1 00:00:00,090 --> 00:00:02,430 The following content is provided under a Creative 2 00:00:02,430 --> 00:00:03,820 Commons license. 3 00:00:03,820 --> 00:00:06,060 Your support will help MIT OpenCourseWare 4 00:00:06,060 --> 00:00:10,150 continue to offer high quality educational resources for free. 5 00:00:10,150 --> 00:00:12,690 To make a donation or to view additional materials 6 00:00:12,690 --> 00:00:16,650 from hundreds of MIT courses, visit MIT OpenCourseWare 7 00:00:16,650 --> 00:00:17,580 at ocw.mit.edu. 8 00:00:25,650 --> 00:00:27,400 CATHERINE DRENNAN: Click in your response. 9 00:00:51,722 --> 00:00:53,910 All right, let's just do ten seconds 10 00:00:53,910 --> 00:00:55,580 on the clicker question. 11 00:01:12,850 --> 00:01:16,150 OK, so 82%. 12 00:01:16,150 --> 00:01:21,250 I like it when we get into the 90s, just FYI, for these. 13 00:01:21,250 --> 00:01:24,580 So since this is similar to one we did last time, 14 00:01:24,580 --> 00:01:27,730 it's just to remind you again of the first part of problem 15 00:01:27,730 --> 00:01:30,130 set one where we have this material 16 00:01:30,130 --> 00:01:33,100 that you need to learn how to cover. 17 00:01:33,100 --> 00:01:36,031 So does anyone want to tell me how they got the right answer? 18 00:01:36,031 --> 00:01:37,780 And this is not going to be a great prize. 19 00:01:37,780 --> 00:01:39,446 There's going to be better prizes later. 20 00:01:39,446 --> 00:01:42,070 This is an American Chemical Society pen for this one, 21 00:01:42,070 --> 00:01:43,990 since we had a similar question last time. 22 00:01:46,465 --> 00:01:47,840 I should've told you it was going 23 00:01:47,840 --> 00:01:48,820 to be a better prize later. 24 00:01:48,820 --> 00:01:50,861 Now no one's going to-- everyone's going to wait. 25 00:01:57,150 --> 00:01:59,470 See if that's turned on. 26 00:01:59,470 --> 00:02:00,500 No, I guess not. 27 00:02:06,640 --> 00:02:08,840 AUDIENCE: So you find the limiting reactant 28 00:02:08,840 --> 00:02:10,690 by dividing the amount of moles for both 29 00:02:10,690 --> 00:02:12,340 by the molar coefficient. 30 00:02:12,340 --> 00:02:16,480 And you see that it's at the O. And then 31 00:02:16,480 --> 00:02:21,160 you just do 12 times the molar fraction, 32 00:02:21,160 --> 00:02:26,779 which is just one AL203 for every 3FEO and you get 4. 33 00:02:26,779 --> 00:02:27,820 CATHERINE DRENNAN: Great. 34 00:02:27,820 --> 00:02:29,740 So if you haven't practiced these yet, 35 00:02:29,740 --> 00:02:30,760 go ahead and practice. 36 00:02:30,760 --> 00:02:31,260 Yeah! 37 00:02:31,260 --> 00:02:34,533 [APPLAUSE] 38 00:02:37,240 --> 00:02:41,140 CATHERINE DRENNAN: OK, so let's think about what we 39 00:02:41,140 --> 00:02:42,990 were talking about on Friday. 40 00:02:42,990 --> 00:02:45,460 We were talking about discovery of the electron 41 00:02:45,460 --> 00:02:50,050 and the nucleus, and realized through the experiments 42 00:02:50,050 --> 00:02:52,870 that the atom is mostly empty space. 43 00:02:52,870 --> 00:02:56,200 But there is this concentrated small part 44 00:02:56,200 --> 00:03:00,370 of the nucleus that can deflect alpha particles, 45 00:03:00,370 --> 00:03:02,110 or ping pong balls. 46 00:03:02,110 --> 00:03:06,610 And this is a really small concentrated part, 47 00:03:06,610 --> 00:03:09,130 so that would be like the head of a pin in a room somewhat 48 00:03:09,130 --> 00:03:13,210 bigger than this, or a pea in the size of a sports arena. 49 00:03:13,210 --> 00:03:16,330 The nucleus is very tiny compared to the atom. 50 00:03:16,330 --> 00:03:19,150 And the atom is, again, mostly empty space. 51 00:03:19,150 --> 00:03:22,750 So this discovery was amazing of these subatomic particles 52 00:03:22,750 --> 00:03:26,470 and was really changing what people were thinking about. 53 00:03:26,470 --> 00:03:28,947 So at that time, they started to realize 54 00:03:28,947 --> 00:03:31,030 in doing these experiments and the experiments I'm 55 00:03:31,030 --> 00:03:33,580 going to tell you today that they needed a different way 56 00:03:33,580 --> 00:03:35,650 of thinking about matter. 57 00:03:35,650 --> 00:03:39,070 And to explain the observations that scientists 58 00:03:39,070 --> 00:03:42,820 were making at the time, they needed to think about the fact 59 00:03:42,820 --> 00:03:46,960 that radiation had both wave-like and particle-like 60 00:03:46,960 --> 00:03:48,010 properties. 61 00:03:48,010 --> 00:03:52,390 And matter had, also, wave-like and particle-like properties. 62 00:03:52,390 --> 00:03:55,330 And also that energy is quantized 63 00:03:55,330 --> 00:03:58,480 into these discrete bundles called photons. 64 00:03:58,480 --> 00:04:02,230 So we're going to be talking about these new discoveries 65 00:04:02,230 --> 00:04:04,210 and the data that didn't fit today, 66 00:04:04,210 --> 00:04:07,720 and how we came up with this new mechanics that 67 00:04:07,720 --> 00:04:11,200 helped to explain the properties that were being observed. 68 00:04:11,200 --> 00:04:14,890 So we're going to start today with a wave particle 69 00:04:14,890 --> 00:04:16,660 duality of light. 70 00:04:16,660 --> 00:04:19,990 So we're going to talk about light as a wave. 71 00:04:19,990 --> 00:04:22,330 And we're going to first talk about the characteristics 72 00:04:22,330 --> 00:04:24,620 of waves, which is a little review. 73 00:04:24,620 --> 00:04:27,560 And then we're going to talk about light as a particle 74 00:04:27,560 --> 00:04:30,340 and get into the photoelectric effect, which 75 00:04:30,340 --> 00:04:33,670 was a really important series of experiments 76 00:04:33,670 --> 00:04:36,830 that help scientists understand what was going on. 77 00:04:36,830 --> 00:04:40,480 So first we'll just have a little review about waves. 78 00:04:40,480 --> 00:04:44,020 So some of you come from places that probably don't have oceans 79 00:04:44,020 --> 00:04:45,940 nearby. 80 00:04:45,940 --> 00:04:49,870 You are now living in a place that does have an ocean nearby, 81 00:04:49,870 --> 00:04:54,100 so you can go take the blue line to Revere Beach. 82 00:04:54,100 --> 00:04:56,740 I always find doing a chemistry problem set is 83 00:04:56,740 --> 00:04:58,600 very relaxing on the beach. 84 00:04:58,600 --> 00:05:00,610 And you can watch the water level 85 00:05:00,610 --> 00:05:05,400 go up and down in this repeated periodic fashion. 86 00:05:05,400 --> 00:05:08,230 So if you haven't experienced water waves, 87 00:05:08,230 --> 00:05:11,640 you should absolutely do that. 88 00:05:11,640 --> 00:05:15,030 So waves have this periodic variation. 89 00:05:15,030 --> 00:05:16,950 So you could have average water level, 90 00:05:16,950 --> 00:05:19,380 the water level will go up and then go down 91 00:05:19,380 --> 00:05:23,430 and go up and go down from high levels to low levels. 92 00:05:23,430 --> 00:05:27,610 This same behavior is observed for other types of waves, 93 00:05:27,610 --> 00:05:29,470 such as sound waves. 94 00:05:29,470 --> 00:05:32,040 So here you would have the average density, 95 00:05:32,040 --> 00:05:35,280 and the sound wave can go to higher density, lower density, 96 00:05:35,280 --> 00:05:37,770 higher density, lower density. 97 00:05:37,770 --> 00:05:42,390 And this same periodic behavior is also observed 98 00:05:42,390 --> 00:05:47,370 with white light or electromagnetic radiation 99 00:05:47,370 --> 00:05:49,890 is also a periodic function. 100 00:05:49,890 --> 00:05:55,030 But here you have this periodic variation of an electric field. 