0:00:00.360,0:00:03.333 - If you shine particular kinds[br]of light on certain metals, 0:00:04.653,0:00:05.726 electrons will be ejected. 0:00:05.726,0:00:08.220 We call this the photoelectric[br]effect because light is photo 0:00:08.220,0:00:10.860 and electrons being ejected is electric. 0:00:10.860,0:00:12.450 And this was one of the key experiments 0:00:12.450,0:00:15.510 that actually helped us[br]discover a completely 0:00:15.510,0:00:17.040 new model of light. 0:00:17.040,0:00:19.560 But how exactly you ask,[br]well, let's find out. 0:00:19.560,0:00:22.200 What's interesting here is[br]that this effect depends on 0:00:22.200,0:00:23.640 the color of light. 0:00:23.640,0:00:27.600 For example, if this metal[br]was, say potassium, okay, 0:00:27.600,0:00:29.490 then if you shine blue light, 0:00:29.490,0:00:31.320 then we will get electrons being ejected, 0:00:31.320,0:00:33.366 photoeletric effect happens. 0:00:33.366,0:00:35.610 But if you were to shine[br]red light on potassium, 0:00:35.610,0:00:38.160 we will not get[br]photoeletric effect at all. 0:00:38.160,0:00:40.020 Regardless of how bright you make it. 0:00:40.020,0:00:42.750 Even if you were to make[br]it blindingly bright, 0:00:42.750,0:00:44.700 we will not get photoeletric effect. 0:00:44.700,0:00:47.700 This is what puzzled physicists. 0:00:47.700,0:00:50.910 I mean, think about the[br]model over here we have atoms 0:00:50.910,0:00:53.430 with electron clouds[br]over here and the nucleus 0:00:53.430,0:00:54.630 at the center, okay? 0:00:54.630,0:00:56.430 When you shine light,[br]the energy of the light 0:00:56.430,0:00:59.400 gets transferred to the electrons[br]and they're able to escape 0:00:59.400,0:01:01.950 the clutches of the nucleus and go out. 0:01:01.950,0:01:04.290 But why can't that happen[br]over here with red light? 0:01:04.290,0:01:06.660 Think about it, I'm shining bright light, 0:01:06.660,0:01:09.210 very high intensity, incredible[br]amount of energy over here, 0:01:09.210,0:01:11.850 and yet electrons are not[br]able to absorb it and get out. 0:01:11.850,0:01:13.920 Why does the photoeletric[br]effect depend on the color? 0:01:13.920,0:01:16.890 That was a big question[br]that didn't make any sense. 0:01:16.890,0:01:18.570 So what do we do over here? 0:01:18.570,0:01:20.670 Well, we do more careful experiments. 0:01:20.670,0:01:22.380 First, let's only look at the color 0:01:22.380,0:01:24.630 and then think about the[br]brightness later, okay. 0:01:24.630,0:01:26.910 So what is color representing[br]electromagnetic waves? 0:01:26.910,0:01:30.960 Remember that color[br]basically depends on the wave 0:01:30.960,0:01:31.793 length of light. 0:01:31.793,0:01:34.920 For example, red color is[br]the wavelength of light 0:01:34.920,0:01:38.130 could be somewhere around 650 nanometers. 0:01:38.130,0:01:40.860 What we find is that at 650 nanometers 0:01:40.860,0:01:43.680 we don't get any photoelectric[br]effect for potassium. 0:01:43.680,0:01:46.590 We don't know why, but[br]now what we can do is 0:01:46.590,0:01:49.230 let's reduce the wavelength[br]and see what happens 0:01:49.230,0:01:51.480 if I keep reducing the[br]wavelength, and let's say 0:01:51.480,0:01:54.900 I come to orange light of 600 nanometers, 0:01:54.900,0:01:56.130 see I've reduced it. 0:01:56.130,0:01:58.560 I still get no photoeletric[br]effect, I don't know why, 0:01:58.560,0:01:59.610 but I'm just doing an experiment. 