1 00:00:00,030 --> 00:00:02,535 The following content is provided under a Creative 2 00:00:02,535 --> 00:00:03,830 Commons license. 3 00:00:03,830 --> 00:00:06,850 Your support will help MIT OpenCourseWare continue to 4 00:00:06,850 --> 00:00:10,510 offer high-quality educational resources for free. 5 00:00:10,510 --> 00:00:13,390 To make a donation or view additional materials from 6 00:00:13,390 --> 00:00:17,490 hundreds of MIT courses, visit MIT OpenCourseWare at 7 00:00:17,490 --> 00:00:18,740 ocw.mit.edu. 8 00:00:20,670 --> 00:00:22,060 PROFESSOR: Let's get right into it. 9 00:00:22,060 --> 00:00:25,160 So last day, we started looking at Roentgen again and 10 00:00:25,160 --> 00:00:28,800 the generation of x-rays, which we saw occurred when we 11 00:00:28,800 --> 00:00:33,780 operate the gas discharge tube at high voltage and low 12 00:00:33,780 --> 00:00:36,640 pressure and this is the image that we saw 13 00:00:36,640 --> 00:00:39,370 of his wife's hand. 14 00:00:39,370 --> 00:00:42,510 Birth of medical radiography. 15 00:00:42,510 --> 00:00:47,060 And we started looking at the origin of x-rays and we 16 00:00:47,060 --> 00:00:51,970 started looking at the energy level diagram of the target 17 00:00:51,970 --> 00:00:58,620 anode in the gas discharge tube and this is the energy 18 00:00:58,620 --> 00:01:00,310 level diagram of the target. 19 00:01:00,310 --> 00:01:03,550 So the ballistic electron-- 20 00:01:03,550 --> 00:01:04,950 I'm calling it incident-- 21 00:01:04,950 --> 00:01:08,810 this is the electron that's making its journey across the 22 00:01:08,810 --> 00:01:10,500 x-ray generating tube. 23 00:01:10,500 --> 00:01:14,500 It left the cathode and it's moving across and crashing 24 00:01:14,500 --> 00:01:15,490 into the anode. 25 00:01:15,490 --> 00:01:18,660 So this is the target or the anode. 26 00:01:18,660 --> 00:01:20,876 So let's label it as such. 27 00:01:20,876 --> 00:01:25,250 So we're looking at one of these mixed metaphors, where 28 00:01:25,250 --> 00:01:32,150 we've got both a Cartesian image here of the electron and 29 00:01:32,150 --> 00:01:33,330 we've got an energy image. 30 00:01:33,330 --> 00:01:38,970 So this is the target or it's the anode and it's charged 31 00:01:38,970 --> 00:01:40,980 positively and so the electrons are 32 00:01:40,980 --> 00:01:41,990 crashing into it. 33 00:01:41,990 --> 00:01:45,635 And we reason that what would happen is, if the electrons, 34 00:01:45,635 --> 00:01:48,260 the ballistic electrons, had high enough energy, they could 35 00:01:48,260 --> 00:01:54,390 actually dislodge 1s electrons and when they do so, they make 36 00:01:54,390 --> 00:01:56,150 the conditions for a cascade. 37 00:01:56,150 --> 00:01:59,730 So you can see electrons falling from n equals 2 to n 38 00:01:59,730 --> 00:02:02,820 equals 1 and when they do so, they emit radiation. 39 00:02:02,820 --> 00:02:06,330 They emit photons and these photons are called K alpha. 40 00:02:06,330 --> 00:02:09,975 K because the final shell number was n equals 1. 41 00:02:09,975 --> 00:02:11,850 And remember, the spectroscopists 42 00:02:11,850 --> 00:02:13,450 prefer to use letters. 43 00:02:13,450 --> 00:02:17,540 So when the electron ends at n equals 1, that's n final, we 44 00:02:17,540 --> 00:02:21,380 call it a K photon. 45 00:02:21,380 --> 00:02:26,670 And furthermore, the subscript alpha means that the delta, 46 00:02:26,670 --> 00:02:30,510 the n initial to n final is only one. 47 00:02:30,510 --> 00:02:33,780 So when you go from 2 to 1, you get the K alpha. 48 00:02:33,780 --> 00:02:37,580 And less likely but still possible is the transition 49 00:02:37,580 --> 00:02:39,180 from 3 down to 1. 50 00:02:39,180 --> 00:02:42,750 So it's still called a K photon because the electron 51 00:02:42,750 --> 00:02:45,930 that generated the photon ended in the K shell, but it 52 00:02:45,930 --> 00:02:50,140 traveled from a delta n of 2, went from 3 to 1 53 00:02:50,140 --> 00:02:51,760 so that's a K beta. 54 00:02:51,760 --> 00:02:54,435 Over here, obviously if we have enough energy to kick out 55 00:02:54,435 --> 00:02:57,800 K shell electrons, we have enough energy to kick out L 56 00:02:57,800 --> 00:02:59,120 shell electrons. 57 00:02:59,120 --> 00:03:01,550 And so if we lose an L shell electron and we have a 58 00:03:01,550 --> 00:03:06,100 cascade, then anything that ends in n equals 2 will be 59 00:03:06,100 --> 00:03:08,120 called an L photon. 60 00:03:08,120 --> 00:03:10,910 If it comes from 3 to 2, that's a delta of 1. 61 00:03:10,910 --> 00:03:12,240 That's the L alpha. 62 00:03:12,240 --> 00:03:14,390 It it falls 4 to 2, that's a delta of 2. 63 00:03:14,390 --> 00:03:15,520 That's the L beta. 64 00:03:15,520 --> 00:03:17,325 And so you can see you get a set of lines. 65 00:03:20,150 --> 00:03:22,790 And that set of lines looks like this. 66 00:03:22,790 --> 00:03:26,370 We can plot a spectrum of this entire set of lines, which 67 00:03:26,370 --> 00:03:30,185 will be characteristic of the identity of the target. 68 00:03:34,520 --> 00:03:37,360 And I think I showed you last day that it's going to look 69 00:03:37,360 --> 00:03:45,650 like this, where we will plot intensity and the intensity on 70 00:03:45,650 --> 00:03:49,410 a spectrum is related to the frequency of occurrence and on 71 00:03:49,410 --> 00:03:52,930 the abscissa, we're going to have some kind of energy 72 00:03:52,930 --> 00:03:53,520 coordinate. 73 00:03:53,520 --> 00:03:57,300 We can put lambda increasing from left to right, which 74 00:03:57,300 --> 00:04:00,020 means energy increases from right to left. 75 00:04:00,020 --> 00:04:02,420 And we saw that we have this family of lines where we have 76 00:04:02,420 --> 00:04:07,610 a discrete line at the energy of K alpha and a second line 77 00:04:07,610 --> 00:04:12,860 at the energy associated with K beta and K beta has a higher 78 00:04:12,860 --> 00:04:15,780 energy, therefore, a lower wavelength, and the relative 79 00:04:15,780 --> 00:04:19,450 heights related to the relative frequency, again, not 80 00:04:19,450 --> 00:04:24,470 to scale, but still the general 81 00:04:24,470 --> 00:04:25,870 relationship is correct. 82 00:04:25,870 --> 00:04:29,040 The likelihood of falling from 2 to 1 is higher than falling 83 00:04:29,040 --> 00:04:32,700 from 3 to 1 and so you'd expect to have the intensity 84 00:04:32,700 --> 00:04:35,060 of the K alpha line greater than the intensity of the K 85 00:04:35,060 --> 00:04:39,040 beta line and the L lines have to be of lower energy because 86 00:04:39,040 --> 00:04:43,570 transitions 3 to 2 are less energy than 2 to 1 so we 87 00:04:43,570 --> 00:04:47,960 expect those lines to be out here and again, a relative 88 00:04:47,960 --> 00:04:52,460 frequency where the L alpha has a slightly lower energy 89 00:04:52,460 --> 00:04:56,220 and L beta has a slightly lower frequency of a current. 90 00:04:56,220 --> 00:04:59,645 So that's the spectrum and this we call characteristic. 91 00:05:03,380 --> 00:05:04,730 Characteristic of what? 92 00:05:04,730 --> 00:05:11,240 Of the identity of the anode or the target. 93 00:05:14,470 --> 00:05:16,510 This is generating the x-rays. 94 00:05:16,510 --> 00:05:20,500 So if I want x-rays of this value of wavelength, I choose 95 00:05:20,500 --> 00:05:22,450 the target appropriately. 