 In the late 1800s, a man named Heinrich Hertz, the person after whom we have named the unit of frequency, Hertz, right? This person accidentally discovered that when you shine, sometimes when you shine light on metals, they eject electrons. And we called this phenomena the photoelectric effect because light is photo and electric means electrons coming out, so photoelectric effect. And this experiment revolutionized physics. It changed the way we thought about this world. And when I first learned about this, I was like, what? I mean, this is such an innocent looking experiment. Like what's so special about, what's so surprising about this? You shine light with energy and you get electrons. So what's the big deal? And so the goal of this video is actually to understand what was the big deal about this experiment and why it shook physics and why we had to come up with a completely new theory of light. Now the answer to this lies in the details of this experiment. And a fun fact is this effect was discovered even before electrons were discovered, which means it took about 10 to 15 years for, you know, and a lot of experiments for us to realize what was going on. But in all these experiments, people were trying to figure out how does changing the properties of light affect the electrons? To be more precise, there are two things that you can change about light. You can change its brightness, brightness, or more technically, you can say the intensity. So that's one thing you can change about the light. You can make it more bright. You can make it dim. The second thing you can change about light is its color. Or again, more technically, we can say its frequency. All right? So you can change these two things. And people wanted to know how does changing these two things affect the photoelectric effect? And the question is now, what are the things that can change in these electrons? Well, again, there are two things that can change in these electrons. So in the photoelectric effect, there are two things that can change. One is it could affect the speed of the electrons. Electrons could either come out faster or they could go slower. So it could affect the kinetic energy of the ejected electrons. The second thing it could affect is the number, I'm just gonna use hash for number, number of electrons. So you could have a lot of electrons coming out per second or you can have a little amount of electrons coming out per second. And so now the stage is set. People wanted to figure out how does changing these two things about the light affected these two things about the electrons? That was what people were trying to figure out. And of course, some of you might be curious and you may be wondering, well, how did we measure these things? I mean, electrons are so tiny. How do you measure their speed or their energy? How do you measure how many electrons are coming out per second? What kind of experiments did people perform? There are some clever experiments and we will talk about all the fun stuff in some other videos, the experiments and all of that. But in this video, let's focus on what the experimental results were and how it shocked everyone. So let's start with the brightness of the light. My question to you is, if you think of light as a wave and if you increase the brightness, increase its intensity, increase the energy of the wave, what would you expect to happen to the kinetic energy of the ejected electrons? Can you pause the video and think a little bit about that? What would you expect to happen? Well, if you're providing more energy to the electrons by increasing the intensity, I would expect electrons coming out with more energy, right? So we would expect more brightness, more kinetic energy, less brightness, less kinetic energy. That kind of makes sense, right? But to everyone's surprise, what they found experimentally is that the kinetic energy of the ejected electrons was independent. This was independent, independent, okay? Off the intensity, off the intensity. And people are like, what? What does that mean? This means you shine bright light, you shine dim light. It doesn't matter. The speed at which electrons come out doesn't change. And I was like, what? How does that make any sense? To understand better, let's think of an example. Instead of thinking about light waves and electrons, let's think about water waves and boats. I'm sure over here you would agree that because you have a huge wave when this comes and hits the boat, it's gonna launch that boat into orbit. And over here, nothing's gonna happen, right? So you see, with the speed at which the boat gets launched should depend on the height of the wave or the intensity of the wave, right? And that's why it was so weird when we found that the kinetic energy of the ejected electrons was independent of the intensity. We're like, how does that make any sense? But we did find that increasing intensity, if you increase the intensity, we did find that the number of electrons that increases. That was our experimental finding. So that increases. But kinetic energy doesn't increase and that's the point, like why? Okay, time for a second shock or something even more weird. They changed the frequency or the color of the light keeping the brightness or keeping the intensity exactly the same. You know what they found? They found that increasing the frequency increases the kinetic energy. What? Let me write that. They found that if you change the frequency, if you increase the frequency, they found that now the kinetic energy increases. If you decrease the frequency, the kinetic energy decreases. And again, people were baffled like, why does the frequency matter? Why if you increase the number of waves per second? For some reason, electrons are getting more energy. I've kept the intensity the same but somehow they're getting now more energy. This was even more weird. And let me tell you what this means. This means if you keep decreasing the frequency, even though you have enough intensity, very bright light, you keep decreasing the frequency, electrons will come out slower and slower and slower. And after one point, electrons won't come out at all. That means if you go below a particular frequency, you get no photoelectric effect. And this frequency, this minimum frequency, even name to it, we call it the threshold frequency. That's just a name, threshold frequency. And what this means is no photoelectric effect below this frequency. Which is so weird. And let me give you an example. If you take copper, its threshold frequency is 10 to the power of 15 hertz. This means even if you shine blindingly bright light below this frequency, you get nothing. All of that energy, nothing. And even if you shine very dim light above this frequency, you get photoelectric effect. Electrons will keep coming out. You increase the frequency, not the intensity, but frequency, you get electrons coming out faster. It's kind of like saying in this case, if you increase the number of waves, that will start increasing the energy with which the boat launches, which makes no sense at all. It's kind of like saying, hey, nothing's gonna happen over here because you only have one wave per second. You need minimum five waves per second to launch the boat. That makes no sense. You would say, why would the frequency matter? Frequency shouldn't have mattered. What should have mattered is just the intensity. So why is there a threshold frequency? Why frequency matters? There should have been a threshold intensity. That's what people were saying. And so that's why this did not make any sense. Physicists were completely baffled at this point. So it's the frequency that decides the kinetic energy and people have no idea why. And brightness does not decide the kinetic energy and people have no idea why. All right, the last one, there's another one, there's a third shocker. This has something to do with the time delay between the incident light and the electrons coming out. See, we thought light was a wave back then and when you're shining this wave of energy, there are billions and trillions of electrons inside over here, right? So we thought that all of that energy will be distributed among all these electrons and each electron will get a very tiny, tiny fraction of the energy, which means for it to gather enough energy, it would take time. It would take some time to gather all the energy needed to finally eject itself from the metal, to gain enough energy to eject itself from the metal. And calculations showed that if you have very dim light, it could take days before the first electron is able to get enough energy to eject out. If you shine very bright light, maybe it would take few hours and maybe if you shine even brighter light, maybe it would take few minutes. But you know what we found? We found that regardless of the brightness, regardless of the frequency, photoelectric effect was always instantaneous. You shine light, you immediately get electrons. You stop shining, no electrons. Doesn't matter whether it's bright light or dim light, doesn't matter if it's high frequency light or low frequency light, you either get photoelectric effect instantly or you won't get it at all. So we found instantaneous effect. This was an instantaneous effect and this also confused people because according to wave theory, the theory back then, we predicted that there should be a time delay, right? So why didn't that happen? And so now you know why this is such an important experiment in the history, one of the most important experiments in the history of physics because it was at a time where we thought physics was complete. We celebrated saying that, light is an electromagnetic wave, we have understood it and then came this experiment which completely shook us. And after this, a few incredible folks like Max Planck, Niels Bohr, Werner Heisenberg and of course our beloved Albert Einstein came along and came up with completely revolutionary ideas of light which you probably know why now as the quantum nature of light to explain this and of course many other phenomena we discovered later. And that's how this is one of the experiments that led to the quantum revolution. We'll talk about all the fun stuff in the future videos.