0:01:59.610,0:02:01.050 This is an observation, okay? 0:02:01.050,0:02:01.883 I keep reducing. 0:02:01.883,0:02:06.883 I keep reducing the wavelength[br]until I hit 541 nanometers. 0:02:06.930,0:02:10.620 At this point, I now start[br]seeing photoeletric effect, 0:02:10.620,0:02:14.220 and in this particular case,[br]electrons are barely ejected 0:02:14.220,0:02:15.660 from the metal. 0:02:15.660,0:02:17.730 That's why I've drawn very[br]tiny arrow marks over here, 0:02:17.730,0:02:19.710 they have hardly have any kinetic energy. 0:02:19.710,0:02:21.610 I just get some photoeletric effect 0:02:22.572,0:02:24.750 and then if I reduce it even[br]further, that's where I get my 0:02:24.750,0:02:27.210 blue light at about say 500 nanometers. 0:02:27.210,0:02:28.620 These are rough numbers, okay? 0:02:28.620,0:02:31.500 At 500 nanometers, I now[br]get photoelectric effect, 0:02:31.500,0:02:35.100 but the electrons coming[br]out with even more energy. 0:02:35.100,0:02:37.590 What happens if I reduce it even further? 0:02:37.590,0:02:42.000 I find that electrons are coming[br]out with even more energy. 0:02:42.000,0:02:43.620 So what's our observation over here? 0:02:43.620,0:02:47.010 We see that if we lower the[br]wavelength, we get more energy 0:02:47.010,0:02:48.930 for the electrons coming out over here. 0:02:48.930,0:02:50.370 We can also talk in terms of frequency. 0:02:50.370,0:02:53.310 Remember, bigger the wavelength,[br]smaller the frequency, 0:02:53.310,0:02:54.510 because if you have big wavelength, 0:02:54.510,0:02:57.090 there are less waves passing per second. 0:02:57.090,0:03:00.240 So this is low frequency[br]and this is high over here. 0:03:00.240,0:03:02.460 So we can say when it comes to frequency, 0:03:02.460,0:03:06.150 more the frequency, more[br]the energy of the electrons. 0:03:06.150,0:03:09.330 And you also have some kind[br]of a cutoff over here, right? 0:03:09.330,0:03:12.390 So for example, if the wave[br]length is above 541 nanometers 0:03:12.390,0:03:15.450 for potassium, for potassium,[br]if it's above 5 41 nanometers, 0:03:15.450,0:03:17.910 no photoeletric effect, only below it, 0:03:17.910,0:03:19.200 we will get photoelectric effect. 0:03:19.200,0:03:21.750 And so every metal will[br]have its own cutoff. 0:03:21.750,0:03:24.090 We call that the threshold wavelength, 0:03:24.090,0:03:25.980 or you can also say threshold frequency. 0:03:25.980,0:03:28.920 But the whole idea is if[br]the wavelength is below 0:03:28.920,0:03:31.999 that threshold wavelength[br]only then you get 0:03:31.999,0:03:33.540 photoelectric effect, if[br]it's about you won't get it. 0:03:33.540,0:03:35.970 Different metals have[br]different threshold wavelengths 0:03:35.970,0:03:38.490 and similarly different[br]threshold frequencies. 0:03:38.490,0:03:40.290 So that's the effect of[br]wavelength of frequency. 0:03:40.290,0:03:42.810 We see that the wavelength[br]of the frequency controls 0:03:42.810,0:03:44.610 the energy with with[br]the electrons come out 0:03:44.610,0:03:46.680 and that cannot be explained as to why. 0:03:46.680,0:03:49.350 Why does the wavelength of[br]the frequency control it? 0:03:49.350,0:03:51.120 Why am I not getting photoeletric effect? 0:03:51.120,0:03:52.890 If it's about the threshold wavelength, 0:03:52.890,0:03:54.180 it doesn't make any sense. 0:03:54.180,0:03:56.250 But anyways, the next[br]question could be for us, 0:03:56.250,0:03:58.500 how does the brightness[br]affect this whole thing? 