96 00:05:22,450 --> 00:05:25,240 That's why we have the relationship here. 97 00:05:25,240 --> 00:05:27,410 And then the other thing is that-- 98 00:05:27,410 --> 00:05:29,920 it's obvious, but I just want to make sure that 99 00:05:29,920 --> 00:05:32,020 we put it up here-- 100 00:05:32,020 --> 00:05:36,330 it's quantized because we're looking at the photons coming 101 00:05:36,330 --> 00:05:40,160 at discrete values of energy. 102 00:05:40,160 --> 00:05:42,910 So that's what we speculated on. 103 00:05:42,910 --> 00:05:44,230 Are there any data? 104 00:05:44,230 --> 00:05:48,130 The answer is yes and the data come from a young man by the 105 00:05:48,130 --> 00:05:49,380 name of Henry Moseley. 106 00:05:52,230 --> 00:05:55,670 He was doing his PhD up in Manchester. 107 00:05:55,670 --> 00:05:57,820 He was is working for Rutherford. 108 00:05:57,820 --> 00:06:06,450 Rutherford was his PhD thesis supervisor and 1913, 1914, he 109 00:06:06,450 --> 00:06:09,600 was making systematic measurements. 110 00:06:09,600 --> 00:06:16,170 He was conducting a systematic study of the characteristic 111 00:06:16,170 --> 00:06:20,750 spectra of no fewer than 38 elements of 112 00:06:20,750 --> 00:06:22,560 the Periodic Table. 113 00:06:22,560 --> 00:06:26,910 And what did we learn from his measurements? 114 00:06:26,910 --> 00:06:30,170 Learned from his measurements that there's was a pattern 115 00:06:30,170 --> 00:06:34,730 here and here's what the pattern is that Moseley found. 116 00:06:34,730 --> 00:06:39,300 He found that if he took the value of-- 117 00:06:39,300 --> 00:06:41,120 I'm going to just take one line. 118 00:06:41,120 --> 00:06:46,170 If he took all of the K alpha lines from 1K alpha per 119 00:06:46,170 --> 00:06:50,810 element and he plotted the value of the wave number of a 120 00:06:50,810 --> 00:06:55,670 particular line, he found that the value of the wave number 121 00:06:55,670 --> 00:06:59,160 of a particular line scaled with the identity of the 122 00:06:59,160 --> 00:07:02,720 element by the square of the proton number. 123 00:07:07,470 --> 00:07:10,420 So, for example, we would have, say here we 124 00:07:10,420 --> 00:07:12,300 started with aluminum. 125 00:07:12,300 --> 00:07:15,890 So aluminum proton numbers 13 so 13 squared. 126 00:07:15,890 --> 00:07:18,030 And he went all the way up to gold. 127 00:07:18,030 --> 00:07:20,670 Didn't do all of them but he did, as I say, 38 128 00:07:20,670 --> 00:07:21,740 from one to the other. 129 00:07:21,740 --> 00:07:23,230 And these are discrete values. 130 00:07:23,230 --> 00:07:26,150 You have different elements here and gold is up here and 131 00:07:26,150 --> 00:07:31,340 he found that he could take the set of data for, say, all 132 00:07:31,340 --> 00:07:34,700 the K alphas and they lie on a line-- 133 00:07:34,700 --> 00:07:36,260 new bar-- 134 00:07:36,260 --> 00:07:40,440 as if proportional to the square of the proton number 135 00:07:40,440 --> 00:07:45,520 and he did the same with other lines-- 136 00:07:45,520 --> 00:07:47,470 say, L alpha and so on-- 137 00:07:47,470 --> 00:07:50,700 found that they lie on a line. 138 00:07:50,700 --> 00:07:54,400 So what does all of this mean? 139 00:07:54,400 --> 00:07:56,090 Well, let's take a look. 140 00:07:56,090 --> 00:07:59,690 Here's the image of the paper. 141 00:07:59,690 --> 00:08:01,740 High-frequency spectra elements. 142 00:08:04,420 --> 00:08:08,040 Henry Moseley, Master's degree, so he's working on his 143 00:08:08,040 --> 00:08:11,970 PhD, and the data were taken as follows in 144 00:08:11,970 --> 00:08:13,780 photographic plates. 145 00:08:13,780 --> 00:08:18,090 And so by trigonometry, you can figure out what the 146 00:08:18,090 --> 00:08:21,120 wavelength would be of the line that would go to this 147 00:08:21,120 --> 00:08:23,760 degree of distortion, et cetera, et cetera. 148 00:08:23,760 --> 00:08:25,120 It's a beautiful set of lines. 149 00:08:25,120 --> 00:08:26,460 Look at how these date are posed. 150 00:08:26,460 --> 00:08:27,700 So there's calcium. 151 00:08:27,700 --> 00:08:29,530 Scandium is frightfully expensive so you 152 00:08:29,530 --> 00:08:30,270 don't see it here. 153 00:08:30,270 --> 00:08:32,990 Then there's titanium, vanadium, chromium, manganese, 154 00:08:32,990 --> 00:08:37,120 and as you change the element, the wave number, the 155 00:08:37,120 --> 00:08:40,820 wavelength, the energy of all of these lines changes 156 00:08:40,820 --> 00:08:44,650 systematically in accordance with the square of the proton 157 00:08:44,650 --> 00:08:47,610 number, and we got over here to copper. 158 00:08:47,610 --> 00:08:51,870 Next one is zinc. Zinc melts at 420 degrees Celsius, which 159 00:08:51,870 --> 00:08:53,320 is relatively low melting. 160 00:08:53,320 --> 00:08:56,530 And under the bombardment of electrons-- 161 00:08:56,530 --> 00:09:00,450 the bombardment of zinc would cause it to melt and so rather 162 00:09:00,450 --> 00:09:03,460 than work with zinc, he instead worked with brass 163 00:09:03,460 --> 00:09:05,750 because brass is an alloy of copper and zinc-- 164 00:09:05,750 --> 00:09:06,890 and look carefully. 165 00:09:06,890 --> 00:09:10,480 You can see that brass has four lines, two lines 166 00:09:10,480 --> 00:09:13,670 identical to the lines of copper and these two new lines 167 00:09:13,670 --> 00:09:16,670 are the lines associated with zinc. And I mentioned last day 168 00:09:16,670 --> 00:09:21,490 that you could actually deconvolve the complex spectra 169 00:09:21,490 --> 00:09:24,920 and identify the constituent elements. 170 00:09:24,920 --> 00:09:29,130 So this was really fantastic, really fantastic. 171 00:09:29,130 --> 00:09:31,580 Si what did we learn from all of this? 172 00:09:31,580 --> 00:09:35,210 What did we learn by these data? 173 00:09:35,210 --> 00:09:44,530 well, first of all, corrected Mendeleyev What 174 00:09:44,530 --> 00:09:45,700 do I mean by that? 175 00:09:45,700 --> 00:09:48,570 Not here to say bad things about Mendeleyev, but 176 00:09:48,570 --> 00:10:00,240 Mendeleyev had told us that periodicity is a function of 177 00:10:00,240 --> 00:10:03,900 the atomic mass. 178 00:10:03,900 --> 00:10:05,670 That's what Mendeleyev said. 179 00:10:05,670 --> 00:10:08,510 Periodicity is a function of atomic mass. 180 00:10:08,510 --> 00:10:11,460 And now Moseley says no. 181 00:10:11,460 --> 00:10:14,480 Moseley says that periodicity is a 182 00:10:14,480 --> 00:10:16,040 function of proton number. 183 00:10:21,980 --> 00:10:24,080 And you can see here-- 184 00:10:26,730 --> 00:10:28,470 I wanted to show you the-- 185 00:10:28,470 --> 00:10:30,810 this is an image taken from his paper. 186 00:10:30,810 --> 00:10:31,970 He worked for Rutherford. 187 00:10:31,970 --> 00:10:34,770 These people were brilliant experimentalists and 188 00:10:34,770 --> 00:10:36,130 so here he's got-- 189 00:10:36,130 --> 00:10:38,500 the cathode is up here at the top. 190 00:10:38,500 --> 00:10:41,007 You see cathode of x-ray tube and the feed through and so 191 00:10:41,007 --> 00:10:44,740 on, so the anode is down here and it's connected and so on, 192 00:10:44,740 --> 00:10:49,020 and rather than take the x-ray tube apart in order to change 193 00:10:49,020 --> 00:10:52,850 the target, he went to the toy store and he bought-- 194 00:10:52,850 --> 00:10:57,330 this is the train from the toy store and he's got different 195 00:10:57,330 --> 00:11:03,080 elements seated next to each other on the flat car of the 196 00:11:03,080 --> 00:11:06,750 train and he's got feedthroughs with silk fishing 197 00:11:06,750 --> 00:11:10,180 line so that after he's done the experiment with one 198 00:11:10,180 --> 00:11:16,790 element, he can pull the train car over and change the anode 199 00:11:16,790 --> 00:11:20,002 without having to take the apparatus apart. 