The photoelectric and photovoltaic effects

0:00- [Instructor] If you shine particular kinds of light on certain metals, electrons will be ejected. We call this the photoelectric effect because light is photo and electrons being ejected is electric, and this was one of the key experiments that actually helped us discover a completely new model of light.

0:17But how exactly you ask? Well, let's find out. What's interesting here is that this effect depends on the color of light. For example, if this metal was, say, potassium, okay, then if you shine blue light then we will get electrons being ejected, photoelectric effect happens.

0:32But if you were to shine red light on potassium, we will not get photoelectric effect at all, regardless of how bright you make it, even if you were to make it blindingly bright, we will not get photoelectric effect. This is what puzzled physicists. I mean, think about the model over here.

0:49We have atoms with electron clouds over here and the nucleus at the center, okay. When you shine light, the energy of the light gets transferred to the electrons and they're able to escape the clutches of the nucleus and go out. But why can't that happen over here with red light?

1:04Think about it. I'm shining bright light, very high intensity, incredible amount of energy over here, and yet electrons are not able to absorb it and get out. Why does the photoelectric effect depend on the color? That was a big question that didn't make any sense.

1:16So what do we do over here? Well, we do more careful experiments. First, let's only look at the color and then think about the brightness later, okay. So what is color representing? Electromagnetic waves. Remember that color basically depends on the wavelength of light.

1:31For example, red color is, the wavelength of light could be somewhere around 650 nanometers. What we find is that at 650 nanometers, we don't get any photoelectric effect for potassium. We don't know why, but now what we can do is let's reduce the wavelength and see what happens.

1:49If I keep reducing the wavelength and let's say I come to orange light of 600 nanometers, see I've reduced it, I still get no photoelectric effect. I don't know why, but I'm just doing an experiment. This is an observation, okay. I keep reducing, I keep reducing the wavelength until I hit 541 nanometers.

2:06At this point, I now start seeing photoelectric effect, and in this particular case, electrons are barely ejected from the metal, that's why I've drawn very tiny arrow marks over here. They have hardly have any kinetic energy. I just get some photoelectric effect.

2:21And then if I reduce it even further, that's where I get my blue light at about say 500 nanometers. These are rough numbers, okay? At 500 nanometers, I now get photoelectric effect, but the electrons coming out with even more energy. What happens if I reduce it even further?

2:37I find that electrons are coming out with even more energy. So what's our observation over here? We see that if we lower the wavelength, we get more energy for the electrons coming out over here. We can also talk in terms of frequency. Remember, bigger the wavelength, smaller the frequency, because if you have big wavelength, there are less waves passing per second.

2:57So this is low frequency and this is high over here. So we can say when it comes to frequency, more the frequency, more the energy of the electrons. And you also have some kind of a cutoff over here, right. So for example, if the wavelength is above 541 nanometers for potassium, for potassium, if it's above 541 nanometers, no photoelectric effect, only below it, we will get photoelectric effect.

3:19And so every metal will have its own cutoff, we call that the threshold wavelength or you can also say threshold frequency. But the whole idea is, if the wavelength is below that threshold wavelength, only then you get photoelectric effect, if it's above, you won't get it.

3:33Different metals have different threshold wavelengths, and similarly different threshold frequencies. So that's the effect of wavelength of frequency. We see that the wavelength of the frequency controls the energy with which the electrons come out and that cannot be explained as to why.

3:46Why does the wavelength of the frequency control it? Why am I not getting photoelectric effect if it's above the threshold wavelength? It doesn't make any sense. But anyways, the next question could be for us, how does the brightness affect this whole thing? Does it have any effect? The answer is yes.

4:01Remember, brightness, or the intensity of light, is basically how big the valleys and the peaks are. So if you were to make the light brighter, then it will look somewhat like this. You can imagine it this way. This is brighter light, okay. Now, what we find is that we get more electrons, okay.