0:03:58.500,0:04:00.090 Does it have any effect? 0:04:00.090,0:04:01.020 The answer is yes. 0:04:01.020,0:04:04.664 Remember, brightness or the[br]intensity of light is basically 0:04:04.664,0:04:07.980 how big the valleys and[br]the peaks are, right? 0:04:07.980,0:04:11.880 So if you were to make the light brighter, 0:04:11.880,0:04:14.400 then it will look somewhat like this. 0:04:14.400,0:04:15.660 You can imagine it this way. 0:04:15.660,0:04:17.640 This is brighter light, okay? 0:04:17.640,0:04:21.953 Now what we find is that we[br]get more electrons, okay? 0:04:23.160,0:04:25.170 It doesn't change the energy[br]with these electrons come out. 0:04:25.170,0:04:28.181 See they're coming out with[br]much the same energy as before, 0:04:28.181,0:04:29.520 but we now get more electrons. 0:04:29.520,0:04:31.967 Of course, if you're above[br]the threshold wavelength, 0:04:31.967,0:04:35.580 you'll not get photo[br]photoelectric effect at all, 0:04:35.580,0:04:36.900 regardless of the brightness. 0:04:36.900,0:04:39.030 It doesn't matter, okay? 0:04:39.030,0:04:41.760 So if you decrease the[br]brightness or intensity, 0:04:41.760,0:04:43.230 you get less electrons. 0:04:43.230,0:04:45.990 If you increase the intensity,[br]you get more electrons. 0:04:45.990,0:04:49.140 So intensity only controls[br]the number of electrons, 0:04:49.140,0:04:52.530 but it's the wavelength of[br]the frequency that controls 0:04:52.530,0:04:54.210 the energy with visual electrons come out, 0:04:54.210,0:04:57.150 it also controls whether we[br]get photoeletric effect or not. 0:04:57.150,0:05:00.600 The big question was why the[br]wave model just cannot explain 0:05:00.600,0:05:03.060 this because according to wave model, 0:05:03.060,0:05:05.370 you should get photoeletric[br]effect for all colors 0:05:05.370,0:05:06.360 of light, right? 0:05:06.360,0:05:08.520 If you make light bright[br]enough, electrons should be able 0:05:08.520,0:05:11.070 to absorb it and just get,[br]you know just get admitted. 0:05:11.070,0:05:12.270 But that doesn't happen. 0:05:12.270,0:05:16.140 And this is why physicists[br]back then were puzzled 0:05:16.140,0:05:19.200 and we were desperately in[br]need of an answer for this. 0:05:19.200,0:05:20.910 So what did we do? 0:05:20.910,0:05:22.920 Well, to explain these[br]observations, we came up with 0:05:22.920,0:05:26.730 a completely brand new model of light. 0:05:26.730,0:05:29.460 Instead of thinking of[br]light as waves that carry 0:05:29.460,0:05:31.980 energy continuously and that can transfer 0:05:31.980,0:05:36.000 energy continuously,[br]we thought maybe light 0:05:36.000,0:05:40.200 is made of discrete packets[br]of energy, not waves, 0:05:40.200,0:05:43.230 but packets of energy,[br]which we call photons. 0:05:43.230,0:05:46.410 And then light is being[br]absorbed by say electrons. 0:05:46.410,0:05:47.940 You also absorb it as packets. 0:05:47.940,0:05:50.640 You'll either absorb no[br]light or you'll absorb 0:05:50.640,0:05:53.190 one packet of light or[br]two packets of light 0:05:53.190,0:05:56.000 and so on and so forth,[br]nothing in between. 0:05:56.000,0:05:58.470 We call this discrete,[br]which is exactly opposite 0:05:58.470,0:06:00.030 of what happens in wave model, 0:06:00.030,0:06:02.940 there you can absorb continuously. 