200 00:11:20,002 --> 00:11:23,160 These guys were very, very good experimentalists. 201 00:11:23,160 --> 00:11:25,620 So here's from his paper. 202 00:11:25,620 --> 00:11:28,620 The author intends first to make a general survey of the 203 00:11:28,620 --> 00:11:31,440 principal types of high-frequency radiation and 204 00:11:31,440 --> 00:11:34,060 then to examine the spectra of a few elements in greater 205 00:11:34,060 --> 00:11:36,120 detail with greater accuracy. 206 00:11:36,120 --> 00:11:39,210 The results already obtained show that such data have an 207 00:11:39,210 --> 00:11:40,840 important bearing on the question of the internal 208 00:11:40,840 --> 00:11:43,700 structure of the atom and strongly support the views of 209 00:11:43,700 --> 00:11:45,420 Rutherford and Bohr. 210 00:11:45,420 --> 00:11:46,670 It's 1913. 211 00:11:46,670 --> 00:11:48,510 Remember, that's when Bohr paper came out. 212 00:11:48,510 --> 00:11:51,270 All these people were working in the lab at the same time, 213 00:11:51,270 --> 00:11:52,800 supporting each other. 214 00:11:52,800 --> 00:11:55,680 You see, this doesn't make any sense with a Plum Pudding 215 00:11:55,680 --> 00:11:58,380 Model, does it? 216 00:11:58,380 --> 00:12:01,640 See, he continues: "We have here a proof that there isn't 217 00:12:01,640 --> 00:12:04,910 the atom of fundamental quantity, which increases by 218 00:12:04,910 --> 00:12:08,370 regular steps as we pass from one element to the next. 219 00:12:08,370 --> 00:12:12,340 This quantity can only be the charge on the central positive 220 00:12:12,340 --> 00:12:16,840 nucleus on the existence of which we already have definite 221 00:12:16,840 --> 00:12:18,470 proof." 222 00:12:18,470 --> 00:12:21,090 See, not only-- remember, the Plum Pudding Model has this 223 00:12:21,090 --> 00:12:23,800 big blob of positive charge nests. 224 00:12:23,800 --> 00:12:25,770 There are no protons. 225 00:12:25,770 --> 00:12:28,890 In the nuclear model, we have a nucleus that has positive 226 00:12:28,890 --> 00:12:32,430 charge and he's saying, I know there's positive charge, and 227 00:12:32,430 --> 00:12:35,150 it increases as you go from one element to the next. 228 00:12:35,150 --> 00:12:37,610 We are therefore led by experiment to view that N-- we 229 00:12:37,610 --> 00:12:40,210 use the letter Z today. 230 00:12:40,210 --> 00:12:41,560 He's using capital N. 231 00:12:41,560 --> 00:12:45,460 N is the same as the number of the place occupied by the 232 00:12:45,460 --> 00:12:47,740 element in the periodic system. 233 00:12:47,740 --> 00:12:50,720 This atomic number-- for the first time, the term atomic 234 00:12:50,720 --> 00:12:51,450 number is used. 235 00:12:51,450 --> 00:12:54,350 That's why I was being coy here and I kept saying proton 236 00:12:54,350 --> 00:12:57,410 number, because that's the way they knew it. 237 00:12:57,410 --> 00:12:59,480 Now he's say, this is the social security 238 00:12:59,480 --> 00:13:01,630 number of the element. 239 00:13:01,630 --> 00:13:05,560 Proton number is then for hydrogen 1, helium 2, lithium 240 00:13:05,560 --> 00:13:08,310 3, calcium 20, zinc 30, et cetera. 241 00:13:08,310 --> 00:13:10,310 We can confidently predict-- 242 00:13:10,310 --> 00:13:13,940 look at-- this is the masters student and 243 00:13:13,940 --> 00:13:14,770 he's going on a limb. 244 00:13:14,770 --> 00:13:17,680 He says: "We can confidently predict that in the few cases 245 00:13:17,680 --> 00:13:21,230 in which the order of the atomic weights A clashes with 246 00:13:21,230 --> 00:13:24,190 the chemical order of the periodic system, the chemical 247 00:13:24,190 --> 00:13:29,850 properties are governed by N, while A itself is probably a 248 00:13:29,850 --> 00:13:34,700 complicated function of N." He's right. 249 00:13:34,700 --> 00:13:36,160 You look at the Periodic Table. 250 00:13:36,160 --> 00:13:38,530 You say, yeah, yeah, they just go in descending mass or 251 00:13:38,530 --> 00:13:39,990 ascending proton number. 252 00:13:39,990 --> 00:13:41,240 Look carefully. 253 00:13:43,090 --> 00:13:46,180 Potassium has a lower mass than argon. 254 00:13:46,180 --> 00:13:47,070 And nobody's to-- 255 00:13:47,070 --> 00:13:48,620 well, if you were Mendeleyev, you'd say, go back 256 00:13:48,620 --> 00:13:49,340 and measure it again. 257 00:13:49,340 --> 00:13:50,310 Well, they have measured it. 258 00:13:50,310 --> 00:13:53,520 These are the accurate values and no one's going to put 259 00:13:53,520 --> 00:13:57,820 potassium underneath neon, but it comes next in the order of 260 00:13:57,820 --> 00:14:00,550 ascending atomic mass. 261 00:14:00,550 --> 00:14:02,290 Cobalt and nickel-- nickel actually 262 00:14:02,290 --> 00:14:04,090 weighs less than cobalt. 263 00:14:04,090 --> 00:14:06,390 But they're transition elements so who cares if you 264 00:14:06,390 --> 00:14:08,760 get those two mixed up? 265 00:14:08,760 --> 00:14:10,080 You find them in stainless steel. 266 00:14:10,080 --> 00:14:11,610 It doesn't matter. 267 00:14:11,610 --> 00:14:12,960 This is a good one. 268 00:14:12,960 --> 00:14:15,350 Iodine is lower mass than tellurium. 269 00:14:15,350 --> 00:14:17,040 Iodine is a halogen. 270 00:14:17,040 --> 00:14:20,420 It belongs under fluorine, chlorine and bromine. 271 00:14:20,420 --> 00:14:21,590 You're not going to put it under 272 00:14:21,590 --> 00:14:25,280 oxygen, sulfur, selenium. 273 00:14:25,280 --> 00:14:29,310 But look, there's the data. 274 00:14:29,310 --> 00:14:33,340 And then last one that Moseley couldn't have known about is 275 00:14:33,340 --> 00:14:37,090 transuranic synthetic element, but that's just the fourth 276 00:14:37,090 --> 00:14:38,390 case in the periodic table. 277 00:14:38,390 --> 00:14:40,580 So that gets you the last wedge in Trivial Pursuit. 278 00:14:40,580 --> 00:14:42,960 What are the four pairs of elements 279 00:14:42,960 --> 00:14:46,460 that are out of sequence? 280 00:14:46,460 --> 00:14:47,470 OK. 281 00:14:47,470 --> 00:14:56,630 So now we can say that proton number is the atomic number. 282 00:14:56,630 --> 00:14:58,200 It's the identity. 283 00:14:58,200 --> 00:15:01,710 So that comes out of Moseley, comes out of his work. 284 00:15:01,710 --> 00:15:07,260 The second thing that he did as a result of his study of 38 285 00:15:07,260 --> 00:15:09,420 elements is he figured out what to do with the 286 00:15:09,420 --> 00:15:11,160 lanthanides. 287 00:15:11,160 --> 00:15:12,410 Remember our friends, the lanthanides. 288 00:15:15,680 --> 00:15:19,650 Place the lanthanides correctly in 289 00:15:19,650 --> 00:15:20,900 the Periodic Table. 290 00:15:25,290 --> 00:15:29,240 Many of the lanthanides have a valence 3 so people were 291 00:15:29,240 --> 00:15:29,840 struggling. 292 00:15:29,840 --> 00:15:33,600 They were putting them underneath aluminum. 293 00:15:33,600 --> 00:15:36,590 They didn't know what to do with them. 294 00:15:36,590 --> 00:15:38,450 He placed the lanthanides correctly in 295 00:15:38,450 --> 00:15:39,680 the Periodic Table. 