1 00:00:00,360 --> 00:00:03,333 - If you shine particular kinds of light on certain metals, 2 00:00:04,653 --> 00:00:05,726 electrons will be ejected. 3 00:00:05,726 --> 00:00:08,220 We call this the photoelectric effect because light is photo 4 00:00:08,220 --> 00:00:10,860 and electrons being ejected is electric. 5 00:00:10,860 --> 00:00:12,450 And this was one of the key experiments 6 00:00:12,450 --> 00:00:15,510 that actually helped us discover a completely 7 00:00:15,510 --> 00:00:17,040 new model of light. 8 00:00:17,040 --> 00:00:19,560 But how exactly you ask, well, let's find out. 9 00:00:19,560 --> 00:00:22,200 What's interesting here is that this effect depends on 10 00:00:22,200 --> 00:00:23,640 the color of light. 11 00:00:23,640 --> 00:00:27,600 For example, if this metal was, say potassium, okay, 12 00:00:27,600 --> 00:00:29,490 then if you shine blue light, 13 00:00:29,490 --> 00:00:31,320 then we will get electrons being ejected, 14 00:00:31,320 --> 00:00:33,366 photoeletric effect happens. 15 00:00:33,366 --> 00:00:35,610 But if you were to shine red light on potassium, 16 00:00:35,610 --> 00:00:38,160 we will not get photoeletric effect at all. 17 00:00:38,160 --> 00:00:40,020 Regardless of how bright you make it. 18 00:00:40,020 --> 00:00:42,750 Even if you were to make it blindingly bright, 19 00:00:42,750 --> 00:00:44,700 we will not get photoeletric effect. 20 00:00:44,700 --> 00:00:47,700 This is what puzzled physicists. 21 00:00:47,700 --> 00:00:50,910 I mean, think about the model over here we have atoms 22 00:00:50,910 --> 00:00:53,430 with electron clouds over here and the nucleus 23 00:00:53,430 --> 00:00:54,630 at the center, okay? 24 00:00:54,630 --> 00:00:56,430 When you shine light, the energy of the light 25 00:00:56,430 --> 00:00:59,400 gets transferred to the electrons and they're able to escape 26 00:00:59,400 --> 00:01:01,950 the clutches of the nucleus and go out. 27 00:01:01,950 --> 00:01:04,290 But why can't that happen over here with red light? 28 00:01:04,290 --> 00:01:06,660 Think about it, I'm shining bright light, 29 00:01:06,660 --> 00:01:09,210 very high intensity, incredible amount of energy over here, 30 00:01:09,210 --> 00:01:11,850 and yet electrons are not able to absorb it and get out. 31 00:01:11,850 --> 00:01:13,920 Why does the photoeletric effect depend on the color? 32 00:01:13,920 --> 00:01:16,890 That was a big question that didn't make any sense. 33 00:01:16,890 --> 00:01:18,570 So what do we do over here? 34 00:01:18,570 --> 00:01:20,670 Well, we do more careful experiments. 35 00:01:20,670 --> 00:01:22,380 First, let's only look at the color 36 00:01:22,380 --> 00:01:24,630 and then think about the brightness later, okay. 37 00:01:24,630 --> 00:01:26,910 So what is color representing electromagnetic waves? 38 00:01:26,910 --> 00:01:30,960 Remember that color basically depends on the wave 39 00:01:30,960 --> 00:01:31,793 length of light. 40 00:01:31,793 --> 00:01:34,920 For example, red color is the wavelength of light 41 00:01:34,920 --> 00:01:38,130 could be somewhere around 650 nanometers. 42 00:01:38,130 --> 00:01:40,860 What we find is that at 650 nanometers 43 00:01:40,860 --> 00:01:43,680 we don't get any photoelectric effect for potassium. 44 00:01:43,680 --> 00:01:46,590 We don't know why, but now what we can do is 45 00:01:46,590 --> 00:01:49,230 let's reduce the wavelength and see what happens 46 00:01:49,230 --> 00:01:51,480 if I keep reducing the wavelength, and let's say 47 00:01:51,480 --> 00:01:54,900 I come to orange light of 600 nanometers, 48 00:01:54,900 --> 00:01:56,130 see I've reduced it. 49 00:01:56,130 --> 00:01:58,560 I still get no photoeletric effect, I don't know why, 50 00:01:58,560 --> 00:01:59,610 but I'm just doing an experiment. 