101 00:05:55,030 --> 00:05:57,600 So we have, whether it's water waves 102 00:05:57,600 --> 00:06:01,200 or if we have sound waves or light waves, 103 00:06:01,200 --> 00:06:04,110 we always have this periodic behavior. 104 00:06:04,110 --> 00:06:06,570 And you can define this periodic behavior 105 00:06:06,570 --> 00:06:09,870 by a number of different terms, which we'll talk about now. 106 00:06:09,870 --> 00:06:12,300 Mostly we'll be focused on light waves today, 107 00:06:12,300 --> 00:06:16,860 but these terms can apply to other types of waves, as well. 108 00:06:16,860 --> 00:06:19,020 So we have amplitude. 109 00:06:19,020 --> 00:06:22,560 And that's the deviation from the average level. 110 00:06:22,560 --> 00:06:25,200 So you can have a positive amplitude 111 00:06:25,200 --> 00:06:27,630 or a negative amplitude here. 112 00:06:27,630 --> 00:06:32,670 So this is the height of the wave. 113 00:06:32,670 --> 00:06:37,530 You also have wavelength where the abbreviation is lambda. 114 00:06:37,530 --> 00:06:41,730 And this is the distance between successive maxima, 115 00:06:41,730 --> 00:06:44,910 so the distance from this maxima to this maxima 116 00:06:44,910 --> 00:06:48,210 is one wavelength. 117 00:06:48,210 --> 00:06:52,200 We also have frequency of the wave, which helps to define it. 118 00:06:52,200 --> 00:06:55,510 Oh, I should say wavelength can be up here or down here. 119 00:06:55,510 --> 00:06:58,260 They should be exactly the same. 120 00:06:58,260 --> 00:07:06,220 So frequency or nu is the number of cycles per unit time. 121 00:07:06,220 --> 00:07:11,680 So we talk about wavelength, amplitude, and frequency. 122 00:07:11,680 --> 00:07:14,370 And as you turn the page, we can also 123 00:07:14,370 --> 00:07:17,610 talk about the period of the wave. 124 00:07:17,610 --> 00:07:23,400 So 1 over the frequency is called the period, 125 00:07:23,400 --> 00:07:27,090 and it's the time it takes for one cycle to occur. 126 00:07:27,090 --> 00:07:30,210 So the time that you go from one maximum 127 00:07:30,210 --> 00:07:33,150 to the other maximum for one cycle 128 00:07:33,150 --> 00:07:36,240 is the period of the wave. 129 00:07:36,240 --> 00:07:39,870 As with most things in chemistry, there are units, 130 00:07:39,870 --> 00:07:43,200 always think about your units. 131 00:07:43,200 --> 00:07:49,200 So units of frequency are cycles per second, 132 00:07:49,200 --> 00:07:52,170 and that's also called a hertz. 133 00:07:52,170 --> 00:07:55,380 So you will often just see per second, 134 00:07:55,380 --> 00:07:57,620 but sometimes you'll see hertz in these problems. 135 00:07:57,620 --> 00:08:02,510 So those can be used interchangeably. 136 00:08:02,510 --> 00:08:06,890 And one other term is intensity, which is 137 00:08:06,890 --> 00:08:09,920 equal to the amplitude squared. 138 00:08:09,920 --> 00:08:11,544 So for all of these waves, there are 139 00:08:11,544 --> 00:08:13,460 these certain characteristics you think about. 140 00:08:13,460 --> 00:08:16,940 The amplitude of the wave, the wavelength, and the frequency, 141 00:08:16,940 --> 00:08:21,920 the period of the wave, and also the intensity of the wave. 142 00:08:21,920 --> 00:08:25,100 So now if we're thinking about light waves, which 143 00:08:25,100 --> 00:08:28,460 we will be most of the time, we can also 144 00:08:28,460 --> 00:08:35,030 think about the speed of that light that is traveling. 145 00:08:35,030 --> 00:08:38,150 And so at time 0 if we're thinking 146 00:08:38,150 --> 00:08:44,330 about the time it is going to take this little orange dot, 147 00:08:44,330 --> 00:08:47,270 to move this pink here that's labled, 148 00:08:47,270 --> 00:08:50,220 to move from here to here, move one wavelength. 149 00:08:50,220 --> 00:08:53,030 So it has moved to this second location. 150 00:08:53,030 --> 00:08:56,750 The time it takes to do that is going 151 00:08:56,750 --> 00:09:00,770 to be 1 over the frequency. 152 00:09:00,770 --> 00:09:04,700 If we think about then the speed at which this will happen, 153 00:09:04,700 --> 00:09:07,280 that it'll go from here to here at time 0 154 00:09:07,280 --> 00:09:12,500 to time 1 over the frequency or one period as we just defined, 155 00:09:12,500 --> 00:09:16,310 now we can fill out this equation. 156 00:09:16,310 --> 00:09:18,230 And see that the speed is going to be 157 00:09:18,230 --> 00:09:20,840 equal to the wavelength, lambda. 158 00:09:20,840 --> 00:09:23,120 So that's the distance traveled, the wave 159 00:09:23,120 --> 00:09:25,610 traveled one wavelength. 160 00:09:25,610 --> 00:09:28,490 And the time it took, the time elapsed 161 00:09:28,490 --> 00:09:32,300 was one period or 1 over the frequency, 162 00:09:32,300 --> 00:09:34,700 and then we can define one of the equations 163 00:09:34,700 --> 00:09:36,380 that you probably don't really need 164 00:09:36,380 --> 00:09:40,040 to have defined for you, which is that the speed of light 165 00:09:40,040 --> 00:09:43,740 is equal to the wavelength times the frequency. 166 00:09:43,740 --> 00:09:48,740 And this is in usually units of meters per second. 167 00:09:48,740 --> 00:09:54,500 So most people already know what this speed of light, 168 00:09:54,500 --> 00:09:58,160 we're talking again about light here, is equal to. 169 00:09:58,160 --> 00:10:00,860 And it has a constant speed. 170 00:10:00,860 --> 00:10:05,240 So electromagnetic radiation has a constant speed, 171 00:10:05,240 --> 00:10:07,690 the speed of light. 172 00:10:07,690 --> 00:10:11,500 And the speed of light is abbreviated c, 173 00:10:11,500 --> 00:10:14,840 equals the wavelength times the frequency, 174 00:10:14,840 --> 00:10:20,290 which is 2.9979 times 10 to the 8th meters per second. 175 00:10:20,290 --> 00:10:23,410 So most of you have seen this before 176 00:10:23,410 --> 00:10:25,180 and are well aware of it. 177 00:10:25,180 --> 00:10:28,420 This also has some of the conversions on here. 178 00:10:28,420 --> 00:10:31,420 Speed of light is quite fast and often 179 00:10:31,420 --> 00:10:34,580 people have the expression how fast [INAUDIBLE]. 180 00:10:34,580 --> 00:10:36,830 You're working at the speed of light. 181 00:10:36,830 --> 00:10:39,260 And that's a pretty valid thing to indicate 182 00:10:39,260 --> 00:10:41,440 that you're doing things very, very quickly, 183 00:10:41,440 --> 00:10:43,000 because that is fast. 184 00:10:43,000 --> 00:10:46,420 And in fact, if we had the earth here and the moon here, 185 00:10:46,420 --> 00:10:49,300 light would go like that. 186 00:10:49,300 --> 00:10:52,840 And it should take about 1.2 seconds to do that. 187 00:10:52,840 --> 00:10:55,660 So speed of light, very quickly. 188 00:10:55,660 --> 00:10:58,180 All right, so what's also important in thinking 189 00:10:58,180 --> 00:11:00,610 about this is that the speed of light 190 00:11:00,610 --> 00:11:04,180 is a constant, which is going to mean 191 00:11:04,180 --> 00:11:06,580 that these terms are related. 192 00:11:06,580 --> 00:11:09,790 So first before we move on, let's do a clicker question. 193 00:11:09,790 --> 00:11:12,310 And you can all test out that your clickers 194 00:11:12,310 --> 00:11:13,340 are working again. 195 00:11:57,140 --> 00:11:58,590 How are we doing? 196 00:11:58,590 --> 00:11:59,938 OK, let's do 10 more seconds. 197 00:12:18,760 --> 00:12:20,700 OK, 92%, that's awesome. 198 00:12:20,700 --> 00:12:23,610 All right, does someone want to explain 199 00:12:23,610 --> 00:12:29,190 how they could eliminate one and two versus three and four 200 00:12:29,190 --> 00:12:30,332 and get this one right? 