4:23It doesn't change the energy with which the electrons come out, see they're coming out with pretty much the same energy as before, but we now get more electrons. Of course, if you're above the threshold wavelength, you'll not get photoelectric effect at all, regardless of the brightness, it doesn't matter, okay.

4:39So if you decrease the brightness or intensity, you get less electrons. If you increase the intensity, you get more electrons. So intensity only controls the number of electrons, but it's the wavelength of the frequency that controls the energy with which the electrons come out, it also controls whether we get photoelectric effect or not.

4:57The big question was why? The wave model just cannot explain this, because according to wave model, you should get photoelectric effect for all colors of light. If you make light bright enough, electrons should be able to absorb it and just get emitted, but that doesn't happen, and this is why physicists back then were puzzled and we were desperately in need of an answer for this.

5:19So what did we do? Well, to explain these observations, we came up with a completely brand new model of light. Instead of thinking of light as waves that carry energy continuously and that can transfer energy continuously, we thought maybe light is made of discrete packets of energy, not waves, but packets of energy, which we call photons.

5:43And then light is being absorbed by say electrons, you also absorb it as packets. You'll either absorb no light or you'll absorb one packet of light or two packets of light and so on and so forth, nothing in between. We call this discrete, which is exactly opposite of what happens in wave model, there you can absorb continuously, okay.

6:03So how does this explain the photoelectric effect, the observations over here? Well, let's see. The key thing over here is that the energy of the photons or the packets, notice, depends on the color. If you're dealing with long wavelength or low frequency light, then we have less energy of the packet, the photons have less energy, and if you're dealing with short wavelength or high frequency light, you can see that the packets have more energy.

6:29So shorter the wavelength or more the frequency, there is more energy in the packet. There is a relationship between energy and the wavelength, which we'll not get into, but let me just give you some rough numbers over here because the numbers is gonna help us over here.

6:43So here are some numbers. So it turns out that if you consider red light of 650 nanometers, the energy of the packet, the energy of the photon is about 1.9 electronvolt. You maybe wondering, shouldn't we be measuring energy in joules? Well, joules turns out to be a big unit of energy, so we use a smaller unit of energy, which we call electronvolts.

7:03Don't worry too much about the units over here, it's just the numbers. You can see these packets have tiny energy, but this packet has much bigger energy, 2.8 electronvolts, you can see that, right. Now for potassium, it turns out, if you want to pluck an electron, if electron needs to be ejected, the minimum energy that you need is about 2.3 electronvolts, this is for potassium.

7:27Now is a great time for you to pause the video, and see if you can try and come up with an explanation over here. Alright, let's see. The big idea over here is that if you want to knock off an electron, I mean like, you know, make that electron escape, then a single photon should have at least this much energy.

7:44If the photons do not have at least this much energy, then the electron will absorb it but it's not enough to escape and so it'll just reradiate it back. And therefore, if you consider red light, a single photon does not carry enough energy, and that's the reason why electrons are not getting ejected over here, and that's why these lights are unable to give you photoelectric effect.

8:07Over here, we have just enough energy for photoelectric effect, and therefore, electrons barely make it out over here, because all of the energy is used up in just releasing the electrons, there's hardly any energy left over here so they'll be hardly moving. But over here, notice, you have more than the necessary energy over here and therefore some residual energy is left, and so electrons after coming out have some extra energy remaining, that goes out as kinetic energy.

8:35And since this has even more energy, each photon has even more energy, well, electrons now eject with even more kinetic energy 'cause there's more residual energy after getting ejected. But what about the intensity? Well, if you increase the intensity in this model, we are increasing the number of photons, that's it.

8:54Over here, notice, if a single photon does not have enough energy, then I don't care how many photons you shine, it's just not going to work, that's why here I will still not get any photoelectric effect. But over here, now I'm shining more number of photons, so more electrons can absorb that energy and therefore more electrons can escape per second and that's why I get more electrons over here.