51 00:01:59,610 --> 00:02:01,050 This is an observation, okay? 52 00:02:01,050 --> 00:02:01,883 I keep reducing. 53 00:02:01,883 --> 00:02:06,883 I keep reducing the wavelength until I hit 541 nanometers. 54 00:02:06,930 --> 00:02:10,620 At this point, I now start seeing photoeletric effect, 55 00:02:10,620 --> 00:02:14,220 and in this particular case, electrons are barely ejected 56 00:02:14,220 --> 00:02:15,660 from the metal. 57 00:02:15,660 --> 00:02:17,730 That's why I've drawn very tiny arrow marks over here, 58 00:02:17,730 --> 00:02:19,710 they have hardly have any kinetic energy. 59 00:02:19,710 --> 00:02:21,610 I just get some photoeletric effect 60 00:02:22,572 --> 00:02:24,750 and then if I reduce it even further, that's where I get my 61 00:02:24,750 --> 00:02:27,210 blue light at about say 500 nanometers. 62 00:02:27,210 --> 00:02:28,620 These are rough numbers, okay? 63 00:02:28,620 --> 00:02:31,500 At 500 nanometers, I now get photoelectric effect, 64 00:02:31,500 --> 00:02:35,100 but the electrons coming out with even more energy. 65 00:02:35,100 --> 00:02:37,590 What happens if I reduce it even further? 66 00:02:37,590 --> 00:02:42,000 I find that electrons are coming out with even more energy. 67 00:02:42,000 --> 00:02:43,620 So what's our observation over here? 68 00:02:43,620 --> 00:02:47,010 We see that if we lower the wavelength, we get more energy 69 00:02:47,010 --> 00:02:48,930 for the electrons coming out over here. 70 00:02:48,930 --> 00:02:50,370 We can also talk in terms of frequency. 71 00:02:50,370 --> 00:02:53,310 Remember, bigger the wavelength, smaller the frequency, 72 00:02:53,310 --> 00:02:54,510 because if you have big wavelength, 73 00:02:54,510 --> 00:02:57,090 there are less waves passing per second. 74 00:02:57,090 --> 00:03:00,240 So this is low frequency and this is high over here. 75 00:03:00,240 --> 00:03:02,460 So we can say when it comes to frequency, 76 00:03:02,460 --> 00:03:06,150 more the frequency, more the energy of the electrons. 77 00:03:06,150 --> 00:03:09,330 And you also have some kind of a cutoff over here, right? 78 00:03:09,330 --> 00:03:12,390 So for example, if the wave length is above 541 nanometers 79 00:03:12,390 --> 00:03:15,450 for potassium, for potassium, if it's above 5 41 nanometers, 80 00:03:15,450 --> 00:03:17,910 no photoeletric effect, only below it, 81 00:03:17,910 --> 00:03:19,200 we will get photoelectric effect. 82 00:03:19,200 --> 00:03:21,750 And so every metal will have its own cutoff. 83 00:03:21,750 --> 00:03:24,090 We call that the threshold wavelength, 84 00:03:24,090 --> 00:03:25,980 or you can also say threshold frequency. 85 00:03:25,980 --> 00:03:28,920 But the whole idea is if the wavelength is below 86 00:03:28,920 --> 00:03:31,999 that threshold wavelength only then you get 87 00:03:31,999 --> 00:03:33,540 photoelectric effect, if it's about you won't get it. 88 00:03:33,540 --> 00:03:35,970 Different metals have different threshold wavelengths 89 00:03:35,970 --> 00:03:38,490 and similarly different threshold frequencies. 90 00:03:38,490 --> 00:03:40,290 So that's the effect of wavelength of frequency. 91 00:03:40,290 --> 00:03:42,810 We see that the wavelength of the frequency controls 92 00:03:42,810 --> 00:03:44,610 the energy with with the electrons come out 93 00:03:44,610 --> 00:03:46,680 and that cannot be explained as to why. 94 00:03:46,680 --> 00:03:49,350 Why does the wavelength of the frequency control it? 95 00:03:49,350 --> 00:03:51,120 Why am I not getting photoeletric effect? 96 00:03:51,120 --> 00:03:52,890 If it's about the threshold wavelength, 97 00:03:52,890 --> 00:03:54,180 it doesn't make any sense. 98 00:03:54,180 --> 00:03:56,250 But anyways, the next question could be for us, 99 00:03:56,250 --> 00:03:58,500 how does the brightness affect this whole thing? 