 While performing experiments on photoelectric effect, people were interested in measuring two things. One is how many electrons are coming out per second. These electrons are called photo electrons because they're coming out due to light. But how many of them are coming out per second? And secondly, with what energies are these electrons coming out? In fact, we wanted to know with what maximum kinetic energy would the electrons have? And as you might know the story, as you've seen in the previous videos, when we saw how the intensity and the frequency of the light affected these two things, it blew our mind. It changed the way we thought about light and it led to the whole quantum revolution. But the question I want to explore in this video and I think we should ask these questions more often in science is, how do we measure these things? What kind of experiments did we do that allowed us to count how many electrons are coming out per second and with what energies they're coming out? And remember, we're talking about the 1900s. And so in this video, we'll focus on building an experiment that will allow us to count how many electrons are coming out per second. And in the future video, we'll look at how to measure with what energies these electrons are coming out. So where do we start? Well, for photoelectric effect, we at least need a metal. So we know we can begin over there. Some metal that shows photoelectric effect. So back then we knew zinc showed photoelectric effect. That's how we discovered it. It was the first metal, I think. So we knew zinc shows photoelectric effect. And we discovered that when you shine ultraviolet light on it, zinc was becoming positive. So from this, we guessed that electrons are coming out from zinc. But now the question is, how do we count how many electrons are coming out? Well, what we can do is we can put another metal over here, which can sort of act as the collector. The goal of this metal would be to collect the electrons. So this would be the collector. And since the zinc is emitting the electrons, that zinc would become, or the metal which we are choosing over here, that would become the emitter. This would be the emitter. But how does that help? How does collecting electrons help us count how many electrons are coming out? Well, imagine the collector is collecting 100 electrons per second. That means there is a current of 100 electrons per second. If we can find a way to measure that current, then that current itself will be an indicator of how many electrons are being collected. How do we measure the current? Well, we can use an ammeter, but we can't use it over here. So what we can do is we can make a path for these collected electrons to go back. So we can make a path like this, right? Because think about as this collector collects electrons, it becomes negative. This becomes more positive, the electrons want to go back. So as this is collecting 100 electrons per second, those 100 electrons will go here. And so there will be 100 electrons going here per second. And I can measure the current here by using an ammeter. Back in the 1900s, we had invented ammeters. So ammeters were there. So if I put an ammeter over here, maybe a milli-ammeter, because the current might be very small. The current here is a direct indicator of how many electrons are being collected per second. Boom! I found a way to measure or count electrons. So let me write that. So this current here is a direct indicator of number of electrons collected, collected per second. But wait, we can't party it because we're not done yet. There's still a problem with this setup. See, this only measures how many electrons are being collected per second. And that's not necessarily the same as the electrons emitted per second, because not all the emitted electrons might reach the collector. Some of the electrons may just not reach the collector. And you can't practically build a collector to collect all the electrons just like that, right? So some of the electrons may not. So this number is not a true indicator. In fact, it should be smaller than actually how many electrons are emitted, because a lot of electrons might be missing the collector. So what do we do? So now the question is, well, now the problem is not all electrons emitted are being collected. So the question is, how do we do that? How can we somehow find a way to ensure that all the emitted electrons are being collected? How do we do that? Well, for that, we need to find a way for the collector to somehow attract electrons towards it, or maybe for emitter to somehow repel or push the electrons away from it. How can we do that? I want you to pause the video and think about what can you do? How can you modify this setup for the collector to start sucking in these electrons? Well, electrons are charged particles. They are negatively charged particles, right? So just make the collector positive. It'll start attracting it. Make the emitter negative. It'll start repelling it. That's all we have to do. But how do we do that? Hey, we can use a battery and put a voltage. So we can put a battery here. Let's put a battery here. And if I make, let's see, I want to make the collector positive. So if I put the positive of the battery here and the negative of the battery here, now my collector starts becoming positive and starts sucking more electrons into it. And I'm getting what I wanted. I'm now able to suck in the emitted electrons. And because