9:16Putting it all together, since the wavelength of the frequency decides the energy of an individual photon, that decides the kinetic energy. Shorter the wavelength, more is the energy of the photon and more is the kinetic energy. If the wavelength is bigger and it's too big, the energy of the photon is very tiny, it'll not be able to knock off anything and you'll not get any photoelectric effect.

9:39And since intensity is basically the number of photons, if you have more number of photons, you'll get more number of electrons coming out. But over here, it doesn't matter how many photons you shine, and therefore it doesn't matter what the brightness is, you will not get photoelectric effect.

9:54Beautiful, isn't it? So wait, does this mean that light is not a wave? It's actually particles? Well, not quiet. You see certain phenomena of light like diffraction or interference means that light must have wave properties. And certain other phenomenons like photoelectric effect, black body radiation, scattering of light, and any other such effects makes us believe that light must also have the particle nature, the photon nature, which means a light must have a dual nature, both particle and waves.

10:29It's not that light sometimes behaves as waves and sometimes behaves as particle. No, no, no. Light has both wave and particle nature. And if you're wondering, well, how does that make any sense? How can something be both waves and particles at the same time? Well, unfortunately, there's no way to really visualize it because in our macroscopic world, we don't have any experience of things having both wave and particle nature.

10:52But this is one of the reasons why sometimes when we are showing photons, we show it this way with a tiny wave packet, but this doesn't mean that the photons are wiggling up and down, okay, that's a misconception that I used to have. It's not like that. A better way to sort of think about this is that light is not a wave in the traditional sense, it's not a particle in the traditional sense, it's a brand new object, which we don't have experience within our daily life.

11:19This object has both wave properties and particle properties, and we call such an object, a quantum object. Now this sounds very theoretical, right? But there are so many applications of the fact that light is a quantum object. Let me tell you one of them, okay.

11:34Now, in photoelectric effect, from light, we get electrons ejected. Now there's a very similar, slightly different, effect in which when you shine light, you can generate voltage. We call such an effect a photovoltaic effect. Now, the way that works is we need to first create a crystal in which there's an already inbuilt electric field.

11:57It's possible to do that. We'll not get too much details of how we build such crystals, but using semiconductors, we can build crystals like that. We don't have to hook it up to any battery or anything, it'll have an inbuilt electric field. The crystal is built in such a way that one side of the crystal has slightly different properties compared to another side of the crystal, and because of the difference in properties, an electric field gets built up.

12:18Now the important point is there are electrons everywhere, but if you focus on this region, there are a lot of electrons, but they're all bonded and they're not free to move. So even if there's an electric field pushing on them, they cannot move because they're stuck in bonds.

12:32You can imagine that this is like the sea of electrons, they're all kind of fixed inside the crystal, they cannot move. But if we shine light in this region, and if the light has the suitable frequency or the suitable wavelength, then the electrons can absorb that energy, but it won't get emitted, okay, that's the difference over here.

12:52In photoelectric effect, it gets emitted, but here, instead of getting emitted, it just gets enough energy to escape the bond. And as a result, now it's free to move, and therefore it'll get accelerated to the left in this diagram, because electric field is to the right, electrons are negatively charged, they experience of force in the opposite direction.

13:08And as a result, it will now come to the left side, and it will leave behind a gap. Now, other electrons under electrons which are bonded can swoop into this gap, which makes the gap go to the right, and then the other electrons can swoop into this gap and so on and so forth.

13:25So it kind of feels that this gap, this vacancy, will move in to the other side. This way a lot of electrons and a lot of vacancies can be created. And so, look, if you can complete this circuit, electrons would love to go from here to here through that external circuit.

13:42In other words, there is a voltage created. And so what we have done is we have used the energy from light to create voltage, photovoltaic effect. If you put a lot of these together, we create a solar panel, that's how solar cells and solar panels work. They work on the photovoltaic effect.

14:04Whether you consider them on the roofs of the houses or you consider the ones which are in the spacecraft, they all use the same idea. At the end of the day, we are using the fact that light is a quantum object to harness the power of light, which we get from the sun.