100 00:03:58,500 --> 00:04:00,090 Does it have any effect? 101 00:04:00,090 --> 00:04:01,020 The answer is yes. 102 00:04:01,020 --> 00:04:04,664 Remember, brightness or the intensity of light is basically 103 00:04:04,664 --> 00:04:07,980 how big the valleys and the peaks are, right? 104 00:04:07,980 --> 00:04:11,880 So if you were to make the light brighter, 105 00:04:11,880 --> 00:04:14,400 then it will look somewhat like this. 106 00:04:14,400 --> 00:04:15,660 You can imagine it this way. 107 00:04:15,660 --> 00:04:17,640 This is brighter light, okay? 108 00:04:17,640 --> 00:04:21,953 Now what we find is that we get more electrons, okay? 109 00:04:23,160 --> 00:04:25,170 It doesn't change the energy with these electrons come out. 110 00:04:25,170 --> 00:04:28,181 See they're coming out with much the same energy as before, 111 00:04:28,181 --> 00:04:29,520 but we now get more electrons. 112 00:04:29,520 --> 00:04:31,967 Of course, if you're above the threshold wavelength, 113 00:04:31,967 --> 00:04:35,580 you'll not get photo photoelectric effect at all, 114 00:04:35,580 --> 00:04:36,900 regardless of the brightness. 115 00:04:36,900 --> 00:04:39,030 It doesn't matter, okay? 116 00:04:39,030 --> 00:04:41,760 So if you decrease the brightness or intensity, 117 00:04:41,760 --> 00:04:43,230 you get less electrons. 118 00:04:43,230 --> 00:04:45,990 If you increase the intensity, you get more electrons. 119 00:04:45,990 --> 00:04:49,140 So intensity only controls the number of electrons, 120 00:04:49,140 --> 00:04:52,530 but it's the wavelength of the frequency that controls 121 00:04:52,530 --> 00:04:54,210 the energy with visual electrons come out, 122 00:04:54,210 --> 00:04:57,150 it also controls whether we get photoeletric effect or not. 123 00:04:57,150 --> 00:05:00,600 The big question was why the wave model just cannot explain 124 00:05:00,600 --> 00:05:03,060 this because according to wave model, 125 00:05:03,060 --> 00:05:05,370 you should get photoeletric effect for all colors 126 00:05:05,370 --> 00:05:06,360 of light, right? 127 00:05:06,360 --> 00:05:08,520 If you make light bright enough, electrons should be able 128 00:05:08,520 --> 00:05:11,070 to absorb it and just get, you know just get admitted. 129 00:05:11,070 --> 00:05:12,270 But that doesn't happen. 130 00:05:12,270 --> 00:05:16,140 And this is why physicists back then were puzzled 131 00:05:16,140 --> 00:05:19,200 and we were desperately in need of an answer for this. 132 00:05:19,200 --> 00:05:20,910 So what did we do? 133 00:05:20,910 --> 00:05:22,920 Well, to explain these observations, we came up with 134 00:05:22,920 --> 00:05:26,730 a completely brand new model of light. 135 00:05:26,730 --> 00:05:29,460 Instead of thinking of light as waves that carry 136 00:05:29,460 --> 00:05:31,980 energy continuously and that can transfer 137 00:05:31,980 --> 00:05:36,000 energy continuously, we thought maybe light 138 00:05:36,000 --> 00:05:40,200 is made of discrete packets of energy, not waves, 139 00:05:40,200 --> 00:05:43,230 but packets of energy, which we call photons. 140 00:05:43,230 --> 00:05:46,410 And then light is being absorbed by say electrons. 141 00:05:46,410 --> 00:05:47,940 You also absorb it as packets. 142 00:05:47,940 --> 00:05:50,640 You'll either absorb no light or you'll absorb 143 00:05:50,640 --> 00:05:53,190 one packet of light or two packets of light 144 00:05:53,190 --> 00:05:56,000 and so on and so forth, nothing in between. 145 00:05:56,000 --> 00:05:58,470 We call this discrete, which is exactly opposite 146 00:05:58,470 --> 00:06:00,030 of what happens in wave model, 147 00:06:00,030 --> 00:06:02,940 there you can absorb continuously. 