201 00:12:42,492 --> 00:12:44,450 AUDIENCE: Sure, well it looks like this problem 202 00:12:44,450 --> 00:12:46,325 is asking us to look at two different things, 203 00:12:46,325 --> 00:12:50,280 the wavelength of these waves and the frequency. 204 00:12:50,280 --> 00:12:53,800 And you can look at the distance between the two peaks 205 00:12:53,800 --> 00:12:58,990 in both of the waves to see how short or long the wavelength 206 00:12:58,990 --> 00:12:59,490 is. 207 00:12:59,490 --> 00:13:01,560 Wavelength A has a much shorter distance 208 00:13:01,560 --> 00:13:03,420 between peaks than B does. 209 00:13:03,420 --> 00:13:06,540 And then you're also looking for nu, the frequency. 210 00:13:06,540 --> 00:13:13,830 So within a period of time you can fit more waves of A than B, 211 00:13:13,830 --> 00:13:18,366 so it has a higher frequency. 212 00:13:18,366 --> 00:13:20,970 [APPLAUSE] 213 00:13:20,970 --> 00:13:29,220 CATHERINE DRENNAN: So, this prize here is LEGO. 214 00:13:29,220 --> 00:13:32,430 And LEGO has decided to make girl chemists. 215 00:13:32,430 --> 00:13:34,560 So this is a little LEGO girl chemist 216 00:13:34,560 --> 00:13:37,760 that comes with pretty colored erlenmeyer flasks. 217 00:13:37,760 --> 00:13:40,160 So that's very special. 218 00:13:42,930 --> 00:13:46,300 OK, yeah, so the trick of that question 219 00:13:46,300 --> 00:13:49,360 you just looked which was the longer wavelength. 220 00:13:49,360 --> 00:13:51,600 And then you could rule out that from the picture 221 00:13:51,600 --> 00:13:54,450 and also realize that there's connections because 222 00:13:54,450 --> 00:13:58,590 of the constant speed of light between the frequency 223 00:13:58,590 --> 00:14:00,250 and the wavelength. 224 00:14:00,250 --> 00:14:04,200 So when you're talking about wavelengths of light, 225 00:14:04,200 --> 00:14:06,690 c is always equal to a product of the wavelength 226 00:14:06,690 --> 00:14:08,110 times the frequency. 227 00:14:08,110 --> 00:14:10,030 So they're not independent of each other. 228 00:14:10,030 --> 00:14:12,210 And if you know one, you will know the other. 229 00:14:12,210 --> 00:14:15,060 So this is something you'll come in very handy as you're 230 00:14:15,060 --> 00:14:18,270 doing these problems. 231 00:14:18,270 --> 00:14:21,480 OK, so let's talk about light for a minute 232 00:14:21,480 --> 00:14:26,280 and look at these different colors of light. 233 00:14:26,280 --> 00:14:30,120 So we go from red at long wavelengths 234 00:14:30,120 --> 00:14:33,510 to violet at the shorter wavelengths. 235 00:14:33,510 --> 00:14:38,130 And so here the wavelength is decreasing. 236 00:14:38,130 --> 00:14:42,570 And then if we think about the corresponding frequencies, 237 00:14:42,570 --> 00:14:45,660 up here then if we have long wavelengths, 238 00:14:45,660 --> 00:14:49,570 what are we going to have in terms of the frequency? 239 00:14:49,570 --> 00:14:53,790 Yeah, so we will have lower or shorter frequencies going up 240 00:14:53,790 --> 00:14:56,400 to higher frequencies up here. 241 00:14:56,400 --> 00:14:58,440 So again, we have this relationship 242 00:14:58,440 --> 00:15:01,140 because the speed of light equals the wavelength 243 00:15:01,140 --> 00:15:03,610 times the frequency. 244 00:15:03,610 --> 00:15:07,290 So you are not responsible for memorizing 245 00:15:07,290 --> 00:15:09,600 all of the wavelengths, but you should 246 00:15:09,600 --> 00:15:11,940 have a sense of the order of the wavelengths 247 00:15:11,940 --> 00:15:15,990 and certainly the relationship between wavelength 248 00:15:15,990 --> 00:15:17,490 and frequency. 249 00:15:17,490 --> 00:15:19,890 And there's going to be a number of problems 250 00:15:19,890 --> 00:15:22,380 later when we're talking about colors of light that 251 00:15:22,380 --> 00:15:25,320 are admitted from things or colors that are absorbed where 252 00:15:25,320 --> 00:15:29,640 it's really convenient to have the order of wavelengths 253 00:15:29,640 --> 00:15:30,780 memorized. 254 00:15:30,780 --> 00:15:32,730 So I thought I would just help you out 255 00:15:32,730 --> 00:15:38,190 with this by this nice little song 256 00:15:38,190 --> 00:15:42,180 from They Must Be Giants in Here Comes Science's album, which 257 00:15:42,180 --> 00:15:45,130 should help you always remember the order of the wavelengths. 258 00:15:45,130 --> 00:15:47,190 So let's just see if this will play. 259 00:15:47,190 --> 00:15:49,933 MUSIC: R is for red. 260 00:15:49,933 --> 00:15:51,905 O is for orange. 261 00:15:51,905 --> 00:15:53,384 Y is for yellow. 262 00:15:53,384 --> 00:15:55,830 And G is for green. 263 00:15:55,830 --> 00:15:57,423 B is for blue. 264 00:15:57,423 --> 00:15:58,776 I for indigo. 265 00:15:58,776 --> 00:16:01,700 And V is for violet. 266 00:16:01,700 --> 00:16:05,964 And that spells Roy G. Biv. 267 00:16:05,964 --> 00:16:08,951 Roy G. Biv is a colorful man. 268 00:16:08,951 --> 00:16:13,280 And he proudly stands at the rainbow's end. 269 00:16:13,280 --> 00:16:18,712 Roy G. Biv is a colorful man and his name spells out 270 00:16:18,712 --> 00:16:20,008 the whole color spectrum. 271 00:16:20,008 --> 00:16:22,620 Roy G. Biv is a-- 272 00:16:22,620 --> 00:16:26,310 CATHERINE DRENNAN: OK, so I think you get the idea there. 273 00:16:26,310 --> 00:16:28,590 And it sticks in your head. 274 00:16:28,590 --> 00:16:32,470 And you may remember this for the rest of your life, 275 00:16:32,470 --> 00:16:33,910 even if you don't want to. 276 00:16:33,910 --> 00:16:35,430 That's a very catchy song. 277 00:16:35,430 --> 00:16:37,260 In fact, my six-year-old daughter 278 00:16:37,260 --> 00:16:39,060 learned it when she was about three, 279 00:16:39,060 --> 00:16:40,860 and then we get very upset if anybody 280 00:16:40,860 --> 00:16:44,040 drew the colors that were not in the appropriate order 281 00:16:44,040 --> 00:16:45,160 of the rainbow. 282 00:16:45,160 --> 00:16:48,990 And she would come around and correct their work. 283 00:16:48,990 --> 00:16:52,020 Anyway, it made her into a little bit of a holy terror 284 00:16:52,020 --> 00:16:54,990 but we're working on that. 285 00:16:54,990 --> 00:16:58,290 So in addition to the visible light, which is actually 286 00:16:58,290 --> 00:17:04,230 a very small part of the range of waves 287 00:17:04,230 --> 00:17:07,980 and so we have visual light in here, 288 00:17:07,980 --> 00:17:10,710 we can also think about other waves that 289 00:17:10,710 --> 00:17:15,810 go from then long wavelength here and low frequency 290 00:17:15,810 --> 00:17:18,569 to short wavelength and high frequency. 291 00:17:18,569 --> 00:17:23,390 So we have radio waves on the long wavelength end 292 00:17:23,390 --> 00:17:26,130 and we have then microwaves. 293 00:17:26,130 --> 00:17:30,120 Microwaves are I think college students' best friends. 294 00:17:30,120 --> 00:17:33,690 And as I am teaching this course, since many of you 295 00:17:33,690 --> 00:17:37,890 are freshmen, I feel the need to compare my many years 296 00:17:37,890 --> 00:17:41,860 of experience at MIT with you. 297 00:17:41,860 --> 00:17:46,500 Point number one, just use the popcorn button 298 00:17:46,500 --> 00:17:48,390 on the microwave. 299 00:17:48,390 --> 00:17:51,990 Don't think about how long is it-- just use the popcorn 300 00:17:51,990 --> 00:17:54,000 button on the microwave. 301 00:17:54,000 --> 00:17:57,480 I used to live in Simmons for a while. 302 00:17:57,480 --> 00:18:02,250 3:00 AM fire drills or fire things because someone 303 00:18:02,250 --> 00:18:05,400 did not push the popcorn button on the microwave 304 00:18:05,400 --> 00:18:06,530 while making popcorn. 305 00:18:06,530 --> 00:18:10,890 Anyway, life lesson number one, microwaves. 