we want to ensure that all the electrons are at least almost all the electrons, even the ones that are ejected, you know, different angles, even they need to get sucked in. What we can do is we can put a very high voltage, right? If the voltage is very high, then there's a stronger pulling force and more, you know, good chance that all the electrons get sucked in. But my question to you is, as you're running this experiment, let's say you put a high voltage, how will you be sure that even at the high voltage, all the electrons have gotten sucked in? I want you to pause and think about this for a while. This is, we have everything, but think experimentally, how will you confirm that all the electrons have been sucked in? All the emitted electrons are definitely being collected. What can you do? I want you to pause and think. Here's what we can do. Put some voltage and measure the current. Now increase that voltage. If the current increases, that means more electrons are getting collected. That means we are not there yet. Further increase the voltage. Again, if the current increases, that means still more electrons are being collected. We're not there yet. As you increase the voltage, finally there comes a point where the current will no longer increase. What does that mean? Ah, that means no more electrons are being collected. That means we must have hit a saturation. Almost all the emitted electrons must now be getting collected. When the current reaches a maximum value, and it doesn't increase after that, now we know for sure that all the electrons that are being emitted, almost, we can say, are definitely getting collected. The moral of the story is, in this experiment, wait for the current to reach maximum. Let me write that over here. Max current, or we can say saturation current, that is equivalent to the number of photo electrons, photo electrons ejected per second or collected per second. Pretty much the same at this point. So the last thing I want you to do, because practically you know how people perform this experiment, they draw graphs. And I know graphs can seem intimidating, but they are really insightful. So what I want you to think about, if you now have the whole story, I want you to predict this graph. Let's draw a graph of voltage on the x-axis, the voltage that you're going to put a battery over here, voltage of the battery versus the current that you detect over here. And the current is often called the photo current because it's the current due to photo electric effect. So over here we'll measure, we'll plot the photo current. I want you to pause and think about as you change the voltage from zero, from when the battery was not there, all the way to increasing the voltage, how would the graph look like? Can you pause? This is the last time I'm asking to pause. Can you pause and try? All right, let's see. So let's start with when the voltage is zero, when we don't have a battery. Do we get zero current? No, because even without the battery, we saw there will be some electrons ejected in the direction over here. They will get collected and there will be a current. So there might be some current to begin with, even when the voltage is zero. Now as I increase the voltage, more and more electrons get start getting sucked in. And so we would expect the graph to increase, go up. And then eventually we know we hit a maximum. And after that the graph won't increase anymore. So you can kind of guess the graph would go somewhat like this. And this point at which we reach saturation that represents our, you know, that represents our photo electrons come number of photo electrons coming out per second. So this means if I repeat the experiment, let's say I change something about my light, I repeat the experiment. And in the new experiment, let's say I get a graph in which the saturation current level is somewhere over here. I now know for sure in this experiment less electrons are emitted per second. In fact, I can by measuring this, I can say, okay, half the amount of electrons are being emitted per second. Or if I change something about my light and now in the new experiment, let's say I plot a graph and I get a graph in which the new saturation current is here. I again know for sure now that more photo electrons are being emitted per second. So you see using this clever experiment, we can just by looking at the maximum value of the photo current, we can know how many photo electrons are coming out per second. The last thing to think about is that if there are air molecules in between, then that will mess up our experiment. So this experiment should ideally be conducted in vacuum. So the electrons are not lost due to my know by colliding with the air molecules. And that's why this experiment is often conducted in some kind of a transparent chamber, transparent because we want the light to pass through. And so we often choose glass or quartz or some other transparent chamber and we suck out all the air and create vacuum. And there we go. This is how pretty clever, right? We can now count how many electrons are coming out per second. And in a future video, we'll see how we can tweak the same experiment and measure with what energy is there coming out. But until then, I want you to think about it, maybe discuss with your friends and teachers of how can you use the same setup, tweak it a little bit and be able to find what's the maximum energy with which the electrons are coming out.