 Even if you shine blindingly bright visible light on the zinc metal surface, you get no photoelectric effect. And even if you shine extremely dim, but ultraviolet light, you get photoelectric effect. Why? Because it turns out that the visible light has just too low frequency. It's too low to cause photoelectric effect. And the ultraviolet light has high enough frequency for zinc to cause photoelectric effect. And so the goal of this video is to see how quantum nature of light helps us understand this weird nature of photoelectric effect as to why the brightness or the intensity doesn't matter, but it's the frequency that matters. So let's begin. So let's consider the wave model and the quantum model separately. Let's start with the wave model. Imagine this is our zinc metal and here's an electron trapped inside the metal. Now for it to escape, it needs to gain some energy. For simplicity, let's say it requires three units of energy. Now, according to the wave model, light is a wave. And when you shine light, so wave, when you shine light, the electron starts absorbing the energy from this wave. And so the electrons energy will start increasing from whatever it is right now. It'll get plus one, plus two. And finally, once it increases beyond three, it has now enough energy. It escapes from that metal. It's happy goes photoelectric effect. And according to this model, it doesn't matter what the frequency is. Any frequency light you shine, the electron must gain enough energy eventually and should just escape. And in fact, according to this model, if you shine brighter light, the electron gains so much more energy and it should be able to escape much more easily, and therefore you should get more photoelectric effect over here. And that's why according to wave model, it doesn't make sense. Why does the frequency matter and not the intensity? But now let's see what the quantum model says. The quantum model says, hey, light is not made of waves. It's made of photons. And we've explored this, right? According to the quantum model, light is made of particles, what we call photons. Now what happens when an electron absorbs this photon? Same electron. Again, even this one needs three units of energy. Let's take some numbers. Let's say the energy of this photon is five units. It has five units of energy. It's a packet that contains five units of energy. Now, when this electron absorbs this photon, it absorbs that five units instantly. What this means is that the energy of the electron instantly jumps plus five and try to understand what that means. It doesn't go plus one, plus two, plus three, plus four, and then plus five. It instantly goes plus five. It's kind of like your car going from zero to 100 kilometers per hour directly without going through plus 10, plus 10 kilometers per hour, 20 kilometers per hour, 30 kilometers. Can you imagine that? Direct jump. You can't imagine that. You might think that's so weird, but that's what quantum mechanics is all about. That's what quantum is all about. You can't get one or two units of energy. You either get all the five at once or you don't get anything at all. That's the weirdest, coolest thing about quantum mechanics. Anyways, once it absorbs that energy, now, because it has gotten plus five instantly, it has more than energy needed to escape. It escapes. It immediately escapes and you get photoelectric effect. But now, what if, what if the photon that we are incidenting does not have five units of energy? It has, say, only two units of energy. Let me draw a smaller photon to represent that. OK, let's say it only has two units of energy. What would happen now? Now, again, the electron would absorb that energy, but it's not enough to escape. And so it will still stay, stay trapped inside the metal. And so the electron does get excited because it has absorbed the photon of energy, but almost instantly it de-excites. It comes back to wherever it was before and it, you know, it releases that energy, maybe in the form of heat or maybe collisions. And so it comes back to where it was. And so now what happens if it if it absorbs another photon? Same thing. It excites and de-excites. Now, these electrons will always absorb one photon at a time. The chances of absorbing two photons at the same time is almost very negligible. And therefore you can see, even if I shine millions of photons on this, it will not cause any photoelectric effect. It's kind of like trying to knock this boat out of the ocean by using ping-pong balls. If you use a ping-pong ball, one ping-pong ball, nothing's going to happen. So even if you shoot multiple ping-pong balls, millions of ping-pong balls on after the other, nothing's going to happen to this boat. The number of ping-pong balls don't matter. What matters is that a single ball, imagine you have a single large cannon ball. One cannon ball is all that it takes to shoot this boat and knock it off. And that's how you should think about it. The number of photons don't matter. It's a single, a single photon should have enough energy to knock it off. Only then photoelectric effect will happen. And what does the energy of a photon depend on? You've seen the energy of any photon depends on, from Planck's equation, the Planck's constant H times the frequency of light. So more frequency of light, more energy of photon. So now can you pause the video and come back and explain why we don't see any photoelectric effect here, but we do see it over here. Pause and can think about it. All right, so what might be happening over here? Because the frequency of the light is too low, that means the energy of the photon over here must also be too low. And so I'm going to draw one, you know, tiny black dot to represent very low energy photon, and it does not have enough energy to knock the electron off. And that's why you get no photoelectric effect. But it's so bright, why doesn't that matter? Because brightness, high intensity means you have a lot of these photons. So let me draw lots of them. OK, my pen slipped over here. So we have a lot of photons falling on a lot of electrons, but none of them have enough energy to knock them off. So you get no photoelectric effect, like lots of ping-pong balls coming and hitting this boat is not going to happen. What's happening here? You have high frequency light and therefore the photon over here, I'm going to draw a big photon over here. The photons have enough energy to knock the electrons off. And so, but it's so dim. So yeah, it's very dim. And therefore there might be very few photons over here, very few. But whenever these photons get absorbed by the electrons, you get photoelectric effect. So you see what's happening? Even if you have a lot of energy over here, it's kind of like diluted into very tiny photons. None of them are able to do anything. And even if you have very weak energy over here, it's very concentrated into large photon packets. Each one of them is able to knock off the electrons. That's why photoelectric effect depends on the frequency whether it's going to happen or not. It depends on the frequency and not the intensity. OK, but let's not stop here. Let's see if we can explain all the results we saw of photoelectric experiment. One of the things that we saw is even if you were to increase the intensity of this light, you make it more energetic. The electrons do not come out with more energy. We saw that the kinetic energy does not increase. Instead, we get more photo electrons. Sorry, we get more electrons coming out. Can you explain why this is happening? OK, when you increase the intensity, you do not change the energy of individual photons. The energy of the individual photons stays the same. And therefore, when an electron absorbs that photon, nothing has changed. And so the electron comes out with pretty much the same energy as before. And therefore, the electrons energy does not change. But when you increase the intensity, you do increase the number of photons. So the number of photons increase. And therefore now more electrons are going to get those photons. And therefore you expect more electrons to come out per second. So now it makes sense. OK, the second thing we saw is if you increase the frequency of the light, then the kinetic energy increases. Electrons now come out with more energy. Can you explain that using the quantum model? Yes, because if you increase the frequency, the energy of the photons start increasing. Electrons gain more energy when they absorb that photon. And so they come out with more energy. Now that makes sense. The final thing we saw over here was photoelectric effect is instantaneous. Even if you shine extremely dim light, as long as it's about threshold frequency, you get photoelectric effect instantly. No time delay. Now, according to Wave Theory, this did not make sense because if you're shining very, very low intensity light, then the electron will take more time to gather that all of those energy. And so it will take more time for the electrons to come out. So according to the wave model, there should have been a delay. But now according to the quantum model, does it make sense? Yes, as long as the photons have enough energy, the interaction is almost instantaneous. So the electron almost instantly gets all that energy. Remember, it doesn't gain energy slowly and steadily. It instantly gains all of that energy. And then it instantly comes out. And so because the interaction between the electron and the photons are almost instantaneous, you get an almost instantaneous effect. And so hardly any time delay. So this is how the quantum model beautifully explains all the weird results of photoelectric effect. It's no longer weird now, right?