximum is get maximum you can get from this equation fine. So, there will be all kinds of electron that will be coming out from 0 energy till maximum kinetic energy but this will happen only when mu is more than threshold frequency fine. So, can you find out how much is the threshold frequency using this equation all you find threshold frequency threshold frequency is when electron just able to come out or kinetic energy is minimum or 0 fine. So, put k 0 here you will get h times mu threshold minus phi naught is equal to 0. So, mu threshold is phi naught by h do you know what is the value of h what is this h called Planck's constant how much is this value 6.6 6.3 34 s i m x or and many times it will be given that energy is in the form many times energy will be given in the form of electron volts. So, do you know what is the relationship between electron volt and joules 1 electron volt is how much this is 1.9 into 1.6 if 10 to the power minus 19 joules this is 1 electron volt how much is 2 electron volt 2 into this 5 electron volt 5 into this like that because you are dealing with very very less amount of energy. So, it makes sense to write energy in the form of electron volt. So, we found out the threshold frequency now can you find out the stopping potential stopping potential write down find the stopping potential of a metal surface stopping potential of a metal surface having what function phi for a frequency mu for a frequency mu and what function phi find out the stopping potential how much it is then k max is what in terms of stopping potential how much it is e v naught. So, you can just write h mu minus phi naught is equal to e v naught. So, you get v naught equal to h by e mu minus phi naught by e. So, you can see that you get a straight line relation between v naught and mu or a linear relation this is what the graph is when we have. When v naught is 0 you get the frequency mu that is the threshold frequency and the slope of the graph is h by e that has nothing to do with what metal it is. One more thing you will see in the numerical is power of radiation what is power what is unit of power power is what and what is what joules per second. So, if power is p watt and every photon has h into mu energy this many joules. So, I will be able to find out number of photons that are coming per second how total energy per second is p joule per second and one photon has h into mu joules fine. So, if n photons are there per second then n into h mu should be equal to power n is number of photons emitted per second. So, this thing comes again and again in the numerical number of photons per second. Aja few more things I mean I am just telling you based on my experience in the numericals. At times you will be able to find little kind energy maximum kind energy of electron and what is electron a charged particle. So, you will have a velocity and you have a charge and if you supply a magnetic field perpendicular to its velocity it will move in a circle you will be able to find out the radius because you will be able to find the velocity radius is m v by q b fine. So, all that can be done in the numerical and the k max which you are finding from this equation is a maximum kind energy when it comes out from the emitter plate fine. When you supply of potential difference it can further gain energy fine. So, if you supply a potential difference of v naught positive potential difference then the total kind energy when it reaches the collector plate will be k max plus e times v naught are you getting it. So, all that you have to be very very careful about and then you will not find many variations other than these. So, that is all I mean if you solve like 30 to 40 question your own this chapter is done. Let us take few of the NCRT questions let us see how it goes. A monochromatic light of frequency 6 into 10 square 14 hertz and also in this chapter there will be lot of calculations. So, be ready for that and you should be able to approximately find the answer at times many times. In fact, you will be asked to find out energy and then you will be like to get doing this calculation and see by lambda and then you have to divide by charge of electron also to get the energy in electron volts fine. So, you write like this 6.63 into 10 raise to power minus 44 into 3 into 10 raise to power 8 divided by charge of electron which is 1.6 into 10 raise to power minus 19 into wavelength. What happens many times I have seen that students will just remember this value h c by e how much is that because this will come again and again. If you remember it that is fine, but then if you do not want to remember then also you can you know this is approximately 4 times, but slightly more than 4. So, 4 is a 12. So, just write down 12.1 or 12.2 and that is it 10 raise to power this is 27 minus 34 how much minus 7 minus 7 this divided by 12. So, you should be quick in the calculation with that not like you ok let me divide 6.63 and you put 1.6 over here and this is a by the time you do all this nonsense people will be solving those equations. And these things like you will learn only when you solve lot of questions. Anyways coming back to the numerical the frequency is this, this frequency is produced by a laser. The power emitted by the laser is 2 into 10. So, minus 3 watt the power is 2 into 10 raise to power minus 3 watt. You need to find energy of the photon in the light beam how much is the energy of the photon and how many photons per second are emitted? Photons per second that are emitted by this laser source. Sir energy moving as we need to find the electron volts or joules? If nothing is said joules, energy is 39.8 into 10 to the power minus 20 and the number of electrons was I think there is 500 more. Not electrons, what are photons? You got it, energy is h into mu just multiply h with mu you will get energy ok number of photons per second into h into mu should be equal to 2 into 10 raise to power minus 3. So, if you divide h into mu from this you get number of photons per second. One more thing in the higher order questions you will see that when electron comes out of metal surface it is a charged particle that are leaving the metal surface. So, the metal will continuously gain charge more and more positive charge at gain ok. So, we are assuming that metal remains neutral right now. If it gains charge with time it will become increasingly difficult for electrons to leave the metal. Are you getting slowly and slowly by itself only electrons will not be able to come out and at times in the numerical it will be given that one electron out of 1 million photons will be able to come out. So, you will be able to roughly find out what is the current like for example, here itself if I say that one right of in the same question if one electron out of 1 billion not billion 1 million. So, one electron is ejected from 1 million photons find out the current. What will our answer 5 into 10 raise to power 15 photons per second. So, how many electrons per second. So, how many electrons per second here by 10 to power 6. So, I will get 5 into 10 raise to power 9 electrons are coming per second how much charge per second that is the current. This into charge of an electron right. So, this into charge of an electron which is 1.6 into 10 raise to power minus 19 ampere. So, this is what 5 6 are 38 into 10 raise to power minus 10 ampere.