148 00:06:02,940 --> 00:06:06,180 Okay, so how does this explain the photoelectric effect, 149 00:06:06,180 --> 00:06:07,620 the observations over here? 150 00:06:07,620 --> 00:06:09,249 Well, let's see. 151 00:06:09,249 --> 00:06:12,105 The key thing over here is that the energy of the photons 152 00:06:12,105 --> 00:06:14,100 or the packets notice depends on the color. 153 00:06:14,100 --> 00:06:15,810 If you're dealing with long wavelength 154 00:06:15,810 --> 00:06:18,790 or low frequency light, then we have less energy 155 00:06:18,790 --> 00:06:22,500 of the packet, the photons have less energy. 156 00:06:22,500 --> 00:06:24,870 And if you're dealing with short wavelength 157 00:06:24,870 --> 00:06:28,050 or high frequency light, you can see that the packets have 158 00:06:28,050 --> 00:06:29,310 more energy. 159 00:06:29,310 --> 00:06:32,910 So shorter the wavelength or more the frequency, 160 00:06:32,910 --> 00:06:35,310 there is more energy in the packet. 161 00:06:35,310 --> 00:06:37,830 There is a relationship between energy and the wavelength, 162 00:06:37,830 --> 00:06:39,150 which we'll not get into. 163 00:06:39,150 --> 00:06:41,370 But lemme just give you some rough numbers over here 164 00:06:41,370 --> 00:06:43,200 because the numbers is gonna help us over here. 165 00:06:43,200 --> 00:06:45,630 So here are some numbers. 166 00:06:45,630 --> 00:06:47,930 So it turns out that if you consider red light 167 00:06:49,253 --> 00:06:51,017 of 650 nanometers, the energy of the packet, the energy 168 00:06:51,017 --> 00:06:53,850 of the photon is about 1.9 electron volt. 169 00:06:53,850 --> 00:06:56,310 Yeah, maybe wondering, shouldn't we be measuring 170 00:06:56,310 --> 00:06:57,330 energy in joules? 171 00:06:57,330 --> 00:07:00,330 Well, joule turns out to be a big unit of energy. 172 00:07:00,330 --> 00:07:01,980 So we use a smaller unit of energy, 173 00:07:01,980 --> 00:07:03,600 which we call electron volts. 174 00:07:03,600 --> 00:07:06,677 Don't worry too much about the units over here, 175 00:07:06,677 --> 00:07:07,759 it's just the numbers. 176 00:07:07,759 --> 00:07:10,560 You can see these packets have tiny energy, 177 00:07:10,560 --> 00:07:12,570 but this packet has much bigger energy. 178 00:07:12,570 --> 00:07:14,550 2.8 electron volts, you can see that, right. 179 00:07:14,550 --> 00:07:17,040 Now for potassium it turns out, 180 00:07:17,040 --> 00:07:18,360 if you want to pluck an electron, 181 00:07:18,360 --> 00:07:21,540 if electron needs to be ejected, the minimum energy 182 00:07:21,540 --> 00:07:26,100 that you need is about 2.3 electron volts. 183 00:07:26,100 --> 00:07:27,750 This is for potassium. 184 00:07:27,750 --> 00:07:29,580 Now is a great time for you to pause the video 185 00:07:29,580 --> 00:07:30,810 and see if you can try and come up 186 00:07:30,810 --> 00:07:32,670 with an explanation over here. 187 00:07:32,670 --> 00:07:33,870 Alright, let's see. 188 00:07:33,870 --> 00:07:37,287 The big idea over here is that if you want to knock off 189 00:07:37,287 --> 00:07:38,772 an electron, I mean like you know, 190 00:07:38,772 --> 00:07:41,370 make that electron escape, then a single photon 191 00:07:41,370 --> 00:07:44,280 should have at least this much energy. 192 00:07:44,280 --> 00:07:47,070 If the photons do not have at least this much energy, 193 00:07:47,070 --> 00:07:48,570 then the electron will absorb it, 194 00:07:48,570 --> 00:07:50,850 but it's not enough to escape, 195 00:07:50,850 --> 00:07:52,950 and so it'll just reradiate it back. 196 00:07:52,950 --> 00:07:56,850 And therefore, if you have consider red light, 197 00:07:56,850 --> 00:07:58,650 it does not have a single photon, 198 00:07:58,650 --> 00:08:00,270 does not carry enough energy. 199 00:08:00,270 --> 00:08:02,190 And that's the reason why electrons are not 200 00:08:02,190 --> 00:08:03,690 getting injected over here. 201 00:08:03,690 --> 00:08:06,090 And that's why these lights are unable 202 00:08:06,090 --> 00:08:07,680 to give you photoelectric effect. 