306 00:18:10,890 --> 00:18:15,930 So molecules behave differently in these different kinds 307 00:18:15,930 --> 00:18:16,550 of waves. 308 00:18:16,550 --> 00:18:20,360 So you can rotate infrared, you're looking at vibrations. 309 00:18:20,360 --> 00:18:22,620 Of course here, visible light. 310 00:18:22,620 --> 00:18:24,690 We also have UV light. 311 00:18:24,690 --> 00:18:27,780 So when you go to the beach on the green line 312 00:18:27,780 --> 00:18:30,384 this weekend to do prom set number two 313 00:18:30,384 --> 00:18:31,800 and look at the waves, you'll want 314 00:18:31,800 --> 00:18:36,230 to wear your sunscreen because UV is, in fact, dangerous. 315 00:18:36,230 --> 00:18:40,640 So wear sunscreen, advice number two. 316 00:18:40,640 --> 00:18:43,230 Then we have x-rays. 317 00:18:43,230 --> 00:18:45,660 Hopefully most of you do not know one of the uses 318 00:18:45,660 --> 00:18:47,790 to detect broken bones. 319 00:18:47,790 --> 00:18:49,830 And as you'll hear later on, x-rays 320 00:18:49,830 --> 00:18:53,190 can also be used to solve structures of molecules 321 00:18:53,190 --> 00:18:54,840 at atomic resolution. 322 00:18:54,840 --> 00:18:57,440 And then on the very short wavelength, 323 00:18:57,440 --> 00:18:59,830 then, we have gamma rays as well. 324 00:18:59,830 --> 00:19:02,460 So again, you're not responsible for memorizing 325 00:19:02,460 --> 00:19:04,350 all of these numbers, but you should 326 00:19:04,350 --> 00:19:09,330 have a sense of the order of these different types of rays 327 00:19:09,330 --> 00:19:13,110 of the electromagnetic spectrum, what's at short, 328 00:19:13,110 --> 00:19:15,510 what's at long wavelengths. 329 00:19:15,510 --> 00:19:18,130 All right, so waves have other properties. 330 00:19:18,130 --> 00:19:21,120 And one of the most important properties of waves 331 00:19:21,120 --> 00:19:24,330 is that they can superimpose. 332 00:19:24,330 --> 00:19:28,020 So if you have two waves, one drawn up here, 333 00:19:28,020 --> 00:19:32,650 and one drawn down here, that are in phase with each other, 334 00:19:32,650 --> 00:19:35,610 which means that their troughs are at the same place, 335 00:19:35,610 --> 00:19:37,910 their peaks are at the same place, 336 00:19:37,910 --> 00:19:41,130 you can get constructive interference 337 00:19:41,130 --> 00:19:43,560 and that would look like this. 338 00:19:43,560 --> 00:19:48,120 So you have these waves come together in phase 339 00:19:48,120 --> 00:19:52,380 and you get this much larger constructive interference. 340 00:19:52,380 --> 00:19:55,230 This property can be very important 341 00:19:55,230 --> 00:19:59,110 as we'll talk a little bit more about later. 342 00:19:59,110 --> 00:20:03,810 You can also have what's called destructive interference, 343 00:20:03,810 --> 00:20:07,130 so when you have out of phase waves. 344 00:20:07,130 --> 00:20:10,940 And the clicker, you can tell me what that should look like. 345 00:20:33,020 --> 00:20:34,010 OK, ten seconds. 346 00:20:38,630 --> 00:20:40,710 Now looks like it's back to being a darker color 347 00:20:40,710 --> 00:20:43,820 box in the corner. 348 00:20:43,820 --> 00:20:45,960 Just likes to vary it up on its own. 349 00:20:45,960 --> 00:20:47,126 OK, that's one second. 350 00:20:50,040 --> 00:20:51,370 Woo! 351 00:20:51,370 --> 00:20:53,420 All right, 98%. 352 00:20:53,420 --> 00:20:55,430 We don't have to explain that one. 353 00:20:55,430 --> 00:20:57,830 That one was pretty clear. 354 00:20:57,830 --> 00:21:01,410 So here you had they were completely out of phase. 355 00:21:01,410 --> 00:21:04,170 So you had total destructive interference. 356 00:21:04,170 --> 00:21:09,860 So that just looks like a straight line. 357 00:21:09,860 --> 00:21:12,680 So the combination of constructive and destructive 358 00:21:12,680 --> 00:21:15,020 interference actually has a number 359 00:21:15,020 --> 00:21:18,830 of practical applications. 360 00:21:18,830 --> 00:21:21,300 So people who are very interested 361 00:21:21,300 --> 00:21:24,250 in constructive and destructive interference 362 00:21:24,250 --> 00:21:29,880 include people who are designing symphony halls or classrooms. 363 00:21:29,880 --> 00:21:31,950 Actually, this is one of the better classrooms 364 00:21:31,950 --> 00:21:33,830 in terms of the acoustics. 365 00:21:33,830 --> 00:21:36,750 And the Boston Symphony is actually, supposedly, 366 00:21:36,750 --> 00:21:39,680 the third best in the world in terms of acoustics. 367 00:21:39,680 --> 00:21:43,290 MIT students get nice discounts, go check it out. 368 00:21:43,290 --> 00:21:47,160 Another practical application was 369 00:21:47,160 --> 00:21:50,550 in designing noise canceling headphones. 370 00:21:50,550 --> 00:21:53,220 I brought a pair if some of you have never tried them 371 00:21:53,220 --> 00:21:57,150 and you want to come down after class, you can give them a try 372 00:21:57,150 --> 00:21:59,730 and see what you think. 373 00:21:59,730 --> 00:22:03,130 These headphones, the Bose headphones, 374 00:22:03,130 --> 00:22:05,490 were developed by a former MIT professor. 375 00:22:05,490 --> 00:22:07,230 He passed away last year. 376 00:22:07,230 --> 00:22:10,710 He taught at MIT, taught acoustics, for many years. 377 00:22:10,710 --> 00:22:14,160 And he was riding on an airplane once and it was just so loud. 378 00:22:14,160 --> 00:22:17,070 And he was thinking, wow, I was wondering if there a way 379 00:22:17,070 --> 00:22:19,800 I can design some good headphones to cancel 380 00:22:19,800 --> 00:22:21,140 this noise. 381 00:22:21,140 --> 00:22:24,090 And he did and many billions and billions and billions 382 00:22:24,090 --> 00:22:25,660 of dollars later. 383 00:22:25,660 --> 00:22:30,060 So as you are an MIT student, you 384 00:22:30,060 --> 00:22:31,862 get a discount on these headphones. 385 00:22:31,862 --> 00:22:34,320 So if you're going to buy them, buy them while you're here. 386 00:22:34,320 --> 00:22:40,260 Also, when he died, the majority share of his stock went to MIT. 387 00:22:40,260 --> 00:22:42,600 So you will buy them, get a discount, 388 00:22:42,600 --> 00:22:44,490 and you'll also be giving MIT money 389 00:22:44,490 --> 00:22:48,390 by buying these because we get a lot of money for this. 390 00:22:48,390 --> 00:22:52,006 So I feel like this brings up a really important point 391 00:22:52,006 --> 00:22:53,130 that I just want to stress. 392 00:22:53,130 --> 00:22:54,546 And I'll probably mention a couple 393 00:22:54,546 --> 00:22:57,960 of times that the material that you learn in your classes 394 00:22:57,960 --> 00:23:00,200 here at MIT, and in this class, you 395 00:23:00,200 --> 00:23:02,520 will learn a lot of really useful things. 396 00:23:02,520 --> 00:23:04,800 Some of that will lead to money. 397 00:23:04,800 --> 00:23:08,430 And as you make lots of money, you 398 00:23:08,430 --> 00:23:13,110 should remember where you learned that and know that I 399 00:23:13,110 --> 00:23:15,155 take both cash and checks. 400 00:23:18,000 --> 00:23:21,630 Another practical application is actually in my own research. 401 00:23:21,630 --> 00:23:23,250 So we have a series that I'm going 402 00:23:23,250 --> 00:23:26,184 to be using I mentioned to bring some different faces in. 403 00:23:26,184 --> 00:23:28,350 The first video I'm going to show you in this series 404 00:23:28,350 --> 00:23:29,190 is actually me. 405 00:23:29,190 --> 00:23:30,810 So it's not a different face. 406 00:23:30,810 --> 00:23:33,300 But when I asked other people to make videos 407 00:23:33,300 --> 00:23:34,800 about how they were using chemical 408 00:23:34,800 --> 00:23:36,869 principles in their research, they said OK, 409 00:23:36,869 --> 00:23:38,785 but you're going to do one, too, right, Cathy? 