0:06:02.940,0:06:06.180 Okay, so how does this explain[br]the photoelectric effect, 0:06:06.180,0:06:07.620 the observations over here? 0:06:07.620,0:06:09.249 Well, let's see. 0:06:09.249,0:06:12.105 The key thing over here is[br]that the energy of the photons 0:06:12.105,0:06:14.100 or the packets notice[br]depends on the color. 0:06:14.100,0:06:15.810 If you're dealing with long wavelength 0:06:15.810,0:06:18.790 or low frequency light,[br]then we have less energy 0:06:18.790,0:06:22.500 of the packet, the[br]photons have less energy. 0:06:22.500,0:06:24.870 And if you're dealing[br]with short wavelength 0:06:24.870,0:06:28.050 or high frequency light, you[br]can see that the packets have 0:06:28.050,0:06:29.310 more energy. 0:06:29.310,0:06:32.910 So shorter the wavelength[br]or more the frequency, 0:06:32.910,0:06:35.310 there is more energy in the packet. 0:06:35.310,0:06:37.830 There is a relationship between[br]energy and the wavelength, 0:06:37.830,0:06:39.150 which we'll not get into. 0:06:39.150,0:06:41.370 But lemme just give you[br]some rough numbers over here 0:06:41.370,0:06:43.200 because the numbers is[br]gonna help us over here. 0:06:43.200,0:06:45.630 So here are some numbers. 0:06:45.630,0:06:47.930 So it turns out that if[br]you consider red light 0:06:49.253,0:06:51.017 of 650 nanometers, the energy[br]of the packet, the energy 0:06:51.017,0:06:53.850 of the photon is about 1.9 electron volt. 0:06:53.850,0:06:56.310 Yeah, maybe wondering,[br]shouldn't we be measuring 0:06:56.310,0:06:57.330 energy in joules? 0:06:57.330,0:07:00.330 Well, joule turns out to[br]be a big unit of energy. 0:07:00.330,0:07:01.980 So we use a smaller unit of energy, 0:07:01.980,0:07:03.600 which we call electron volts. 0:07:03.600,0:07:06.677 Don't worry too much[br]about the units over here, 0:07:06.677,0:07:07.759 it's just the numbers. 0:07:07.759,0:07:10.560 You can see these[br]packets have tiny energy, 0:07:10.560,0:07:12.570 but this packet has much bigger energy. 0:07:12.570,0:07:14.550 2.8 electron volts, you[br]can see that, right. 0:07:14.550,0:07:17.040 Now for potassium it turns out, 0:07:17.040,0:07:18.360 if you want to pluck an electron, 0:07:18.360,0:07:21.540 if electron needs to be[br]ejected, the minimum energy 0:07:21.540,0:07:26.100 that you need is about 2.3 electron volts. 0:07:26.100,0:07:27.750 This is for potassium. 0:07:27.750,0:07:29.580 Now is a great time for[br]you to pause the video 0:07:29.580,0:07:30.810 and see if you can try and come up 0:07:30.810,0:07:32.670 with an explanation over here. 0:07:32.670,0:07:33.870 Alright, let's see. 0:07:33.870,0:07:37.287 The big idea over here is[br]that if you want to knock off 0:07:37.287,0:07:38.772 an electron, I mean like you know, 0:07:38.772,0:07:41.370 make that electron escape,[br]then a single photon 0:07:41.370,0:07:44.280 should have at least this much energy. 0:07:44.280,0:07:47.070 If the photons do not have[br]at least this much energy, 0:07:47.070,0:07:48.570 then the electron will absorb it, 0:07:48.570,0:07:50.850 but it's not enough to escape, 0:07:50.850,0:07:52.950 and so it'll just reradiate it back. 0:07:52.950,0:07:56.850 And therefore, if you[br]have consider red light, 0:07:56.850,0:07:58.650 it does not have a single photon, 0:07:58.650,0:08:00.270 does not carry enough energy. 0:08:00.270,0:08:02.190 And that's the reason[br]why electrons are not 0:08:02.190,0:08:03.690 getting injected over here. 0:08:03.690,0:08:06.090 And that's why these lights are unable 0:08:06.090,0:08:07.680 to give you photoelectric effect. 