203 00:08:07,680 --> 00:08:10,590 Over here, we have just enough energy 204 00:08:10,590 --> 00:08:13,067 for photoelectric effect and therefore electrons 205 00:08:13,067 --> 00:08:16,402 barely make it out over here, because all of the energy 206 00:08:16,402 --> 00:08:18,810 is used up in just releasing the electrons. 207 00:08:18,810 --> 00:08:20,820 There's hardly any energy left over here, 208 00:08:20,820 --> 00:08:22,800 so there'll be hardly moving. 209 00:08:22,800 --> 00:08:26,220 But over here, notice you have more than the necessary 210 00:08:26,220 --> 00:08:30,030 energy over here and therefore some residual energy is left. 211 00:08:30,030 --> 00:08:33,690 And so electrons after coming out have some extra energy 212 00:08:33,690 --> 00:08:36,675 remaining that goes out as kinetic energy. 213 00:08:36,675 --> 00:08:38,190 And since this has even more energy, 214 00:08:38,190 --> 00:08:41,250 each photon has even more energy while electrons now 215 00:08:41,250 --> 00:08:43,200 eject with even more kinetic energy, 216 00:08:43,200 --> 00:08:46,050 'cause there's more residual energy after getting ejected. 217 00:08:46,050 --> 00:08:47,880 But what about the intensity? 218 00:08:47,880 --> 00:08:51,060 Well, if you increase the intensity in this model, 219 00:08:51,060 --> 00:08:54,450 we are increasing the number of photons, that's it. 220 00:08:54,450 --> 00:08:57,150 Over here notice if a single photon does not have 221 00:08:57,150 --> 00:08:59,566 enough energy, then I don't care how many photons you shine, 222 00:08:59,566 --> 00:09:01,590 it's just not going to work. 223 00:09:01,590 --> 00:09:03,884 That's why here I will still not get any, 224 00:09:03,884 --> 00:09:05,430 you know, photoelectric effect. 225 00:09:05,430 --> 00:09:08,940 But over here now I'm shining more number of photons, 226 00:09:08,940 --> 00:09:11,290 so more electrons can absorb that energy 227 00:09:12,183 --> 00:09:13,650 and therefore more electrons can escape per second. 228 00:09:13,650 --> 00:09:16,110 And that's why I get more electrons over here, 229 00:09:16,110 --> 00:09:17,640 putting it all together. 230 00:09:17,640 --> 00:09:20,310 Since the wave of the frequency decides the energy 231 00:09:20,310 --> 00:09:23,700 of an individual photon that decides the kinetic energy, 232 00:09:23,700 --> 00:09:27,060 shorter the wavelength, stronger, more is the energy 233 00:09:27,060 --> 00:09:30,330 of the photon and more is the kinetic energy. 234 00:09:30,330 --> 00:09:32,907 If the wavelength is bigger and it's too big, 235 00:09:32,907 --> 00:09:35,160 the energy of the photon is very tiny, 236 00:09:35,160 --> 00:09:36,930 it'll not be able to knock off anything 237 00:09:36,930 --> 00:09:39,210 and you'll not get any photoelectric effect. 238 00:09:39,210 --> 00:09:42,810 And since intensity is basically the number of photons, 239 00:09:42,810 --> 00:09:45,930 if you have more number of photons, you'll get more number 240 00:09:45,930 --> 00:09:47,280 of electrons coming out. 241 00:09:47,280 --> 00:09:50,430 But over here, it doesn't matter how many photons you shine, 242 00:09:50,430 --> 00:09:52,740 and therefore it doesn't matter what the brightness is, 243 00:09:52,740 --> 00:09:54,660 you will not get photoelectric effect. 244 00:09:54,660 --> 00:09:56,250 Beautiful, isn't it? 245 00:09:56,250 --> 00:09:59,040 So wait, does this mean that light is not a wave? 246 00:09:59,040 --> 00:10:01,080 It's actually particles? 247 00:10:01,080 --> 00:10:02,400 Well, not quiet. 