410 00:23:38,785 --> 00:23:41,010 So I was like, yes, I guess I'm going to do one. 411 00:23:41,010 --> 00:23:42,750 And that just happens to be the first one 412 00:23:42,750 --> 00:23:43,990 that I'm going to show. 413 00:23:43,990 --> 00:23:48,560 So another practical use of constructive and destructive 414 00:23:48,560 --> 00:23:50,760 interference has to do with x-rays. 415 00:23:50,760 --> 00:23:53,880 And you can use constructive and destructive interference 416 00:23:53,880 --> 00:23:56,520 to determine the structures of very tiny things, protein 417 00:23:56,520 --> 00:23:59,240 molecules or nucleic acids in your body. 418 00:23:59,240 --> 00:24:00,990 So I'm going to try to run this movie now. 419 00:24:00,990 --> 00:24:02,600 We'll see, this as a demo. 420 00:24:02,600 --> 00:24:07,590 Good to try it out with me and see how this is going to work. 421 00:24:13,325 --> 00:24:15,450 CATHERINE DRENNAN (VIDEO): My name is Cathy Drennan 422 00:24:15,450 --> 00:24:18,720 and I'm a Professor of Chemistry and Biology at MIT. 423 00:24:18,720 --> 00:24:21,210 And I'm also a Professor and Investigator with the Howard 424 00:24:21,210 --> 00:24:22,580 Hughes Medical Institute. 425 00:24:22,580 --> 00:24:27,660 And my lab uses the principles of diffraction in our research. 426 00:24:27,660 --> 00:24:29,790 A wave shoots through some kind of grating. 427 00:24:29,790 --> 00:24:32,640 You can have light, say light waves, shooting through. 428 00:24:32,640 --> 00:24:36,080 And when the light waves hit the metal lattice, 429 00:24:36,080 --> 00:24:37,450 they'll be diffracted. 430 00:24:37,450 --> 00:24:40,320 And some of those waves will be in phase with each other 431 00:24:40,320 --> 00:24:42,030 and will constructively interfere 432 00:24:42,030 --> 00:24:45,150 and you get a bright spot in a diffraction pattern. 433 00:24:45,150 --> 00:24:48,000 Other waves will be out of phase and they'll destructively 434 00:24:48,000 --> 00:24:50,760 interfere and you'll see nothing as a result of those waves. 435 00:24:54,450 --> 00:24:57,240 From this pattern of spots and no spots, 436 00:24:57,240 --> 00:24:59,370 you can understand something about the structure 437 00:24:59,370 --> 00:25:01,900 of the grating that it went through. 438 00:25:01,900 --> 00:25:03,510 So if I had two different gratings, 439 00:25:03,510 --> 00:25:05,760 the diffraction patterns would be different for these. 440 00:25:05,760 --> 00:25:07,718 And so from looking at the diffraction pattern, 441 00:25:07,718 --> 00:25:12,430 you can figure out how the metal or whatever was arranged 442 00:25:12,430 --> 00:25:14,850 that generated that pattern. 443 00:25:14,850 --> 00:25:17,040 This property works whether it's a metal 444 00:25:17,040 --> 00:25:22,260 grating or a lattice that's made up of protein molecules. 445 00:25:22,260 --> 00:25:24,100 Because the protein molecules are small 446 00:25:24,100 --> 00:25:26,290 and crystals are small, we use x-rays 447 00:25:26,290 --> 00:25:28,380 and short wavelength, high energy. 448 00:25:28,380 --> 00:25:30,240 But because everything is really tiny, 449 00:25:30,240 --> 00:25:33,450 we need really bright x-rays to do this. 450 00:25:33,450 --> 00:25:38,050 And I don't mean high energy bright, I mean intensity. 451 00:25:38,050 --> 00:25:41,364 So we need more photons per second. 452 00:25:41,364 --> 00:25:42,780 So we have to go to a place called 453 00:25:42,780 --> 00:25:44,880 the synchrotron, a research facility, that 454 00:25:44,880 --> 00:25:47,130 has really intense x-rays. 455 00:25:47,130 --> 00:25:49,490 And then we shoot those x-rays through our crystal, 456 00:25:49,490 --> 00:25:51,610 collect this diffraction pattern, 457 00:25:51,610 --> 00:25:54,310 and then figure out what the shapes of these molecules are. 458 00:25:57,070 --> 00:25:59,830 The structure can tell you so much. 459 00:25:59,830 --> 00:26:01,747 I mean, there can be big questions of field 460 00:26:01,747 --> 00:26:02,830 about how something works. 461 00:26:02,830 --> 00:26:05,204 And all of a sudden, you see what the molecule looks like 462 00:26:05,204 --> 00:26:07,600 and you're like, ha, of course. 463 00:26:07,600 --> 00:26:09,730 Sometimes there's a problem with the DNA 464 00:26:09,730 --> 00:26:13,360 that then gets translated into a defect in the protein. 465 00:26:13,360 --> 00:26:15,760 But people often don't know why does 466 00:26:15,760 --> 00:26:18,790 it matter, why does it matter that the protein is 467 00:26:18,790 --> 00:26:20,070 this or that? 468 00:26:20,070 --> 00:26:22,270 But we can look at it and figure it out. 469 00:26:22,270 --> 00:26:25,360 So we can compare what the protein structure looks 470 00:26:25,360 --> 00:26:28,360 like for a healthy individual with the protein structure 471 00:26:28,360 --> 00:26:29,890 from someone who has a mutation. 472 00:26:29,890 --> 00:26:31,540 All of a sudden you might see, wow, 473 00:26:31,540 --> 00:26:34,480 the vitamin that this protein needs can't bind anymore. 474 00:26:34,480 --> 00:26:36,340 So we can have a sense of what's wrong. 475 00:26:36,340 --> 00:26:37,881 And then sometimes you can figure out 476 00:26:37,881 --> 00:26:40,674 how to treat it once you know what the problem is. 477 00:26:40,674 --> 00:26:42,340 Sometimes there'll be a protein molecule 478 00:26:42,340 --> 00:26:43,720 that everyone knows is important. 479 00:26:43,720 --> 00:26:46,080 Everyone wants to know what it looks like. 480 00:26:46,080 --> 00:26:48,040 But you might be the one who does it. 481 00:26:48,040 --> 00:26:50,480 You might be the one to figure out what it looks like. 482 00:26:50,480 --> 00:26:52,130 And you'll be the first one. 483 00:26:52,130 --> 00:26:54,380 You'll see these patterns, these diffraction patterns, 484 00:26:54,380 --> 00:26:56,629 and you'll build this model and all of a sudden you'll 485 00:26:56,629 --> 00:26:59,170 be like wow, that's not what people expected. 486 00:26:59,170 --> 00:27:01,210 And it'll just be this incredible discovery. 487 00:27:01,210 --> 00:27:04,420 So you're an explorer of the molecular world 488 00:27:04,420 --> 00:27:05,986 when you're a crystallographer. 489 00:27:09,880 --> 00:27:11,152 [APPLAUSE] 490 00:27:11,152 --> 00:27:12,360 CATHERINE DRENNAN: Thank you! 491 00:27:18,470 --> 00:27:21,760 OK, so that's the first in the series. 492 00:27:21,760 --> 00:27:24,616 We're going to have another one this week, 493 00:27:24,616 --> 00:27:26,365 as well, because we have a lot of history. 494 00:27:26,365 --> 00:27:29,560 So we've got to counter it with a lot of current research. 495 00:27:29,560 --> 00:27:33,760 And so you'll be learning about quantum dots also this week. 496 00:27:33,760 --> 00:27:38,140 OK, so those are some of the characteristics of waves 497 00:27:38,140 --> 00:27:42,190 that are really important and we talked about light as a wave. 498 00:27:42,190 --> 00:27:45,060 Now we're going to talk about light as a particle. 499 00:27:45,060 --> 00:27:47,740 Light as a wave is a little bit easier to grasp. 500 00:27:47,740 --> 00:27:51,530 Light as a particle is a little bit more confusing. 501 00:27:51,530 --> 00:27:54,910 And it took a while for people to really appreciate 502 00:27:54,910 --> 00:27:57,170 that light had particle-like properties, i.e. 503 00:27:57,170 --> 00:27:58,840 It was quantized. 504 00:27:58,840 --> 00:28:02,320 And this really came out of the photoelectric effect. 505 00:28:02,320 --> 00:28:03,700 So what were people doing? 