296 00:15:39,680 --> 00:15:41,350 And it's not as though these were brand new. 297 00:15:41,350 --> 00:15:46,650 Lanthanum itself had been first isolated in 1839, and 298 00:15:46,650 --> 00:15:48,970 all through the 1800s, they were picking them up and 299 00:15:48,970 --> 00:15:53,600 finally, lutetium was the last one discovered, 300 00:15:53,600 --> 00:15:57,390 categorized in 1907. 301 00:15:57,390 --> 00:15:59,500 But people didn't know what to do with them and remember, 302 00:15:59,500 --> 00:16:02,360 they were obsessed with atomic mass measurements. 303 00:16:02,360 --> 00:16:03,300 So I'm going to give you this. 304 00:16:03,300 --> 00:16:05,190 I'll give you this piece of information. 305 00:16:05,190 --> 00:16:08,340 There's the atomic mass of lanthanum, and the atomic mass 306 00:16:08,340 --> 00:16:13,350 of lutetium is 174.97. 307 00:16:13,350 --> 00:16:14,940 So what? 308 00:16:14,940 --> 00:16:16,790 I don't know what to do with that information. 309 00:16:16,790 --> 00:16:18,430 That doesn't help me at all. 310 00:16:18,430 --> 00:16:21,720 But now comes Moseley and he says, we're not talking about 311 00:16:21,720 --> 00:16:22,710 atomic mass. 312 00:16:22,710 --> 00:16:24,450 We're talking about atomic number. 313 00:16:24,450 --> 00:16:28,460 So now I tell you this atomic number is 57. 314 00:16:28,460 --> 00:16:30,090 So where do you put it? 315 00:16:30,090 --> 00:16:33,150 Duh, you put it right next to barium. 316 00:16:33,150 --> 00:16:34,830 There's no debate. 317 00:16:34,830 --> 00:16:38,800 And furthermore, this one is 71-- 318 00:16:38,800 --> 00:16:40,310 atomic number. 319 00:16:40,310 --> 00:16:43,850 Well, if this is 71 and this is 57, I can 320 00:16:43,850 --> 00:16:45,210 tell you with impunity-- 321 00:16:45,210 --> 00:16:47,980 how many lanthanides are there? 322 00:16:47,980 --> 00:16:50,650 There's 14 of them from here to there. 323 00:16:50,650 --> 00:16:51,370 OK. 324 00:16:51,370 --> 00:16:58,390 14 elements, which makes sense because in s we've got one 325 00:16:58,390 --> 00:17:02,790 orbital, in p we've got three orbitals, and d we've got 5 326 00:17:02,790 --> 00:17:05,690 orbitals and these are f and there's 7 orbitals. 327 00:17:05,690 --> 00:17:07,360 7 times 2 is 14. 328 00:17:07,360 --> 00:17:08,610 Everything makes sense. 329 00:17:11,980 --> 00:17:17,110 And the last thing that's a corollary to the item two-- 330 00:17:17,110 --> 00:17:20,470 we were able to give uranium its proper 331 00:17:20,470 --> 00:17:24,110 atomic number is 92. 332 00:17:24,110 --> 00:17:27,930 So now I ask you, how many elements are there up to 333 00:17:27,930 --> 00:17:29,910 uranium starting with hydrogen? 334 00:17:29,910 --> 00:17:31,090 92. 335 00:17:31,090 --> 00:17:36,180 All of this comes as the result of Moseley's 336 00:17:36,180 --> 00:17:36,890 experiments. 337 00:17:36,890 --> 00:17:39,420 But it's not over yet because he worked for Rutherford, and 338 00:17:39,420 --> 00:17:42,040 Rutherford pushed his people really hard. 339 00:17:42,040 --> 00:17:45,160 He wasn't abusive, but he brought out the best in them. 340 00:17:45,160 --> 00:17:47,130 So who else was in the building? 341 00:17:47,130 --> 00:17:49,260 Bohr, and what were they doing? 342 00:17:49,260 --> 00:17:54,000 They were doing theory and so he said, well, that's nice, 343 00:17:54,000 --> 00:17:56,960 but he says, I want those things fit to an equation. 344 00:17:56,960 --> 00:17:59,610 So Moseley said, all right. 345 00:17:59,610 --> 00:18:00,400 I'm going to use-- 346 00:18:00,400 --> 00:18:03,090 there's already an equation in the building for a new bar. 347 00:18:03,090 --> 00:18:04,390 It's the Rydberg equation. 348 00:18:04,390 --> 00:18:07,200 So I'm going to use a Rydberg-like equation. 349 00:18:07,200 --> 00:18:08,320 So this is what he does. 350 00:18:08,320 --> 00:18:09,550 This is Moseley's fit. 351 00:18:09,550 --> 00:18:14,490 He goes the wave number of whatever the line is-- whether 352 00:18:14,490 --> 00:18:17,550 it's K alpha, L alpha, what have you-- is going to go as 353 00:18:17,550 --> 00:18:20,790 the product of the Rydberg constant-- 354 00:18:20,790 --> 00:18:25,760 1/NF squared minus 1/NI squared. 355 00:18:25,760 --> 00:18:28,700 So far that's just the Rydberg equation. 356 00:18:28,700 --> 00:18:31,730 But now comes Moseley's contribution. 357 00:18:31,730 --> 00:18:38,260 We're going to put z squared, but one more piece. 358 00:18:38,260 --> 00:18:39,970 Notice the way I've drawn those lines. 359 00:18:39,970 --> 00:18:42,100 They don't go through the origin, do they? 360 00:18:42,100 --> 00:18:44,830 There's an offset. 361 00:18:44,830 --> 00:18:47,140 So he said, let's allow for the offset-- 362 00:18:47,140 --> 00:18:50,120 z minus sigma quantity square. 363 00:18:50,120 --> 00:18:52,120 And this is known as Moseley's Law. 364 00:18:58,440 --> 00:19:02,160 And you've got values for sigma. 365 00:19:02,160 --> 00:19:16,270 sigma for K alpha equals 1 and for L alpha equals 7.4. 366 00:19:16,270 --> 00:19:17,860 So now I can-- 367 00:19:17,860 --> 00:19:21,030 with impunity, you give me the wavelength. 368 00:19:21,030 --> 00:19:23,670 I can get the wave number-- it's 1 over the wavelength and 369 00:19:23,670 --> 00:19:27,260 I can plug into this equation and identify the element or 370 00:19:27,260 --> 00:19:31,495 turn it around if I want to get wavelength radiation of, 371 00:19:31,495 --> 00:19:35,910 say, 1.25 angstroms, I can plug into this equation and 372 00:19:35,910 --> 00:19:38,640 come up with the z, which will tell me what the target 373 00:19:38,640 --> 00:19:40,890 element choice should be. 374 00:19:40,890 --> 00:19:44,020 So let's go and plug this in because we're only going to 375 00:19:44,020 --> 00:19:45,390 look at these two. 376 00:19:45,390 --> 00:19:49,390 So we can say that the wave number for K alpha-- 377 00:19:49,390 --> 00:19:53,780 it's always going to be 1/2 squared minus 1/1 squared, 378 00:19:53,780 --> 00:19:55,900 which is always going to come up 3/4. 379 00:19:55,900 --> 00:20:01,600 So it's 3/4 times Rydberg z minus 1 squared. 380 00:20:01,600 --> 00:20:09,150 So this is for the 2:1 or, if you like, L to K transition 381 00:20:09,150 --> 00:20:12,670 and then the other one of interest here 382 00:20:12,670 --> 00:20:15,670 is nu bar of L alpha. 383 00:20:15,670 --> 00:20:19,060 nu bar of L alpha's going to be 1/3 squared minus 1/2 384 00:20:19,060 --> 00:20:26,270 squared, which becomes 536 times the Rydberg constant z 385 00:20:26,270 --> 00:20:31,540 minus 7.4 squared and this is for the transition 3:2 386 00:20:31,540 --> 00:20:38,100 or KLM, M to L. 387 00:20:38,100 --> 00:20:39,150 OK. 388 00:20:39,150 --> 00:20:41,480 So there it is. 389 00:20:41,480 --> 00:20:44,430 And now, what's the significance of all of this? 390 00:20:47,720 --> 00:20:51,960 I try to give a physical significance of this sigma, 391 00:20:51,960 --> 00:20:55,940 which is known as the screening factor. 392 00:20:55,940 --> 00:21:00,590 We're going to call sigma here the screening factor. 393 00:21:00,590 --> 00:21:02,840 You'll see why in a second. 394 00:21:02,840 --> 00:21:04,740 Why are we calling it the screening factor? 395 00:21:11,620 --> 00:21:15,620 So let's consider what's happening in the case of the K 396 00:21:15,620 --> 00:21:17,190 alpha lines. 397 00:21:17,190 --> 00:21:20,010 Let's look at K alpha generation. 