248 00:10:02,400 --> 00:10:05,130 You see certain phenomena of light like diffraction 249 00:10:05,130 --> 00:10:09,600 or interference means that light must have wave properties 250 00:10:09,600 --> 00:10:13,440 and certain other phenomenon like photoelectric effect, 251 00:10:13,440 --> 00:10:17,327 black body radiation, scattering of light, 252 00:10:17,327 --> 00:10:19,140 and other such effects makes us believe 253 00:10:19,140 --> 00:10:22,409 that light must also have the particle nature, 254 00:10:22,409 --> 00:10:26,520 the photon nature, which means a light must have 255 00:10:26,520 --> 00:10:29,760 a dual nature, both particle and waves. 256 00:10:29,760 --> 00:10:31,850 It's not that light sometimes behaves as waves 257 00:10:31,850 --> 00:10:33,671 and sometimes wears as particle. 258 00:10:33,671 --> 00:10:34,629 No, no, no. 259 00:10:34,629 --> 00:10:36,660 Light has both wave and particle nature. 260 00:10:36,660 --> 00:10:38,580 And if you're wondering, well, how does that make any sense? 261 00:10:38,580 --> 00:10:40,110 How can something be both waves 262 00:10:40,110 --> 00:10:42,150 and particles at the same time? 263 00:10:42,150 --> 00:10:45,780 Well, unfortunately, there's no way to really visualize it 264 00:10:45,780 --> 00:10:47,220 because in our macroscopic world, 265 00:10:47,220 --> 00:10:50,220 we don't have any experience of things having both wave 266 00:10:50,220 --> 00:10:52,500 and particle nature. 267 00:10:52,500 --> 00:10:54,180 But this is one of the reasons why sometimes when we are 268 00:10:54,180 --> 00:10:56,040 showing photons, we show it this way 269 00:10:56,040 --> 00:10:59,970 with a tiny wave packet, but this doesn't mean 270 00:10:59,970 --> 00:11:01,920 that the photons are wiggling up and down. 271 00:11:01,920 --> 00:11:03,900 Okay, that's a misconception that I used to have. 272 00:11:03,900 --> 00:11:05,868 It's not like that. 273 00:11:05,868 --> 00:11:07,710 A better way to sort of think about this is that 274 00:11:07,710 --> 00:11:11,041 light is not a wave in the traditional sense. 275 00:11:11,041 --> 00:11:13,830 It's not a particle in the traditional sense, 276 00:11:13,830 --> 00:11:17,110 it's a brand new object, which we don't have 277 00:11:17,110 --> 00:11:19,410 experience within our daily life. 278 00:11:19,410 --> 00:11:21,270 This object has both wave properties 279 00:11:21,270 --> 00:11:24,240 and particle properties, and we call such an object, 280 00:11:24,240 --> 00:11:26,010 a quantum object. 281 00:11:26,010 --> 00:11:28,260 Now this sounds very theoretical, right? 282 00:11:28,260 --> 00:11:30,120 But there are so many applications of the fact 283 00:11:30,120 --> 00:11:31,709 that light is a quantum object. 284 00:11:31,709 --> 00:11:34,230 Let me tell you one of them, okay? 285 00:11:34,230 --> 00:11:36,210 Now, in photoelectric effect from light, 286 00:11:36,210 --> 00:11:38,133 we get electrons ejected, right? 287 00:11:39,221 --> 00:11:40,054 Now there's a very similar, 288 00:11:40,054 --> 00:11:42,720 slightly different effect called (indistinct) 289 00:11:42,720 --> 00:11:45,330 when you shine light, you can generate voltage. 290 00:11:45,330 --> 00:11:49,410 We call such an effect, a photovoltaic effect. 291 00:11:49,410 --> 00:11:53,520 Now, the way that works is we need to first create a crystal 292 00:11:53,520 --> 00:11:57,030 in which there's an already inbuilt electric field. 293 00:11:57,030 --> 00:11:58,350 It's possible to do that. 294 00:11:58,350 --> 00:12:00,270 We'll not get too much details of how we build 295 00:12:00,270 --> 00:12:02,280 such crystals, but using semiconductors, 296 00:12:02,280 --> 00:12:04,080 we can build crystals like that. 297 00:12:04,080 --> 00:12:06,600 We don't have to hook it up to any battery or anything. 298 00:12:06,600 --> 00:12:08,910 It'll have an inbuilt electric field. 299 00:12:08,910 --> 00:12:11,280 The crystals is built in such a way that one side 300 00:12:11,280 --> 00:12:13,350 of the crystal has slightly different properties compared 301 00:12:13,350 --> 00:12:14,940 to another side of the crystal. 