506 00:28:03,700 --> 00:28:06,250 And this is around the time of the discovery of the electron 507 00:28:06,250 --> 00:28:07,530 and the nucleus. 508 00:28:07,530 --> 00:28:09,880 And what scientists were having fun doing 509 00:28:09,880 --> 00:28:13,990 were taking beams of UV light and hitting metal surfaces, 510 00:28:13,990 --> 00:28:16,335 and seeing if they could inject electrons, 511 00:28:16,335 --> 00:28:17,460 which have been discovered. 512 00:28:17,460 --> 00:28:19,150 It's like, let's get some electrons out 513 00:28:19,150 --> 00:28:20,590 of those metal surfaces. 514 00:28:20,590 --> 00:28:24,130 We know they're there, let's see them come off and characterize 515 00:28:24,130 --> 00:28:25,770 their properties. 516 00:28:25,770 --> 00:28:32,110 So they found that if they had some UV light and the frequency 517 00:28:32,110 --> 00:28:35,770 of that UV light was below something, below a threshold 518 00:28:35,770 --> 00:28:39,810 that they referred to as the threshold frequency. 519 00:28:39,810 --> 00:28:42,850 So we have a new zero here. 520 00:28:42,850 --> 00:28:46,450 If the frequency was lower than this magic number 521 00:28:46,450 --> 00:28:50,230 for frequency, nothing would happen. 522 00:28:50,230 --> 00:28:52,930 But if they increased the frequency, 523 00:28:52,930 --> 00:28:57,610 if it was greater than or equal to this threshold frequency, 524 00:28:57,610 --> 00:28:59,950 then all of a sudden they would see something. 525 00:28:59,950 --> 00:29:03,100 They would see an electron being ejected. 526 00:29:03,100 --> 00:29:05,410 And the electron would come off with a certain amount 527 00:29:05,410 --> 00:29:08,740 of kinetic energy, K.E. And kinetic energy 528 00:29:08,740 --> 00:29:13,690 is equal to a half times mass times the velocity squared. 529 00:29:13,690 --> 00:29:16,830 All right, so they decided let's characterize this. 530 00:29:16,830 --> 00:29:21,560 Lets very some parameters and see what happens. 531 00:29:21,560 --> 00:29:25,150 So they looked at constant intensity of this light 532 00:29:25,150 --> 00:29:28,000 and they changed the frequency. 533 00:29:28,000 --> 00:29:30,490 And then they looked at the number of electrons 534 00:29:30,490 --> 00:29:32,650 that were coming off. 535 00:29:32,650 --> 00:29:36,190 And so below this threshold frequency, 536 00:29:36,190 --> 00:29:37,960 no electrons came off. 537 00:29:37,960 --> 00:29:40,180 I just told you about that. 538 00:29:40,180 --> 00:29:42,700 Then when they were at the threshold, 539 00:29:42,700 --> 00:29:45,400 they saw electrons coming off and then they 540 00:29:45,400 --> 00:29:47,920 increased the frequency even more, 541 00:29:47,920 --> 00:29:51,560 but they weren't getting any more electrons. 542 00:29:51,560 --> 00:29:52,560 Hm. 543 00:29:52,560 --> 00:29:54,980 OK, this was interesting. 544 00:29:54,980 --> 00:29:57,520 So they thought what else can we measure here. 545 00:29:57,520 --> 00:29:59,560 And they knew how to measure the kinetic energy. 546 00:29:59,560 --> 00:30:01,150 So it's like, let's start measuring 547 00:30:01,150 --> 00:30:06,430 the kinetic energy of these electrons that are coming off. 548 00:30:06,430 --> 00:30:10,750 So they plotted kinetic energy of the ejected electrons 549 00:30:10,750 --> 00:30:14,980 as a function of the frequency of the incoming light. 550 00:30:14,980 --> 00:30:18,640 And again, below the threshold, they saw nothing. 551 00:30:18,640 --> 00:30:23,090 But above the threshold, they saw the kinetic energy increase 552 00:30:23,090 --> 00:30:28,270 proportionally to the increase in the frequency of the light. 553 00:30:28,270 --> 00:30:30,130 And this didn't really make any sense 554 00:30:30,130 --> 00:30:32,380 from what they knew at the time. 555 00:30:32,380 --> 00:30:36,910 They didn't have a way to relate kinetic energy and frequency. 556 00:30:36,910 --> 00:30:40,560 So they really weren't sure what this was about, 557 00:30:40,560 --> 00:30:42,490 but they were having fun doing experiments. 558 00:30:42,490 --> 00:30:48,310 It's like, let's keep going, let's vary more properties. 559 00:30:48,310 --> 00:30:52,630 So then they decided to look at how 560 00:30:52,630 --> 00:30:55,360 the kinetic energy of the electron 561 00:30:55,360 --> 00:30:58,240 was affected by changing the intensity of the light. 562 00:30:58,240 --> 00:31:00,790 And they thought that if you increase the intensity, 563 00:31:00,790 --> 00:31:03,060 you'd have more energy in your system, 564 00:31:03,060 --> 00:31:06,130 you should have more kinetic energy. 565 00:31:06,130 --> 00:31:10,510 But that did not seem to be the case. 566 00:31:10,510 --> 00:31:13,960 They increased the intensity, the kinetic energy 567 00:31:13,960 --> 00:31:15,970 stayed the same. 568 00:31:15,970 --> 00:31:18,430 They were having trouble wrapping their head around it. 569 00:31:18,430 --> 00:31:23,290 But they said, all right, well let's collect some more data. 570 00:31:23,290 --> 00:31:28,540 So now they decided to look at the number of ejected electrons 571 00:31:28,540 --> 00:31:31,264 as a function of the intensity. 572 00:31:31,264 --> 00:31:32,680 And they really didn't think there 573 00:31:32,680 --> 00:31:33,980 should be much difference. 574 00:31:33,980 --> 00:31:36,640 Increase the intensity, number of electrons 575 00:31:36,640 --> 00:31:37,810 should be the same. 576 00:31:37,810 --> 00:31:41,950 But experiments showed otherwise. 577 00:31:41,950 --> 00:31:45,160 So when they increased the intensity, 578 00:31:45,160 --> 00:31:49,010 more electrons came off. 579 00:31:49,010 --> 00:31:51,029 And this is where they were in the field. 580 00:31:51,029 --> 00:31:52,570 It almost seemed like everything they 581 00:31:52,570 --> 00:31:55,270 did was opposite of what they expected. 582 00:31:55,270 --> 00:31:57,910 It's a pretty exciting time actually in science 583 00:31:57,910 --> 00:32:00,530 when you're getting results that are unexpected. 584 00:32:00,530 --> 00:32:03,430 And some of this data sat around for a while. 585 00:32:03,430 --> 00:32:06,070 And then Einstein decided to take a little look at it 586 00:32:06,070 --> 00:32:09,026 and see what he thought of this data. 587 00:32:09,026 --> 00:32:10,900 And people were studying all sorts of things. 588 00:32:10,900 --> 00:32:13,580 They're taking different metals, you have a different metal, 589 00:32:13,580 --> 00:32:15,850 you have a different threshold frequency. 590 00:32:15,850 --> 00:32:20,080 And so people were characterizing different metals 591 00:32:20,080 --> 00:32:21,980 and figuring out the threshold frequency 592 00:32:21,980 --> 00:32:23,590 for all the different metals. 593 00:32:23,590 --> 00:32:27,310 And then plotting the kinetic energy of the ejected 594 00:32:27,310 --> 00:32:30,640 electrons as a function of the frequency. 595 00:32:30,640 --> 00:32:34,300 And you look at this, you realize huh, 596 00:32:34,300 --> 00:32:36,260 there's different threshold frequencies 597 00:32:36,260 --> 00:32:38,620 for the different metals, but they all 598 00:32:38,620 --> 00:32:41,290 seem to have these straight lines that all 599 00:32:41,290 --> 00:32:43,964 seem to have the same slope. 600 00:32:43,964 --> 00:32:45,880 And sometimes when you look at the discoveries 601 00:32:45,880 --> 00:32:48,610 of really amazing people like Einstein, 602 00:32:48,610 --> 00:32:52,160 you're thinking well, basically what 603 00:32:52,160 --> 00:32:55,010 he did was solve the equation for a straight line. 604 00:32:55,010 --> 00:32:59,000 You realize hey, maybe I can contribute to science, as well. 605 00:32:59,000 --> 00:33:04,010 So we had a whole lot of straight lines here. 606 00:33:04,010 --> 00:33:07,910 And when you have that, you can solve for the slope. 