398 00:21:20,010 --> 00:21:22,115 So not to scale. 399 00:21:25,420 --> 00:21:27,480 Let's draw-- 400 00:21:27,480 --> 00:21:32,840 here's the nucleus with all of its z positive charges and 401 00:21:32,840 --> 00:21:37,640 then I'm going to draw the K shell, and its got two 402 00:21:37,640 --> 00:21:41,480 electrons in it, and I'm going to show there's a hole here. 403 00:21:41,480 --> 00:21:43,350 Because without this hole, there's no 404 00:21:43,350 --> 00:21:45,060 reason for the cascade. 405 00:21:45,060 --> 00:21:49,490 And then I'm going to draw the L shell and it's got-- what? 406 00:21:49,490 --> 00:21:54,920 There's 2 from the s, 2s, and then 6 from the 2p. 407 00:21:54,920 --> 00:21:57,860 At most. I know this is a terrible model. 408 00:21:57,860 --> 00:22:00,820 It violates all kinds of things, but it's as complex as 409 00:22:00,820 --> 00:22:03,530 it needs to be for the explanation I'm about to give 410 00:22:03,530 --> 00:22:05,900 and I don't want to load you down with a whole bunch of 411 00:22:05,900 --> 00:22:07,540 extraneous information. 412 00:22:07,540 --> 00:22:11,210 So consider the electrons in the L shell. 413 00:22:11,210 --> 00:22:15,380 They see the electron vacancy in the K shell and they're 414 00:22:15,380 --> 00:22:16,570 going to fall down. 415 00:22:16,570 --> 00:22:20,330 What's the Coulombic pull of the nucleus 416 00:22:20,330 --> 00:22:22,620 on the L shell electrons? 417 00:22:22,620 --> 00:22:27,110 Can you see that it's not z plus, but at z plus screened 418 00:22:27,110 --> 00:22:29,200 by 1 minus. 419 00:22:29,200 --> 00:22:32,900 So the nuclear charge is mediated-- 420 00:22:32,900 --> 00:22:37,210 in other words, it's reduced by 1 thanks to the presence of 421 00:22:37,210 --> 00:22:39,360 the one negative charge here. 422 00:22:39,360 --> 00:22:44,000 So these electrons in L shell feel z less 1 and hence, we 423 00:22:44,000 --> 00:22:46,850 get z minus 1 as screening factor. 424 00:22:46,850 --> 00:22:48,180 It's plausible. 425 00:22:48,180 --> 00:22:50,360 It's at least physically consistent. 426 00:22:50,360 --> 00:22:51,780 Now let's look at the next one. 427 00:22:51,780 --> 00:22:55,230 That's the transition from M to L. 428 00:22:55,230 --> 00:22:55,850 OK. 429 00:22:55,850 --> 00:22:56,660 So we get to M. 430 00:22:56,660 --> 00:23:01,270 We've got 18 electrons, up to maximum of 18, and if they're 431 00:23:01,270 --> 00:23:05,740 going to fall down to n equals 2, there needs to be at least 432 00:23:05,740 --> 00:23:07,810 one vacancy here. 433 00:23:07,810 --> 00:23:10,000 So now let's use the same logic. 434 00:23:10,000 --> 00:23:14,120 So an electron in the m shell sees the positive charge of 435 00:23:14,120 --> 00:23:19,890 the nucleus mediated by the electrons between the shell n 436 00:23:19,890 --> 00:23:22,030 equals 3 and the nucleus. 437 00:23:22,030 --> 00:23:24,210 Now, I don't know how many electrons there are. 438 00:23:24,210 --> 00:23:27,080 What's the extreme case here? 439 00:23:27,080 --> 00:23:32,700 Two, four, six, seven, maybe eight, nine. 440 00:23:32,700 --> 00:23:35,910 Because this would still give me the conditions to generate 441 00:23:35,910 --> 00:23:37,360 the transition 3 to 2. 442 00:23:37,360 --> 00:23:38,980 I need a vacancy in two. 443 00:23:38,980 --> 00:23:41,050 I don't need a vacancy in one. 444 00:23:41,050 --> 00:23:42,640 So this is the maximum. 445 00:23:42,640 --> 00:23:45,620 so that would be two, four, six, eight, nine. 446 00:23:45,620 --> 00:23:49,990 So it could be z minus nine, but that would mean that all 447 00:23:49,990 --> 00:23:54,080 of these electrons are on the same side and I don't have any 448 00:23:54,080 --> 00:23:55,000 vacancies lower. 449 00:23:55,000 --> 00:23:56,620 I only have one vacancy here. 450 00:23:56,620 --> 00:23:57,265 That's an extreme. 451 00:23:57,265 --> 00:23:58,880 It's not observed. 452 00:23:58,880 --> 00:24:02,920 And the other extreme is, we blow away all the electrons. 453 00:24:02,920 --> 00:24:07,130 So somewhere in between 9 and 0, it turns out it's 7.4. 454 00:24:07,130 --> 00:24:11,670 I can't predict that it's 7.4, but 7.4 makes sense. 455 00:24:11,670 --> 00:24:16,490 If the number were greater than 9, I would be distressed. 456 00:24:16,490 --> 00:24:18,063 So it makes sense, rationalizes. 457 00:24:20,920 --> 00:24:22,320 OK. 458 00:24:22,320 --> 00:24:28,570 And by the way, if you use this formula, Moseley's Law-- 459 00:24:28,570 --> 00:24:30,380 let's bring that back down. 460 00:24:30,380 --> 00:24:33,520 Remember, I told you I can still wake up in the middle of 461 00:24:33,520 --> 00:24:37,540 the night and quote you the wavelength of copper K alpha 462 00:24:37,540 --> 00:24:39,590 radiation of five significant figures. 463 00:24:39,590 --> 00:24:41,690 I had it drilled into me in my junior year. 464 00:24:41,690 --> 00:24:46,550 It's 1.5418 angstroms. That's the lambda. 465 00:24:46,550 --> 00:24:50,660 So you can use this formula for new bar and you know that 466 00:24:50,660 --> 00:24:54,520 new bar is equal to 1 over lambda. 467 00:24:54,520 --> 00:24:57,170 So I can use Moseley's Law. 468 00:24:57,170 --> 00:25:00,030 But true value, lambda-- 469 00:25:00,030 --> 00:25:00,910 I love this one. 470 00:25:00,910 --> 00:25:01,340 Watch. 471 00:25:01,340 --> 00:25:03,360 I'm going to do a triple subscript. 472 00:25:03,360 --> 00:25:05,790 Lambda of copper K alpha. 473 00:25:05,790 --> 00:25:06,640 Isn't that cool? 474 00:25:06,640 --> 00:25:13,880 Lambda of copper K alpha is equal to 1.5418 angstroms. And 475 00:25:13,880 --> 00:25:19,165 if you use Moseley's law, you get 1.546 and the delta here 476 00:25:19,165 --> 00:25:20,600 is 1/3 of 1%. 477 00:25:24,970 --> 00:25:29,800 This man was a positive genius. 478 00:25:29,800 --> 00:25:34,210 He was heading pell-mell for the Nobel Prize, but 479 00:25:34,210 --> 00:25:35,530 he never got it. 480 00:25:35,530 --> 00:25:37,530 Why not? 481 00:25:37,530 --> 00:25:42,190 World War I broke out in 1914 and Moseley was passionately 482 00:25:42,190 --> 00:25:44,420 concerned about World War I. 483 00:25:44,420 --> 00:25:45,460 He wanted to fight. 484 00:25:45,460 --> 00:25:48,710 He wanted to fight for the Allied cause and he enlisted 485 00:25:48,710 --> 00:25:49,250 in the Army. 486 00:25:49,250 --> 00:25:50,990 Rutherford was furious. 487 00:25:50,990 --> 00:25:55,850 Rutherford called the Minister of War, which is analogous to 488 00:25:55,850 --> 00:25:58,850 the Secretary of Defense and said, give him a desk job. 489 00:25:58,850 --> 00:26:03,570 Put him up in Oxford at a military laboratory. 490 00:26:03,570 --> 00:26:05,400 And Moseley said, no, I refuse. 491 00:26:05,400 --> 00:26:06,580 I'm going to fight. 492 00:26:06,580 --> 00:26:08,440 And so he was sent to Gallipoli. 493 00:26:08,440 --> 00:26:12,040 Gallipoli, as you may know, was one of the bloodiest sites 494 00:26:12,040 --> 00:26:12,950 of World War I. 495 00:26:12,950 --> 00:26:16,300 A quarter of a million Allied troops and 1/3 sort of a 496 00:26:16,300 --> 00:26:20,650 million Turkish troops were killed at Gallipoli. 497 00:26:20,650 --> 00:26:25,190 Almost 2/3 of a million people died for that 498 00:26:25,190 --> 00:26:27,080 little piece of land. 499 00:26:27,080 --> 00:26:32,850 And on August 10th, 1915, at the age of 27, Henry Moseley 500 00:26:32,850 --> 00:26:37,910 was killed in the battle of Suvla Bay, and they don't give 501 00:26:37,910 --> 00:26:44,250 Nobel Prizes posthumously, and so that was the way it ended. 