0:08:07.680,0:08:10.590 Over here, we have just enough energy 0:08:10.590,0:08:13.067 for photoelectric effect[br]and therefore electrons 0:08:13.067,0:08:16.402 barely make it out over here,[br]because all of the energy 0:08:16.402,0:08:18.810 is used up in just[br]releasing the electrons. 0:08:18.810,0:08:20.820 There's hardly any energy left over here, 0:08:20.820,0:08:22.800 so there'll be hardly moving. 0:08:22.800,0:08:26.220 But over here, notice you[br]have more than the necessary 0:08:26.220,0:08:30.030 energy over here and therefore[br]some residual energy is left. 0:08:30.030,0:08:33.690 And so electrons after coming[br]out have some extra energy 0:08:33.690,0:08:36.675 remaining that goes out as kinetic energy. 0:08:36.675,0:08:38.190 And since this has even more energy, 0:08:38.190,0:08:41.250 each photon has even more[br]energy while electrons now 0:08:41.250,0:08:43.200 eject with even more kinetic energy, 0:08:43.200,0:08:46.050 'cause there's more residual[br]energy after getting ejected. 0:08:46.050,0:08:47.880 But what about the intensity? 0:08:47.880,0:08:51.060 Well, if you increase the[br]intensity in this model, 0:08:51.060,0:08:54.450 we are increasing the number[br]of photons, that's it. 0:08:54.450,0:08:57.150 Over here notice if a[br]single photon does not have 0:08:57.150,0:08:59.566 enough energy, then I don't[br]care how many photons you shine, 0:08:59.566,0:09:01.590 it's just not going to work. 0:09:01.590,0:09:03.884 That's why here I will still not get any, 0:09:03.884,0:09:05.430 you know, photoelectric effect. 0:09:05.430,0:09:08.940 But over here now I'm shining[br]more number of photons, 0:09:08.940,0:09:11.290 so more electrons can absorb that energy 0:09:12.183,0:09:13.650 and therefore more electrons[br]can escape per second. 0:09:13.650,0:09:16.110 And that's why I get[br]more electrons over here, 0:09:16.110,0:09:17.640 putting it all together. 0:09:17.640,0:09:20.310 Since the wave of the[br]frequency decides the energy 0:09:20.310,0:09:23.700 of an individual photon that[br]decides the kinetic energy, 0:09:23.700,0:09:27.060 shorter the wavelength,[br]stronger, more is the energy 0:09:27.060,0:09:30.330 of the photon and more[br]is the kinetic energy. 0:09:30.330,0:09:32.907 If the wavelength is[br]bigger and it's too big, 0:09:32.907,0:09:35.160 the energy of the photon is very tiny, 0:09:35.160,0:09:36.930 it'll not be able to knock off anything 0:09:36.930,0:09:39.210 and you'll not get any[br]photoelectric effect. 0:09:39.210,0:09:42.810 And since intensity is[br]basically the number of photons, 0:09:42.810,0:09:45.930 if you have more number of[br]photons, you'll get more number 0:09:45.930,0:09:47.280 of electrons coming out. 0:09:47.280,0:09:50.430 But over here, it doesn't matter[br]how many photons you shine, 0:09:50.430,0:09:52.740 and therefore it doesn't[br]matter what the brightness is, 0:09:52.740,0:09:54.660 you will not get photoelectric effect. 0:09:54.660,0:09:56.250 Beautiful, isn't it? 0:09:56.250,0:09:59.040 So wait, does this mean[br]that light is not a wave? 0:09:59.040,0:10:01.080 It's actually particles? 0:10:01.080,0:10:02.400 Well, not quiet. 