 Then there were these two scientists, Hallouac and Lennard, they have constructed a setup, experimental setup to understand what is going on in this particular phenomenon, okay. So let us try to see what is that setup. Once you understand the observation made by Hallouac and Lennard, you will be like this chapter is almost over there, okay. So this experiment is very, very important, right. So first we will draw the experimental setup, you also draw with me, Hallouac and Lennard. So I will draw here, they have setup like this, they have this submarine sort of structure. This is evacuated glass tube, why this is evacuated? Whatever happens inside this, I do not want it to get influenced by the particles around. So I want it to be perfect vacuum but that is not possible, so near vacuum. This is quads window, so this is a transparent window through which from a source radiation can be transmitted, fine. So radiation can travel through the transparent window, getting it? Then you have a photosensitive plate, this is, what is a photosensitive plate? A metal, okay, a piece of metal from where the electron can easily come out, nothing like this. Very sensitive, okay. So if the radiation falls on it, the electrons will come out easily, that is why it is called photosensitive. This plate is also called emitter plate, this is emitter. Electrons can get emitted from this plate, fine. This plate is called collector, fine, this is collector, is this clear? So if the radiation falls on it, electrons will come out, fine, so this electron starts to move like this. So there will be several electrons that will then start moving, they will come out, they will have kinetic energy and they will move towards the collector. Now if I make this collector a positive potential, what will happen to electron? They will get attracted, fine. If I make it negative potential, they will get repelled, fine. So I can vary the potential and reverse the potential and see experimentally what all things can change, so I can study the entire behavior. I can not only do that, I can change the intensity of the radiation that is falling on it, fine. Several things that can change if I have this experimenter, fine. So I will just complete the circuit here, this micro emitter, micro emitter will detect any current it will have and here you have a rheostatic, this is the voltmeter. You can change the resistance and this is a high-tension battery, you can say very high voltage you can throw. So you can change the value of potential difference between these two points by changing the rheostat resistance, getting it. You will not only do that, this is 1, 2, 3, 4, 5, 6, 7 and 8. See suppose you connect 1 and 3, 4 and 8, 2 and 5 and 6 and 7, then 2 will be at the positive potential, if you connect these two and these two, 2 will be at the positive potential and 1 and 3 and 4 and 8 if you connect, there will be negative potential, fine. But if you connect 2 and 3 and 4 and 7, it will become negative potential, fine. So using this device is called commutator, using commutator you can reverse the polarity. So you can reverse the polarity and you can change the voltage level also, is this thing clear, fine. So what I will do here, I will suppose there is a, it is like a capacitor plate you know, this is positive and this is negative initially. I can make it more and more positive and I can make it more and more negative and I can reverse the potential also and if I make it more and more positive, electrons will get attracted more and more, fine. What happens is when the radiation falls on this surface, not all electrons will have the same kinetic energy, fine. Some electron will be able to reach till here, some electron will just die down, their velocity will become so less that they can get again attracted back towards the emitter, fine. But if you have a positive potential, then there is a pulling force for the electron to come. But at any moment, suppose 10 electrons are ejected from here, how many it can catch max? 10 only. So this positive charge will not be able to increase number of electron that are coming out from here, but it can increase number of electron it catches, getting it, fine. So we have water parameters to play with, we have write down intensity of radiation that we can vary and see what will happen. We have potential difference to check, we have current to check, then what? We have frequency, fine. So these are the four factors we will see in this particular experiment, fine. Now why this is detecting a current micro emitter, this is an open circuit from here to here, but electrons from here, if it manages to reach here, then the circuit is closed, fine. So this micrometer will detect a current only when electrons from the collector, sorry electrons from the emitter is caught by the collector, fine. So if there is zero current means electrons are not able to reach the collector, right. So there will be a potential, there will be a potential in which this is negative potential and this will be positive potential. If you make this negative potential high enough, current will stop, it will ripple all the electrons, getting it, initially energy of electron will be so high that small negative potential will not be able to deter them, electron will be still able to reach there. But if you make it very high, electrons will be stopped and that is called the stopping potential, fine. What you have to do that? You have to make sure that electron which has a maximum current energy, that is not able to reach there. If you are able to stop the maximum current energy, others will automatically stop, okay. Few of the observation we will see, write down variation of current with intensity. You are changing the intensity and seeing how the current is changing, okay. Suppose this is photocurrent, what is the photocurrent electrons that are coming out from here, current due to that, it is called photocurrent, micrometer detects a photocurrent problem and this is intensity, okay. What is seen is a straight line, in fact this is not so small, fine. So what does it say, what does it mean? Directly proportional, it is directly proportional, so if the intensity is not changed, photocurrent will not change, no matter what you do, if the intensity is same, photocurrent