502 00:26:44,250 --> 00:26:48,920 There's a shot of Henry Moseley with one of his books. 503 00:26:48,920 --> 00:26:51,150 The tributes poured in from all over the world. 504 00:26:51,150 --> 00:26:53,620 The physics community was devastated because they knew 505 00:26:53,620 --> 00:26:56,620 all of this stuff. 506 00:26:56,620 --> 00:27:01,100 This is the one that really, I think, says it best. This was 507 00:27:01,100 --> 00:27:03,520 written by Robert Milliken, an American. 508 00:27:03,520 --> 00:27:08,880 He's the one that gave us the elementary charge of the 509 00:27:08,880 --> 00:27:12,620 electron, from the University of Chicago at the time. 510 00:27:12,620 --> 00:27:14,780 This is what Milliken wrote-- wrote this to 511 00:27:14,780 --> 00:27:16,920 Rutherford to read. 512 00:27:16,920 --> 00:27:19,520 "In a research which is destined to rank as one of the 513 00:27:19,520 --> 00:27:22,590 dozen most brilliant in conception, skillful in 514 00:27:22,590 --> 00:27:27,140 execution, and illuminating in results, a young man 26 years 515 00:27:27,140 --> 00:27:30,410 old threw open the windows through which we can glimpse 516 00:27:30,410 --> 00:27:35,030 the subatomic world with a definiteness and a certainty 517 00:27:35,030 --> 00:27:36,880 never dreamed before. 518 00:27:36,880 --> 00:27:40,750 Had the European War no other result than the snuffing out 519 00:27:40,750 --> 00:27:43,600 of this young life, that alone would make it one of the most 520 00:27:43,600 --> 00:27:47,330 hideous and most irreparable crimes in history." That's 521 00:27:47,330 --> 00:27:49,870 when American scientists knew to write. 522 00:27:49,870 --> 00:27:51,490 It's beautiful writing. 523 00:27:51,490 --> 00:27:54,470 What a tribute. 524 00:27:54,470 --> 00:27:54,850 OK. 525 00:27:54,850 --> 00:27:56,630 So let's move on. 526 00:27:56,630 --> 00:27:58,555 Now that we've got Moseley's Law, we've straightened out 527 00:27:58,555 --> 00:28:00,910 the Periodic Table. 528 00:28:00,910 --> 00:28:01,750 So we keep moving. 529 00:28:01,750 --> 00:28:09,280 And I say, well, I gave you the drawing of the spectrum. 530 00:28:09,280 --> 00:28:10,890 Remember the spectrum? 531 00:28:10,890 --> 00:28:12,140 The spectrum was here. 532 00:28:15,710 --> 00:28:16,670 You see it? 533 00:28:16,670 --> 00:28:18,400 I'll put it up real quick again. 534 00:28:18,400 --> 00:28:19,650 This was the spectrum. 535 00:28:22,850 --> 00:28:23,140 OK. 536 00:28:23,140 --> 00:28:31,350 This is intensity and this is wavelength and this is K alpha 537 00:28:31,350 --> 00:28:35,800 and this is K beta and this is L alpha and this is L beta. 538 00:28:35,800 --> 00:28:39,920 These are the data coming from the x-ray tube. 539 00:28:39,920 --> 00:28:42,030 This happens to be a molybdenum target. 540 00:28:42,030 --> 00:28:45,970 Well, it doesn't quite look like what I've drawn, does it? 541 00:28:45,970 --> 00:28:49,340 It looks a little bit like it, but not quite. 542 00:28:49,340 --> 00:28:51,500 This is definitely there. 543 00:28:51,500 --> 00:28:54,070 You can see some of these lines so I'm going to call 544 00:28:54,070 --> 00:28:59,220 this spectrum A and it looks like spectrum A has been added 545 00:28:59,220 --> 00:29:01,370 on top of something else. 546 00:29:01,370 --> 00:29:04,440 I'm going to call the something else spectrum B and 547 00:29:04,440 --> 00:29:05,980 the spectrum B looks like this. 548 00:29:10,200 --> 00:29:12,100 And in New England-- 549 00:29:12,100 --> 00:29:14,180 this curve has a shape because it's New England. 550 00:29:14,180 --> 00:29:17,790 This is called whale-shaped. 551 00:29:17,790 --> 00:29:20,780 I don't know what they call it in the rest of the country, 552 00:29:20,780 --> 00:29:22,170 but it even looks like a whale. 553 00:29:22,170 --> 00:29:24,440 So you can go on a whale watch. 554 00:29:24,440 --> 00:29:25,990 So it starts-- 555 00:29:25,990 --> 00:29:29,840 it's a sharp front, goes straight up, hits the maximum 556 00:29:29,840 --> 00:29:30,810 and then-- are you ready for this? 557 00:29:30,810 --> 00:29:32,780 It tails off, all right? 558 00:29:32,780 --> 00:29:36,480 Whales have tails, yes. 559 00:29:36,480 --> 00:29:38,160 So what's the difference here? 560 00:29:38,160 --> 00:29:43,820 Well, the spectrum A, we already observed is quantized, 561 00:29:43,820 --> 00:29:44,790 whereas this one isn't. 562 00:29:44,790 --> 00:29:47,800 This one's continuous. 563 00:29:47,800 --> 00:29:53,330 It has continuous values up to this minimum value or, if you 564 00:29:53,330 --> 00:29:57,320 like, maximum value of energy, minimum value of wavelengths. 565 00:29:57,320 --> 00:29:59,650 So we got that. 566 00:29:59,650 --> 00:30:00,650 It's not quantized. 567 00:30:00,650 --> 00:30:03,830 And the other thing that's interesting is-- 568 00:30:03,830 --> 00:30:08,250 spectrum A, we said, is a function of the identity of 569 00:30:08,250 --> 00:30:08,910 the target. 570 00:30:08,910 --> 00:30:13,030 z of the target, whereas this one, it's a function of the 571 00:30:13,030 --> 00:30:14,270 plate voltage. 572 00:30:14,270 --> 00:30:17,080 So you can see in the diagram I've shown you, as the plate 573 00:30:17,080 --> 00:30:24,730 voltage changes, we get a series of enveloping curves. 574 00:30:24,730 --> 00:30:28,080 So this is V1 and this is V2-- 575 00:30:28,080 --> 00:30:29,780 greater than V1. 576 00:30:29,780 --> 00:30:31,980 So this whale-shaped thing is somehow 577 00:30:31,980 --> 00:30:33,245 related to plate voltage. 578 00:30:35,770 --> 00:30:38,770 And then beyond a certain critical value, can you see 579 00:30:38,770 --> 00:30:43,410 when you're down here at 15 or 20,000 volts, all you got is 580 00:30:43,410 --> 00:30:44,780 the whale-shaped curve? 581 00:30:44,780 --> 00:30:48,510 But somewhere between 20 and 25,000 volts, you hit a 582 00:30:48,510 --> 00:30:53,070 threshold and now you switch on the characteristic lines. 583 00:30:53,070 --> 00:30:56,170 So with low voltage, you don't get the characteristic lines. 584 00:30:56,170 --> 00:30:58,260 You only get the whale-shaped curve. 585 00:30:58,260 --> 00:31:01,320 And then beyond a certain critical value of V, it's as 586 00:31:01,320 --> 00:31:04,020 though all of a sudden these lines appear. 587 00:31:04,020 --> 00:31:05,650 Low voltage, no lines. 588 00:31:05,650 --> 00:31:08,015 High voltage, you get the characteristic lines. 589 00:31:11,520 --> 00:31:13,960 And what can we do here? 590 00:31:13,960 --> 00:31:19,840 Well, we can compute K alpha, L alpha by Moseley's Law. 591 00:31:24,150 --> 00:31:27,000 The continuous spectrum can't do anything with that, with 592 00:31:27,000 --> 00:31:28,560 one exception-- 593 00:31:28,560 --> 00:31:29,430 here. 594 00:31:29,430 --> 00:31:38,460 So let's look inside and figure out what's going on. 595 00:31:38,460 --> 00:31:42,860 We have a proposal here of what's going on inside that 596 00:31:42,860 --> 00:31:43,800 target atom. 597 00:31:43,800 --> 00:31:44,830 So I'm going to make-- 598 00:31:44,830 --> 00:31:47,460 this could be the molybdenum target up there. 599 00:31:47,460 --> 00:31:53,760 So these are molybdenum atoms. And just as 600 00:31:53,760 --> 00:31:55,520 in Moseley's figure-- 601 00:31:55,520 --> 00:31:56,990 so this is body centered cubic-- 602 00:31:56,990 --> 00:31:59,030 I can look that up on the Periodic Table. 