0:10:02.400,0:10:05.130 You see certain phenomena[br]of light like diffraction 0:10:05.130,0:10:09.600 or interference means that[br]light must have wave properties 0:10:09.600,0:10:13.440 and certain other phenomenon[br]like photoelectric effect, 0:10:13.440,0:10:17.327 black body radiation, scattering of light, 0:10:17.327,0:10:19.140 and other such effects makes us believe 0:10:19.140,0:10:22.409 that light must also[br]have the particle nature, 0:10:22.409,0:10:26.520 the photon nature, which[br]means a light must have 0:10:26.520,0:10:29.760 a dual nature, both particle and waves. 0:10:29.760,0:10:31.850 It's not that light[br]sometimes behaves as waves 0:10:31.850,0:10:33.671 and sometimes wears as particle. 0:10:33.671,0:10:34.629 No, no, no. 0:10:34.629,0:10:36.660 Light has both wave and particle nature. 0:10:36.660,0:10:38.580 And if you're wondering, well,[br]how does that make any sense? 0:10:38.580,0:10:40.110 How can something be both waves 0:10:40.110,0:10:42.150 and particles at the same time? 0:10:42.150,0:10:45.780 Well, unfortunately, there's[br]no way to really visualize it 0:10:45.780,0:10:47.220 because in our macroscopic world, 0:10:47.220,0:10:50.220 we don't have any experience[br]of things having both wave 0:10:50.220,0:10:52.500 and particle nature. 0:10:52.500,0:10:54.180 But this is one of the reasons[br]why sometimes when we are 0:10:54.180,0:10:56.040 showing photons, we show it this way 0:10:56.040,0:10:59.970 with a tiny wave packet,[br]but this doesn't mean 0:10:59.970,0:11:01.920 that the photons are wiggling up and down. 0:11:01.920,0:11:03.900 Okay, that's a misconception[br]that I used to have. 0:11:03.900,0:11:05.868 It's not like that. 0:11:05.868,0:11:07.710 A better way to sort of[br]think about this is that 0:11:07.710,0:11:11.041 light is not a wave in[br]the traditional sense. 0:11:11.041,0:11:13.830 It's not a particle in[br]the traditional sense, 0:11:13.830,0:11:17.110 it's a brand new object,[br]which we don't have 0:11:17.110,0:11:19.410 experience within our daily life. 0:11:19.410,0:11:21.270 This object has both wave properties 0:11:21.270,0:11:24.240 and particle properties,[br]and we call such an object, 0:11:24.240,0:11:26.010 a quantum object. 0:11:26.010,0:11:28.260 Now this sounds very theoretical, right? 0:11:28.260,0:11:30.120 But there are so many[br]applications of the fact 0:11:30.120,0:11:31.709 that light is a quantum object. 0:11:31.709,0:11:34.230 Let me tell you one of them, okay? 0:11:34.230,0:11:36.210 Now, in photoelectric effect from light, 0:11:36.210,0:11:38.133 we get electrons ejected, right? 0:11:39.221,0:11:40.054 Now there's a very similar, 0:11:40.054,0:11:42.720 slightly different effect[br]called (indistinct) 0:11:42.720,0:11:45.330 when you shine light,[br]you can generate voltage. 0:11:45.330,0:11:49.410 We call such an effect,[br]a photovoltaic effect. 0:11:49.410,0:11:53.520 Now, the way that works is we[br]need to first create a crystal 0:11:53.520,0:11:57.030 in which there's an already[br]inbuilt electric field. 0:11:57.030,0:11:58.350 It's possible to do that. 0:11:58.350,0:12:00.270 We'll not get too much[br]details of how we build 0:12:00.270,0:12:02.280 such crystals, but using semiconductors, 0:12:02.280,0:12:04.080 we can build crystals like that. 0:12:04.080,0:12:06.600 We don't have to hook it up[br]to any battery or anything. 0:12:06.600,0:12:08.910 It'll have an inbuilt electric field. 0:12:08.910,0:12:11.280 The crystals is built in[br]such a way that one side 0:12:11.280,0:12:13.350 of the crystal has slightly[br]different properties compared 0:12:13.350,0:12:14.940 to another side of the crystal. 