will be same, okay. There is a direct correlation between photocurrent and the intensity of the radiation, fine. Next what we are going to do here is variation of photocurrent with potential. On the y axis, again there is a photocurrent and this is potential, can you guess what kind of graph it will be, you are changing the potential, right, you are, assume that if it is positive, it means positive potential, if it changes the polarity, it is negative potential, try to see, I mean try to draw yourself. If you increase the potential to very high, what will happen to current, if current is very high, sorry voltage is very high, it will catch all the electron that are coming out from here, okay. But then after some time, no matter what is the potential, current will be same, okay. So current will be uniform for very high voltage, but if it decrease the voltage, current will dip little bit, why, because not all electrons will be able to reach the collector, fine. And if you further decrease the voltage, you make it negative, then the current will stop at a particular potential, which is called the stopping potential, we know. This is let us say intensity I1. Now if you change the intensity, if you change the intensity to I2, where I2 is more than I1, what do you think will happen, it will go up, current, saturation currently more than one weird thing will happen is the stopping potential will be same, current is, current is of course increased because the intensity has been increased, all right. This current is called saturation current, saturation current for I1 and this is saturation current for I2, okay. Now one weird thing is the stopping potential is same, no matter what is the intensity, right. What you are doing is you are maintaining the frequency, frequency is same, you are not changing the frequency, when you are changing the intensity, okay. What does it mean? If the stopping potential is same, what does it mean? The maximum cutting energy that comes out from here is let us say K max and if stopping potential is V0, how these two are related, K max and stopping potential, you know right, conservation of energy, K1 plus U1, cutting energy plus potential energy is equal to cutting energy plus potential energy and K1 minus K2, you can write it as U2 minus U1. The change in potential is what? Charge into potential difference, E times V0, K1 is K max, K2 is what? 0, it stops. So K max should be E times V0, fine. So this observation says that the maximum cutting energy of the electron that comes out from here does not depend on intensity. No matter what intensity of radiation you fall on the metal surface, the maximum cutting energy of the electron will be same. This is actually slightly weird but then the other observations are you know little bit more insightful. Like for example, next one is variation of current with V0 only but now you are not changing intensity, you are changing frequency. Just draw this, this is potential, this is photogram. So for a particular frequency and intensity, the graph is like this. Let us say this is for frequency mu1, right. Now I am keeping intensity same, intensity is not changing but I am changing frequency now. What do you think the graph will be? Will the saturation current be different? Saturation current will be same or not? Yes. It will be same? Because intensity is directly proportional to photo current, fine. Saturation current will be same. So the graph will start like this only but slowly and slowly it will deviate and now the stopping potential is different. Stopping potential for two frequencies are different. This is for mu1 and that is for mu2 where mu2 is greater than mu1. This is again a weird finding that what does this mean? Maximum kinetic energy does not depend on intensity but it depends on frequency, fine. But total number of electrons that are coming out are same. Just that they have more energy now if you change the frequency. The final graph is stopping potential versus frequency. Stopping potential is what? A potential at which a current stops. So this is v0 and this is mu. It is found to be a straight line like this. This is for metal 1 and for metal 2 you get a parallel line. This is for metal 2, okay. Sorry, on y-axis there is stopping potential and x-axis mu. Now this is the most weird observation of all. Here you can see that the stopping potential is increasing with frequency. That is we have already observed that if you increase the frequency, stopping potential increases. So this is alright that stopping potential is increasing with the frequency. Then like this. But one thing that is weird is this point is zero potential is required to stop. What does it mean? No electrons are coming out. From here to here, this is a range of frequency and this point is called threshold frequency for metal 1. And this is the threshold frequency for metal 2. So it says that if your frequency is less than the threshold frequency, what will happen? Electrons won't be able to come out. No matter what is your intensity. It doesn't matter what is the intensity, electrons will not be coming out. A frequency is less than the threshold frequency. Okay. This is very, very weird because imagine the situation that frequency is very less. Okay. You are supplying energy to the electron with a very less frequency radiation. Fine. So if the frequency is less and electron is getting less energy, it can wait for few milliseconds to acquire sufficient energy and then should come out. But it is not doing that. Even if you put the radiation for hours and hours with a frequency less than the threshold frequency, then also it will never come out. Okay. And if your frequency is greater than the threshold frequency, even if a density is very, very less, then also electrons will come out. This is actually weird. Hence, using wave theory, you will not be able to understand what is going on. As if it is a wave, the electron should absorb sufficient energy from the wave, wait for some time and then come out. Even if frequency is less than the threshold frequency, that is not happening. Fine. So to understand this, Einstein has given a particle behavior of light theory. And that particle Einstein used to call it photons. Okay. So this is like the observation that has led to that theory.