The photoelectric and photovoltaic effects | Physics | Khan Academy

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. This was one of the key experiments that actually helped us discover a completely new model of light. But 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. The photoelectric effect happens. But if you were to shine red light on potassium, we will not get the photoelectric effect at all, regardless of how bright you make it. Even if you were to make it blindingly bright, we will not get the photoelectric effect.

This is what puzzled physicists. I mean, think about the model over here. We 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 with red light? Think 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.

So, 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 does color represent in electromagnetic waves? Remember that color basically depends on the wavelength of light. For 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.

If 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 observation, okay? I keep reducing, I keep reducing the wavelength until I hit 541 nanometers. At this point, I now start seeing the 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 hardly have any kinetic energy. I just get some photoelectric effect. 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 the photoelectric effect, but the electrons coming out with even more energy. What happens if I reduce it even further? I 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. We can also talk in terms of frequency. Remember, bigger the wavelength, smaller the frequency, because if you have a big wavelength, there are fewer waves passing per second. So, this is low frequency, and this is high frequency over here. We can say when it comes to frequency, more of the frequency means more energy of the electrons.

You also have some kind of a cutoff over here, right? So, for example, if the wavelength is above 541 nanometers for potassium, if it's above 541 nanometers, no photoelectric effect. Only below it will we get the photoelectric effect. 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 do you get the photo effect. If it's above, you won't get it.

Different metals have different threshold wavelengths and similarly different threshold frequencies. So, that's the effect of wavelength or frequency. We see that the wavelength or the frequency controls the energy with which the electrons come out, and that cannot be explained as to why. Why does the wavelength or the frequency control it? Why am I not getting the 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 brightness affect this whole thing? Does it have any effect? The answer is yes! Remember, brightness or the intensity of light is basically how big the valleys and the peaks are, right? 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. It doesn't change the energy with which these electrons come out. See, they're coming out pretty much with the same energy as before, but we now get more electrons.

Of course, if you're above the threshold wavelength, you will not get the photoelectric effect at all, regardless of the brightness. It doesn't matter, okay? So, if you decrease the brightness or intensity, you get fewer electrons. If you increase the intensity, you get more electrons. So, intensity only controls the number of electrons, but it's the wavelength or the frequency that controls the energy with which electrons come out. It also controls whether we get the photoelectric effect or not.

The big question was why. The wave model just cannot explain this because, according to the wave model, you should get the photo effect for all colors of light, right? 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. We were desperately in need of an answer for this.

So 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. When 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 the wave model; there, you can absorb continuously.

So, 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. So, the shorter the wavelength or the more the frequency, there's more energy in the packet.

There is a relationship between energy and the wavelength which we will not get into, but let me just give you some rough numbers over here because the numbers are going to help us. So here are some numbers: 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 electron volts. You may be wondering: shouldn’t we be measuring energy in joules? Well, joule turns out to be a big unit of energy, so we use a smaller unit of energy, which we call electron volts. Don't worry too much about the units over here; just the numbers.

You can see these packets have tiny energy, but this packet has much bigger energy—2.8 electron volts. You can see that, right? Now for potassium, it turns out if you want to pluck an electron, the minimum energy that you need is about 2.3 electron volts. This is for potassium.

Now 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—make that electron escape—then a single photon should have at least this much energy. If 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 radiate it back.

And there, if you consider red light, it does not have a single photon that carries enough energy. That's the reason why electrons are not getting ejected, and that's why these lights are unable to give you the photoelectric effect over here. We have just enough energy for the photoelectric effect, and therefore electrons barely make it out over here because all of that 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, and therefore some residual energy is left, so electrons after coming out have some extra energy remaining that goes out as kinetic energy. And since this has even more energy, each photon has even more energy, all electrons now eject with even more kinetic energy because 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. Over 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'll 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. That's why I get more electrons over here.

Putting it all together, since the frequency decides the energy of an individual photon, that decides the kinetic energy. The shorter the wavelength, the stronger, the more 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 won't be able to knock off anything, and you'll not get any photoelectric effect.

And since intensity is basically the number of photons, if you have a more number of photons, you will get a more number of electrons coming out. But over here, it doesn't matter how many photons you shine; therefore, it doesn't matter what the brightness is—you will not get the photoelectric effect. Beautiful, isn't it?

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

It's not that light sometimes behaves as waves and sometimes behaves as a particle. No, no, no! Light has both wave and particle nature. 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.

But this is one of the reasons why sometimes when we show 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 with in our daily life. This 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, now in the photoelectric effect from light, we get electrons ejected, right? Now there's a very similar, slightly different effect called the photovoltaic effect, in which when you shine light, you can generate voltage.

We call such an effect a photovoltaic effect, and the way that works is we need to first create a crystal in which there's an already inbuilt electric field. It's possible to do that, and we won't get too much into the 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 will 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.

Because of the difference in properties, an electric field gets built up. Now, 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. You 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, in the photo effect, it gets emitted, but here, instead of getting emitted, it just gets enough energy to escape the bond. As a result, now it is free to move, and therefore it'll get accelerated to the left in this diagram because the electric field is to the right.

Electrons are negatively charged; they'll experience a force in the opposite direction, and as a result, it will now come to the left side. It will leave behind a gap. Now other bonded electrons 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.

So it kind of feels that this gap will move to the other side. This way, a lot of electrons and a lot of voltage can be created. Look, if you can complete this circuit, electrons would love to go from here to there through that external circuit. In other words, there is a voltage created. 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. Whether 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're using the fact that light is a quantum object to harness the power of light, which you get from the Sun.