603 00:31:59,030 --> 00:32:02,990 I know this is BCC crystal structure, molybdenum atom 604 00:32:02,990 --> 00:32:03,940 sitting here. 605 00:32:03,940 --> 00:32:04,950 This is the anode. 606 00:32:04,950 --> 00:32:08,230 It's charged positive and way up top, I get the cathode. 607 00:32:08,230 --> 00:32:13,080 So the cathode is shooting off ballistic electrons and they 608 00:32:13,080 --> 00:32:17,510 go zooming across the gap and crash into anode, but up until 609 00:32:17,510 --> 00:32:19,800 now, we've just said the anode is some material. 610 00:32:19,800 --> 00:32:22,580 Now we're going to take one more peel off the onion and 611 00:32:22,580 --> 00:32:26,020 say that these are discrete atoms. It's not continuous 612 00:32:26,020 --> 00:32:27,470 molybdenum. 613 00:32:27,470 --> 00:32:28,530 It's molybdenum atoms. 614 00:32:28,530 --> 00:32:31,440 And what do we know the molybdenum atom looks like? 615 00:32:31,440 --> 00:32:35,030 It's got a dense nucleus where all the positive charge 616 00:32:35,030 --> 00:32:40,460 resides and then there's this almost vacuum-like zone with 617 00:32:40,460 --> 00:32:41,760 the negative charge. 618 00:32:41,760 --> 00:32:45,440 So when the electron comes in, this electron sees the 619 00:32:45,440 --> 00:32:47,560 negative charge around the molybdenum atom. 620 00:32:47,560 --> 00:32:48,850 What happens? 621 00:32:48,850 --> 00:32:50,610 It's deflected. 622 00:32:50,610 --> 00:32:56,360 Maybe it comes in on a closer angle and it's scattered 623 00:32:56,360 --> 00:32:57,790 through a higher angle. 624 00:32:57,790 --> 00:33:00,550 Maybe it comes in almost in between and it 625 00:33:00,550 --> 00:33:02,030 hardly moves at all. 626 00:33:02,030 --> 00:33:05,740 Now can you see that when you have a charged species that 627 00:33:05,740 --> 00:33:09,330 changes direction, that's called an acceleration, and an 628 00:33:09,330 --> 00:33:13,140 acceleration gives rise to an emission of radiation? 629 00:33:13,140 --> 00:33:17,580 So because the angle of deflection-- 630 00:33:17,580 --> 00:33:24,660 so this is low-angle deflection, this is high-angle 631 00:33:24,660 --> 00:33:25,170 deflection. 632 00:33:25,170 --> 00:33:31,020 So low-angle deflection means low energy emission. 633 00:33:31,020 --> 00:33:35,590 High angle means high-energy emission, and the result is 634 00:33:35,590 --> 00:33:40,580 this continue spectrum that I've shown you here. 635 00:33:40,580 --> 00:33:45,800 So this is the result of low-angle scattering of the 636 00:33:45,800 --> 00:33:47,130 ballistic electrons. 637 00:33:47,130 --> 00:33:52,580 This is the result of high-angle scattering of the 638 00:33:52,580 --> 00:33:55,230 ballistic electrons, and somewhere in the middle here 639 00:33:55,230 --> 00:33:58,670 is the dominant angle. 640 00:33:58,670 --> 00:34:01,880 I can't calculate this curve with one exception. 641 00:34:01,880 --> 00:34:06,150 Imagine the electron comes from the cathode, and with all 642 00:34:06,150 --> 00:34:11,750 of its energy is dead on and stops, gives up all of its 643 00:34:11,750 --> 00:34:14,070 kinetic energy to a photon. 644 00:34:14,070 --> 00:34:17,440 That's the maximum amount of energy possible. 645 00:34:17,440 --> 00:34:18,590 Let's look at that. 646 00:34:18,590 --> 00:34:24,420 That's the case where an electron comes in, stops dead 647 00:34:24,420 --> 00:34:28,930 here and then emits the photon. 648 00:34:28,930 --> 00:34:31,370 Sp the kinetic energy is translated 649 00:34:31,370 --> 00:34:34,420 into the photon energy. 650 00:34:34,420 --> 00:34:36,420 So we can do that one. 651 00:34:36,420 --> 00:34:38,150 That's a straightforward calculation. 652 00:34:38,150 --> 00:34:44,280 So the energy of the incident electron-- 653 00:34:44,280 --> 00:34:49,720 E of the incident electron is equal to product of the charge 654 00:34:49,720 --> 00:34:51,950 on the electron and the plate voltage. 655 00:34:51,950 --> 00:34:54,440 Well, the charge on the electron is the elementary 656 00:34:54,440 --> 00:34:57,300 charge and the plate voltage is whatever it is, and I'm 657 00:34:57,300 --> 00:35:04,740 going to equate that with the energy of the emitted photon, 658 00:35:04,740 --> 00:35:08,520 and that's equal to hc over lambda. 659 00:35:08,520 --> 00:35:12,730 So now I can cross multiply and I can call this the lambda 660 00:35:12,730 --> 00:35:16,250 of the shortest wavelength. 661 00:35:16,250 --> 00:35:19,120 That's the shortest wavelength on the whale-shaped curve. 662 00:35:19,120 --> 00:35:22,100 So by algebra, you're going to get the product of the plane 663 00:35:22,100 --> 00:35:24,550 constant times the speed of light divided by the 664 00:35:24,550 --> 00:35:27,930 elementary charge times the plate voltage, which turns out 665 00:35:27,930 --> 00:35:33,140 to be 12,400 divided by plate voltage where the wavelength's 666 00:35:33,140 --> 00:35:36,470 given in angstroms. 667 00:35:36,470 --> 00:35:37,130 Just try it. 668 00:35:37,130 --> 00:35:40,520 If you put 10,000 volts, you're going to get 1.24 669 00:35:40,520 --> 00:35:44,460 angstroms, which is smack dab in the middle of the x-region 670 00:35:44,460 --> 00:35:49,930 of the spectrum, and this is called the Duane-Hunt Law. 671 00:35:49,930 --> 00:35:54,343 That's the only thing we can compute in 672 00:35:54,343 --> 00:35:57,020 the continuous spectrum. 673 00:35:57,020 --> 00:35:58,060 So I can get this one. 674 00:35:58,060 --> 00:36:02,030 Lambda shortest wavelength, because shortest wavelength is 675 00:36:02,030 --> 00:36:05,380 maximum energy. 676 00:36:05,380 --> 00:36:11,660 Now there's a fancier name for this whale-shaped curve, and 677 00:36:11,660 --> 00:36:14,300 it's the scientific community's term, and it's a 678 00:36:14,300 --> 00:36:15,010 German word. 679 00:36:15,010 --> 00:36:16,260 It's called bremsstrahlung. 680 00:36:21,070 --> 00:36:22,320 I love it. 681 00:36:24,360 --> 00:36:25,750 You need to know this. 682 00:36:25,750 --> 00:36:29,350 You can impress your friends at parties. 683 00:36:29,350 --> 00:36:31,060 What does bremsstrahlung mean? 684 00:36:31,060 --> 00:36:35,650 Brems is the German word for brake, as when you put on the 685 00:36:35,650 --> 00:36:37,180 brakes of a car. 686 00:36:37,180 --> 00:36:42,230 And strahl, strahl is the word for ray. 687 00:36:42,230 --> 00:36:44,910 And ung is like ing. 688 00:36:44,910 --> 00:36:47,950 So this is raying radiation and this is the 689 00:36:47,950 --> 00:36:50,250 radiation of braking. 690 00:36:50,250 --> 00:36:53,080 So the electrons are coming in and when they come up against 691 00:36:53,080 --> 00:36:57,450 the negative charge of the outer shell the target atom, 692 00:36:57,450 --> 00:36:59,820 they are slamming on the brakes and skidding 693 00:36:59,820 --> 00:37:00,330 everywhere. 694 00:37:00,330 --> 00:37:04,020 So this is what bremsstrahlung means: braking radiation. 695 00:37:07,940 --> 00:37:08,620 So we're going to call-- 696 00:37:08,620 --> 00:37:10,840 I'm going to put a B here-- 697 00:37:10,840 --> 00:37:11,340 see that? 698 00:37:11,340 --> 00:37:12,170 I was thinking ahead. 699 00:37:12,170 --> 00:37:13,420 B is bremsstrahlung. 700 00:37:15,910 --> 00:37:16,620 OK. 701 00:37:16,620 --> 00:37:20,900 So we've got a lot going here. for us. 702 00:37:20,900 --> 00:37:23,020 I think we've explained a fair bit. 703 00:37:23,020 --> 00:37:25,700 Now I want to talk about modern x-ray tubes. 