0:12:14.940,0:12:16.320 And because of the[br]difference in properties, 0:12:16.320,0:12:18.180 an electric field gets built up. 0:12:18.180,0:12:20.640 Now the important point is[br]there are electrons everywhere, 0:12:20.640,0:12:23.460 but if you focus on this[br]region, there are a lot of 0:12:23.460,0:12:26.940 electrons, but they're[br]all bonded and they're not 0:12:26.940,0:12:27.773 free to move. 0:12:27.773,0:12:29.700 So even if there's an electric[br]field pushing on them, 0:12:29.700,0:12:32.580 they cannot move because[br]they're stuck in bonds. 0:12:32.580,0:12:35.040 You can imagine that this is[br]like the sea of electrons. 0:12:35.040,0:12:37.830 They're all kind of[br]fixed inside the crystal, 0:12:37.830,0:12:39.025 they cannot move. 0:12:39.025,0:12:43.650 But if we shine light in this region, 0:12:43.650,0:12:45.300 and if the light has[br]the suitable frequency 0:12:45.300,0:12:48.930 or the suitable wavelength,[br]then the electrons can absorb 0:12:48.930,0:12:51.300 that energy, but it won't get emitted. 0:12:51.300,0:12:53.228 Okay, that's the difference over here. 0:12:53.228,0:12:54.492 In photo effect, it gets emitted. 0:12:54.492,0:12:56.122 But here, instead of getting emitted, 0:12:56.122,0:12:58.020 it just gets enough[br]energy to escape the bond. 0:12:58.020,0:13:01.170 And as a result, now it's[br]free to move and therefore 0:13:01.170,0:13:03.540 it'll get accelerated to[br]the left in this diagram, 0:13:03.540,0:13:05.700 because electric field to the[br]right electrons are negatively 0:13:05.700,0:13:08.040 charged the experience of force[br]in the opposite direction. 0:13:08.040,0:13:12.210 And as a result, it will[br]now come to the left side 0:13:12.210,0:13:14.880 and it'll leave behind a gap. 0:13:14.880,0:13:17.700 Now, other electrons, under[br]electrons, which are bonded, 0:13:17.700,0:13:21.300 can swoop into this gap,[br]which makes the gap go 0:13:21.300,0:13:23.730 to the right, and then the[br]other electrons can swoop into 0:13:23.730,0:13:25.110 this gap and so on and so forth. 0:13:25.110,0:13:29.490 So it kind of feels that this[br]gap, this latency will move in 0:13:29.490,0:13:30.870 to the other side. 0:13:30.870,0:13:34.105 This way, a lot of electrons[br]and a lot of vacancies 0:13:34.105,0:13:35.910 can be created. 0:13:35.910,0:13:38.310 And so look, if you can[br]complete this circuit, 0:13:38.310,0:13:40.710 electrons would love[br]to go from here to here 0:13:40.710,0:13:42.270 through that external circuit. 0:13:42.270,0:13:45.690 In other words, there[br]is a voltage created. 0:13:45.690,0:13:49.320 And so what we have done is[br]we have used the energy from 0:13:49.320,0:13:53.490 light to create voltage[br]photovoltaic effect. 0:13:53.490,0:13:58.490 If you put a lot of these[br]together, we create a solar panel. 0:13:58.800,0:14:01.495 That's how solar cells[br]and solar panels work. 0:14:01.495,0:14:04.530 They work on the photovoltaic effect. 0:14:04.530,0:14:07.140 Whether you consider them[br]on the roofs of the houses 0:14:07.140,0:14:09.720 or you consider the ones[br]which are in the spacecraft, 0:14:09.720,0:14:11.610 they all use the same idea. 0:14:11.610,0:14:13.500 At the end of the day,[br]we are using the fact 0:14:13.500,0:14:17.100 that light is a quantum[br]object to harness the power 0:14:17.100,0:14:19.293 of light, which we get from the sun.