302 00:12:14,940 --> 00:12:16,320 And because of the difference in properties, 303 00:12:16,320 --> 00:12:18,180 an electric field gets built up. 304 00:12:18,180 --> 00:12:20,640 Now the important point is there are electrons everywhere, 305 00:12:20,640 --> 00:12:23,460 but if you focus on this region, there are a lot of 306 00:12:23,460 --> 00:12:26,940 electrons, but they're all bonded and they're not 307 00:12:26,940 --> 00:12:27,773 free to move. 308 00:12:27,773 --> 00:12:29,700 So even if there's an electric field pushing on them, 309 00:12:29,700 --> 00:12:32,580 they cannot move because they're stuck in bonds. 310 00:12:32,580 --> 00:12:35,040 You can imagine that this is like the sea of electrons. 311 00:12:35,040 --> 00:12:37,830 They're all kind of fixed inside the crystal, 312 00:12:37,830 --> 00:12:39,025 they cannot move. 313 00:12:39,025 --> 00:12:43,650 But if we shine light in this region, 314 00:12:43,650 --> 00:12:45,300 and if the light has the suitable frequency 315 00:12:45,300 --> 00:12:48,930 or the suitable wavelength, then the electrons can absorb 316 00:12:48,930 --> 00:12:51,300 that energy, but it won't get emitted. 317 00:12:51,300 --> 00:12:53,228 Okay, that's the difference over here. 318 00:12:53,228 --> 00:12:54,492 In photo effect, it gets emitted. 319 00:12:54,492 --> 00:12:56,122 But here, instead of getting emitted, 320 00:12:56,122 --> 00:12:58,020 it just gets enough energy to escape the bond. 321 00:12:58,020 --> 00:13:01,170 And as a result, now it's free to move and therefore 322 00:13:01,170 --> 00:13:03,540 it'll get accelerated to the left in this diagram, 323 00:13:03,540 --> 00:13:05,700 because electric field to the right electrons are negatively 324 00:13:05,700 --> 00:13:08,040 charged the experience of force in the opposite direction. 325 00:13:08,040 --> 00:13:12,210 And as a result, it will now come to the left side 326 00:13:12,210 --> 00:13:14,880 and it'll leave behind a gap. 327 00:13:14,880 --> 00:13:17,700 Now, other electrons, under electrons, which are bonded, 328 00:13:17,700 --> 00:13:21,300 can swoop into this gap, which makes the gap go 329 00:13:21,300 --> 00:13:23,730 to the right, and then the other electrons can swoop into 330 00:13:23,730 --> 00:13:25,110 this gap and so on and so forth. 331 00:13:25,110 --> 00:13:29,490 So it kind of feels that this gap, this latency will move in 332 00:13:29,490 --> 00:13:30,870 to the other side. 333 00:13:30,870 --> 00:13:34,105 This way, a lot of electrons and a lot of vacancies 334 00:13:34,105 --> 00:13:35,910 can be created. 335 00:13:35,910 --> 00:13:38,310 And so look, if you can complete this circuit, 336 00:13:38,310 --> 00:13:40,710 electrons would love to go from here to here 337 00:13:40,710 --> 00:13:42,270 through that external circuit. 338 00:13:42,270 --> 00:13:45,690 In other words, there is a voltage created. 339 00:13:45,690 --> 00:13:49,320 And so what we have done is we have used the energy from 340 00:13:49,320 --> 00:13:53,490 light to create voltage photovoltaic effect. 341 00:13:53,490 --> 00:13:58,490 If you put a lot of these together, we create a solar panel. 342 00:13:58,800 --> 00:14:01,495 That's how solar cells and solar panels work. 343 00:14:01,495 --> 00:14:04,530 They work on the photovoltaic effect. 344 00:14:04,530 --> 00:14:07,140 Whether you consider them on the roofs of the houses 345 00:14:07,140 --> 00:14:09,720 or you consider the ones which are in the spacecraft, 346 00:14:09,720 --> 00:14:11,610 they all use the same idea. 347 00:14:11,610 --> 00:14:13,500 At the end of the day, we are using the fact 348 00:14:13,500 --> 00:14:17,100 that light is a quantum object to harness the power 349 00:14:17,100 --> 00:14:19,293 of light, which we get from the sun.