607 00:33:07,910 --> 00:33:10,540 So that's what he did, he solved for the slope. 608 00:33:10,540 --> 00:33:14,380 And he got this number, 6.626 times 10 609 00:33:14,380 --> 00:33:18,640 to the -34th Joule seconds. 610 00:33:18,640 --> 00:33:21,190 And he saw that number and he's like, I've 611 00:33:21,190 --> 00:33:24,550 heard that number before. 612 00:33:24,550 --> 00:33:28,730 Planck came up with that number when he was studying black body 613 00:33:28,730 --> 00:33:30,740 radiation. 614 00:33:30,740 --> 00:33:33,220 So totally different phenomenon, but yet 615 00:33:33,220 --> 00:33:35,050 the number comes up again, Planck's 616 00:33:35,050 --> 00:33:38,990 constant, also known as h. 617 00:33:38,990 --> 00:33:43,150 Now that seemed like a really strange coincidence. 618 00:33:43,150 --> 00:33:47,290 So there must be something to this number. 619 00:33:47,290 --> 00:33:49,490 So if you look at this plot, we can also 620 00:33:49,490 --> 00:33:53,380 think about what the y-axis is. 621 00:33:53,380 --> 00:33:59,590 So the y-intercept is minus Planck's constant times 622 00:33:59,590 --> 00:34:02,570 that threshold frequency. 623 00:34:02,570 --> 00:34:04,490 And when you have all of this, you 624 00:34:04,490 --> 00:34:09,020 can now write the equation for the straight line in terms 625 00:34:09,020 --> 00:34:12,020 of all of these variables. 626 00:34:12,020 --> 00:34:15,850 And so y-axis here is kinetic energy. 627 00:34:15,850 --> 00:34:20,770 So we'll solve that equation now in terms of kinetic energy. 628 00:34:20,770 --> 00:34:24,110 So kinetic energy is going to be equal. 629 00:34:24,110 --> 00:34:26,050 We have our x-axis here. 630 00:34:26,050 --> 00:34:28,630 The x-axis is frequency. 631 00:34:28,630 --> 00:34:31,600 And again, now, the slope of the line we know 632 00:34:31,600 --> 00:34:35,080 is Planck's constant, so that's h. 633 00:34:35,080 --> 00:34:41,139 And the y-intercept, or b, was -h 634 00:34:41,139 --> 00:34:45,820 times the threshold frequency. 635 00:34:45,820 --> 00:34:49,270 And this is a very important equation. 636 00:34:49,270 --> 00:34:52,179 So just to define those terms again, 637 00:34:52,179 --> 00:34:56,290 we have the frequency here, Planck's constant. 638 00:34:56,290 --> 00:35:01,660 Planck's constant times the frequency is an energy. 639 00:35:01,660 --> 00:35:04,010 It is the energy of the incident light 640 00:35:04,010 --> 00:35:08,240 or the incoming light, e sub i. 641 00:35:08,240 --> 00:35:12,610 And on this side over here, we have that threshold frequency 642 00:35:12,610 --> 00:35:13,420 again. 643 00:35:13,420 --> 00:35:15,370 We also have Planck's constant. 644 00:35:15,370 --> 00:35:19,660 So Planck's constant times a threshold frequency 645 00:35:19,660 --> 00:35:23,350 is another energy term, which is called the threshold 646 00:35:23,350 --> 00:35:28,490 energy, or more commonly a work function. 647 00:35:28,490 --> 00:35:32,950 So the kinetic energy equals the incident energy, 648 00:35:32,950 --> 00:35:38,470 the energy of the incoming light minus the work function, 649 00:35:38,470 --> 00:35:40,780 which has to do with the threshold frequency which 650 00:35:40,780 --> 00:35:43,780 depends on the metal in question. 651 00:35:43,780 --> 00:35:47,800 So this was a really important equation. 652 00:35:47,800 --> 00:35:52,270 And Einstein realized that the energy of light 653 00:35:52,270 --> 00:35:56,560 is proportional to its frequency and it's proportional 654 00:35:56,560 --> 00:35:58,220 by Planck's constant. 655 00:35:58,220 --> 00:35:59,890 And this really changed how people 656 00:35:59,890 --> 00:36:02,660 had been thinking about energy. 657 00:36:02,660 --> 00:36:06,200 And all of a sudden, a lot of those observations made sense. 658 00:36:06,200 --> 00:36:10,870 Now, Einstein of course had many important discoveries 659 00:36:10,870 --> 00:36:12,340 in his career. 660 00:36:12,340 --> 00:36:15,410 But this one was the one that he personally felt 661 00:36:15,410 --> 00:36:17,800 was the most revolutionary. 662 00:36:17,800 --> 00:36:18,659 I don't know. 663 00:36:18,659 --> 00:36:20,200 People are always their worst critic, 664 00:36:20,200 --> 00:36:24,990 but even he realized that this was important. 665 00:36:24,990 --> 00:36:29,730 And in terms of units, because units are always important. 666 00:36:29,730 --> 00:36:33,770 Notice usually you'll see energy in joules or kilojoules. 667 00:36:33,770 --> 00:36:37,830 And Planck's constant has the units of joule seconds. 668 00:36:37,830 --> 00:36:41,390 And frequency is per seconds or hertz. 669 00:36:41,390 --> 00:36:46,640 So in this equation, your units work out. 670 00:36:46,640 --> 00:36:50,160 So from this idea then we have this notion 671 00:36:50,160 --> 00:36:52,920 that light is made up of these energy packets which 672 00:36:52,920 --> 00:36:57,750 people call photons where the energy of that photon 673 00:36:57,750 --> 00:37:01,840 depends on its frequency. 674 00:37:01,840 --> 00:37:04,310 So we can go back to the photoelectric effect 675 00:37:04,310 --> 00:37:07,580 now and start thinking about those observations 676 00:37:07,580 --> 00:37:12,770 and try to rationalize now what we had been seeing. 677 00:37:12,770 --> 00:37:17,010 So here's this new model for the photoelectric effect 678 00:37:17,010 --> 00:37:20,430 where we can think about the energy of that incoming photon, 679 00:37:20,430 --> 00:37:23,870 or that incident photon. 680 00:37:23,870 --> 00:37:26,430 If it's greater than the work function, 681 00:37:26,430 --> 00:37:32,660 then you'll eject an electron from the metal. 682 00:37:32,660 --> 00:37:36,600 And any left over energy is the kinetic energy 683 00:37:36,600 --> 00:37:38,550 of that ejected electron. 684 00:37:38,550 --> 00:37:40,470 So we can think about that here. 685 00:37:40,470 --> 00:37:43,260 That here we have the energy coming in 686 00:37:43,260 --> 00:37:45,660 of the incident photon. 687 00:37:45,660 --> 00:37:48,000 We have to get over that threshold frequency 688 00:37:48,000 --> 00:37:50,040 or overcome that work function. 689 00:37:50,040 --> 00:37:53,580 So you have this minus this, and the leftover 690 00:37:53,580 --> 00:37:55,470 is the kinetic energy. 691 00:37:55,470 --> 00:37:57,540 So we can also write it this way, 692 00:37:57,540 --> 00:38:00,080 that the kinetic energy equals the incident 693 00:38:00,080 --> 00:38:05,010 energy minus the work function. 694 00:38:05,010 --> 00:38:08,660 So if you just have enough energy to do this, 695 00:38:08,660 --> 00:38:10,290 you have very small kinetic energy. 696 00:38:10,290 --> 00:38:12,810 But if you have a lot of extra energy 697 00:38:12,810 --> 00:38:14,540 once you overcome the work function, 698 00:38:14,540 --> 00:38:17,470 you'll have more kinetic energy. 699 00:38:17,470 --> 00:38:20,040 We can also write the equation this way, 700 00:38:20,040 --> 00:38:23,250 that the incident energy equals the kinetic energy 701 00:38:23,250 --> 00:38:26,730 plus the work function. 702 00:38:26,730 --> 00:38:30,146 OK, so let's just try a clicker question on this. 703 00:38:30,146 --> 00:38:32,270 And we're going to go back and look at those graphs 704 00:38:32,270 --> 00:38:33,561 and think about what they mean. 705 00:39:26,035 --> 00:39:27,576 All right, let's do ten more seconds. 706 00:39:45,850 --> 00:39:48,530 Yeah, OK. 707 00:39:48,530 --> 00:39:53,010 So here the trick was to think about what the work function is 708 00:39:53,010 --> 00:39:56,370 and how much energy was coming in of the photon. 709 00:39:56,370 --> 00:40:01,350 But here the energy is lower than the threshold needed, 710 00:40:01,350 --> 00:40:05,660 so you're not going to get any electrons ejected. 711 00:40:05,660 --> 00:40:06,642 Let's try one more. 712 00:40:06,642 --> 00:40:08,100 Let's see if you can get up to 90%. 713 00:40:13,110 --> 00:40:15,760 So now we've changed the energies. 714 00:40:15,760 --> 00:40:18,908 Or the energy, but not the threshold energy. 715 00:40:39,700 --> 00:40:40,747 OK, ten more seconds. 716 00:40:57,570 --> 00:40:59,280 Yeah, 98%. 717 00:40:59,280 --> 00:41:02,920 Yeah, so now we're over the threshold energy. 718 00:41:02,920 --> 00:41:06,090 So we need to subtract the threshold energy 719 00:41:06,090 --> 00:41:08,800 and the remaining is the kinetic energy. 720 00:41:08,800 --> 00:41:12,340 OK, so these are the kinds of questions we have on this. 721 00:41:12,340 --> 00:41:16,420 And now let's go back and think about these plots again. 722 00:41:16,420 --> 00:41:18,970 So we don't have these a second time in your notes, 723 00:41:18,970 --> 00:41:20,480 but you have them one time. 724 00:41:20,480 --> 00:41:24,220 And let's just think about how this makes sense now 725 00:41:24,220 --> 00:41:28,750 with the new equations that Einstein helped us achieve. 726 00:41:28,750 --> 00:41:33,570 So in the top here, we have the surprising observation 727 00:41:33,570 --> 00:41:36,550 that when you increase the frequency of the light 728 00:41:36,550 --> 00:41:38,800 that the kinetic energy increase. 729 00:41:38,800 --> 00:41:41,470 Well, now this makes sense, because if you're 730 00:41:41,470 --> 00:41:43,720 increasing the frequency of the light, 731 00:41:43,720 --> 00:41:47,800 you're increasing the incident energy of the photons coming 732 00:41:47,800 --> 00:41:48,400 in. 733 00:41:48,400 --> 00:41:52,260 And so if you're increasing this frequency, 734 00:41:52,260 --> 00:41:54,520 so you're increasing the incident energy. 735 00:41:54,520 --> 00:41:57,480 And once you're above the threshold energy, 736 00:41:57,480 --> 00:42:01,780 you'll have more extra kinetic energy coming off 737 00:42:01,780 --> 00:42:05,710 as the frequency, or the energy of the incident light comes up. 738 00:42:05,710 --> 00:42:07,190 So that makes sense. 739 00:42:07,190 --> 00:42:09,190 All right, what about intensity? 740 00:42:09,190 --> 00:42:11,870 We haven't really talked so much about intensity. 741 00:42:11,870 --> 00:42:14,500 So let's consider intensity for a minute 742 00:42:14,500 --> 00:42:18,230 and think about the number of electrons, as well. 743 00:42:18,230 --> 00:42:20,900 So both of these are about intensity. 744 00:42:20,900 --> 00:42:23,280 So let's think about the number of electrons 745 00:42:23,280 --> 00:42:24,814 ejected from a metal surface. 746 00:42:24,814 --> 00:42:26,980 We're going to come back to those plots in a minute, 747 00:42:26,980 --> 00:42:30,010 sorry to make you go back and forth in your notes. 748 00:42:30,010 --> 00:42:31,930 And that's going to be proportional 749 00:42:31,930 --> 00:42:33,740 to the number of photons. 750 00:42:33,740 --> 00:42:37,170 So the more photons you have coming in, 751 00:42:37,170 --> 00:42:39,670 the more electrons are going to have coming out. 752 00:42:39,670 --> 00:42:44,320 That is, if the photons have the appropriate amount, 753 00:42:44,320 --> 00:42:49,750 if they're over the threshold frequency over the threshold 754 00:42:49,750 --> 00:42:52,990 energy, then you're going to have an electron ejected 755 00:42:52,990 --> 00:42:54,360 from the metal surface. 756 00:42:54,360 --> 00:42:59,110 So for each photon that has greater incident 757 00:42:59,110 --> 00:43:01,150 energy than the threshold, you'll 758 00:43:01,150 --> 00:43:03,610 have an electron being ejected. 759 00:43:03,610 --> 00:43:05,470 So what is intensity? 760 00:43:05,470 --> 00:43:10,190 Well, the intensity is really the photons per second. 761 00:43:10,190 --> 00:43:13,240 So it's proportional to the number of photons being 762 00:43:13,240 --> 00:43:15,610 absorbed by the metal, and therefore, 763 00:43:15,610 --> 00:43:20,080 the number of electrons coming out of the metal. 764 00:43:20,080 --> 00:43:23,070 So intensity's units are often in watts. 765 00:43:23,070 --> 00:43:27,380 You also can do a conversion in joules per second. 766 00:43:27,380 --> 00:43:31,780 So the higher the intensity, the more 767 00:43:31,780 --> 00:43:36,160 photons with the appropriate amount of energy 768 00:43:36,160 --> 00:43:40,860 to overcome that work function, the more electrons coming off. 769 00:43:40,860 --> 00:43:44,980 So now we can go back and think about these plots again. 770 00:43:44,980 --> 00:43:49,720 So here the relationship between intensity and kinetic energy 771 00:43:49,720 --> 00:43:50,440 was flat. 772 00:43:50,440 --> 00:43:52,270 And that was unexpected. 773 00:43:52,270 --> 00:43:55,630 But the kinetic energy doesn't change here 774 00:43:55,630 --> 00:44:00,760 since the intensity means more photons per second, not 775 00:44:00,760 --> 00:44:03,150 more energy per photon. 776 00:44:03,150 --> 00:44:06,280 So you're just increasing the number of photons. 777 00:44:06,280 --> 00:44:09,550 If none of the photons have the appropriate energy, 778 00:44:09,550 --> 00:44:12,880 you're not going to have any electrons coming off. 779 00:44:12,880 --> 00:44:18,610 But if you have more photons per second, 780 00:44:18,610 --> 00:44:21,900 and they have the appropriate amount of energy, 781 00:44:21,900 --> 00:44:26,530 then you are going to see these electrons coming off. 782 00:44:26,530 --> 00:44:29,400 So the number of electrons admitted 783 00:44:29,400 --> 00:44:33,400 does change since high intensity means more photons. 784 00:44:33,400 --> 00:44:37,150 More photons, more electrons. 785 00:44:37,150 --> 00:44:40,840 So these things that Einstein helped us with, 786 00:44:40,840 --> 00:44:44,730 these equations, now made sense of the data 787 00:44:44,730 --> 00:44:46,190 that was being observed. 788 00:44:46,190 --> 00:44:48,520 So the photoelectric effect was really 789 00:44:48,520 --> 00:44:51,970 important in helping derive these relationships 790 00:44:51,970 --> 00:44:54,250 between energy and frequency. 791 00:44:54,250 --> 00:44:57,630 And so in particular, the really important points 792 00:44:57,630 --> 00:45:00,880 here is that light is made up of these photons, 793 00:45:00,880 --> 00:45:03,820 these discrete energy packages. 794 00:45:03,820 --> 00:45:08,230 And each one of those photons has to have enough energy in it 795 00:45:08,230 --> 00:45:11,500 to overcome the threshold to emit an electron. 796 00:45:11,500 --> 00:45:14,710 So energy is proportional to frequency, 797 00:45:14,710 --> 00:45:18,100 which was a really new and exciting idea at the time. 798 00:45:18,100 --> 00:45:21,480 E equals Planck's constant times frequency. 799 00:45:21,480 --> 00:45:24,420 And the intensity of light has to do with the number 800 00:45:24,420 --> 00:45:26,540 of photons hitting per second. 801 00:45:26,540 --> 00:45:28,590 And if you keep these things in mind, 802 00:45:28,590 --> 00:45:31,900 you'll do really well finishing up a number of the problems 803 00:45:31,900 --> 00:45:36,340 on the photoelectric effect, which are in problem set one. 804 00:45:36,340 --> 00:45:37,860 OK, see you on Wednesday. 805 00:45:37,860 --> 00:45:41,820 We're going to do a demo of the photoelectric effect.