704 00:37:25,700 --> 00:37:29,030 What do modern x-ray tubes have that these primitive ones 705 00:37:29,030 --> 00:37:33,900 that Moseley worked with and Roentgen 706 00:37:33,900 --> 00:37:36,480 worked with didn't have? 707 00:37:36,480 --> 00:37:40,300 So I want to show you that the modern x-ray tube is the 708 00:37:40,300 --> 00:37:44,890 result of improvements made by an MIT alum. 709 00:37:44,890 --> 00:37:53,880 His name was William Coolidge, and he's the class of '96-- 710 00:37:53,880 --> 00:37:59,490 1896. 711 00:37:59,490 --> 00:38:04,260 And he made a number of improvements. 712 00:38:04,260 --> 00:38:07,680 He actually taught for awhile then eventually spent a good 713 00:38:07,680 --> 00:38:09,940 part of his professional career working as a research 714 00:38:09,940 --> 00:38:13,030 scientist at the General Electric Labs out in 715 00:38:13,030 --> 00:38:13,850 Schenectady. 716 00:38:13,850 --> 00:38:17,470 If you go down to the lobby of Building 6, on the south side 717 00:38:17,470 --> 00:38:19,910 of the lobby, there's a showcase. 718 00:38:19,910 --> 00:38:21,130 Look inside the showcase. 719 00:38:21,130 --> 00:38:27,780 You'll see there's a little display in honor of Coolidge. 720 00:38:27,780 --> 00:38:29,630 So what's the first thing Coolidge did? 721 00:38:29,630 --> 00:38:31,640 He was an engineer so he was thinking about making things 722 00:38:31,640 --> 00:38:32,250 more efficient. 723 00:38:32,250 --> 00:38:35,270 First thing he did is he turned the discharge tube into 724 00:38:35,270 --> 00:38:38,010 a vacuum tube. 725 00:38:38,010 --> 00:38:40,090 Remember, Roentgen worked at low pressure, but there was 726 00:38:40,090 --> 00:38:42,910 still gas, and he was blinded by the light and so on. 727 00:38:42,910 --> 00:38:45,730 This means you don't get any visible light, and secondly, 728 00:38:45,730 --> 00:38:50,650 it's more efficient because if you've got the tube like this 729 00:38:50,650 --> 00:38:53,510 with the two electrodes, the feedthroughs and you've got 730 00:38:53,510 --> 00:38:56,190 gas inside, some of the electrons are crashing into 731 00:38:56,190 --> 00:38:58,310 the gas molecules, and we don't care 732 00:38:58,310 --> 00:38:59,290 about the gas molecules. 733 00:38:59,290 --> 00:39:03,130 We want to get the electrons crashing into the target. 734 00:39:03,130 --> 00:39:05,575 So by going to vacuum, this improves the efficiency. 735 00:39:09,590 --> 00:39:13,485 No glow in the visible and higher energy efficiency. 736 00:39:18,770 --> 00:39:22,930 More x-rays out per unit power put in. 737 00:39:22,930 --> 00:39:24,410 What's the second thing he did? 738 00:39:24,410 --> 00:39:26,887 Second think he did was hot cathode. 739 00:39:30,590 --> 00:39:32,610 Remember, you're trying to rip the electrons out of the 740 00:39:32,610 --> 00:39:34,690 cathode and send them on their journey. 741 00:39:34,690 --> 00:39:37,670 So Coolidge reasoned that if you heated the cathode, you'd 742 00:39:37,670 --> 00:39:41,110 weaken the bonds, and the electrons would come off. 743 00:39:41,110 --> 00:39:44,780 They'd boil off much more readily, OK? 744 00:39:44,780 --> 00:39:54,630 Raise temperature to reduce bond energy, to reduce binding 745 00:39:54,630 --> 00:39:58,284 energy of the electrons. 746 00:39:58,284 --> 00:40:04,450 The electrons in the cathode makes them easier to boil off. 747 00:40:04,450 --> 00:40:06,200 Think I've got a cartoon of that. 748 00:40:06,200 --> 00:40:07,310 Yeah, here it is. 749 00:40:07,310 --> 00:40:09,490 So here's the tube lying on its side. 750 00:40:09,490 --> 00:40:11,300 So the cathode is over here to the left. 751 00:40:11,300 --> 00:40:12,120 It's negative. 752 00:40:12,120 --> 00:40:13,430 Here's the anode to the right. 753 00:40:13,430 --> 00:40:16,770 That's this purple thing here and the electrons are moving 754 00:40:16,770 --> 00:40:18,590 from left to right and so he's got-- 755 00:40:18,590 --> 00:40:21,090 see, you can have multiple electrical signals going 756 00:40:21,090 --> 00:40:23,070 through the same conductor. 757 00:40:23,070 --> 00:40:23,750 It's not the same. 758 00:40:23,750 --> 00:40:25,860 You can't have an AC waveform and a DC 759 00:40:25,860 --> 00:40:27,410 waveform in the same conductor. 760 00:40:27,410 --> 00:40:30,950 So you've got a big DC voltage between the cathode and the 761 00:40:30,950 --> 00:40:34,440 anode and you'd got a separate little circuit going through 762 00:40:34,440 --> 00:40:37,990 the cathode, running almost like a toaster to 763 00:40:37,990 --> 00:40:39,790 make it super hot. 764 00:40:39,790 --> 00:40:43,860 So the potential between here and here is 35,000 volts. 765 00:40:43,860 --> 00:40:46,860 The potential along this stretch of real estate might 766 00:40:46,860 --> 00:40:49,250 be several hundred volts, and furthermore, this 767 00:40:49,250 --> 00:40:52,200 could be an AC signal. 768 00:40:52,200 --> 00:40:52,720 It's Coolidge. 769 00:40:52,720 --> 00:40:53,940 He was smart. 770 00:40:53,940 --> 00:40:57,530 So this makes this hot and now for per given voltage, you get 771 00:40:57,530 --> 00:41:00,610 much more yield of the electrons. 772 00:41:00,610 --> 00:41:01,810 So that was pretty good. 773 00:41:01,810 --> 00:41:03,610 I like that. 774 00:41:03,610 --> 00:41:05,210 Smart. 775 00:41:05,210 --> 00:41:10,160 Third thing he did was he heated the cathode and he 776 00:41:10,160 --> 00:41:11,720 cooled the anode. 777 00:41:11,720 --> 00:41:12,970 Water-cooled anode. 778 00:41:18,190 --> 00:41:18,675 Why? 779 00:41:18,675 --> 00:41:20,280 You got to dissipate the heat. 780 00:41:20,280 --> 00:41:22,830 All these electrons crashing into the anode. 781 00:41:22,830 --> 00:41:25,250 You raise the temperature of that anode so high you'll melt 782 00:41:25,250 --> 00:41:29,010 it and so they had to run the tubes intermittently. 783 00:41:29,010 --> 00:41:31,480 They just get a decent signal. 784 00:41:31,480 --> 00:41:33,400 You have to take these x-ray measurements over a long 785 00:41:33,400 --> 00:41:35,510 period of time because the amount of x-rays you get is 786 00:41:35,510 --> 00:41:37,680 small, but you have to keep shutting the thing down. 787 00:41:37,680 --> 00:41:39,260 Otherwise, you melt through the anode. 788 00:41:39,260 --> 00:41:42,840 So he put that on a water-cooled copper hearth and 789 00:41:42,840 --> 00:41:44,990 was able to run continuous. 790 00:41:44,990 --> 00:41:46,240 Continuous operation. 791 00:41:50,400 --> 00:41:51,650 No more pulse current. 792 00:41:51,650 --> 00:41:53,870 But sometimes when nature hands you a 793 00:41:53,870 --> 00:41:55,450 lemon, you make lemonade. 794 00:41:55,450 --> 00:41:56,730 So I'm going to turn this thing around. 795 00:41:56,730 --> 00:41:57,790 I'll say, hey, wait a minute. 796 00:41:57,790 --> 00:42:00,070 I don't want any water-cooled copper anode. 797 00:42:00,070 --> 00:42:02,300 Suppose I want to weld some titanium. 798 00:42:02,300 --> 00:42:06,240 Titanium melts at 1675 degrees Centigrade and it's got a 799 00:42:06,240 --> 00:42:08,190 voracious appetite for oxygen. 800 00:42:08,190 --> 00:42:10,270 Well, this thing's a vacuum tube. 801 00:42:10,270 --> 00:42:12,950 So what if I were to take my part and I put the two pieces 802 00:42:12,950 --> 00:42:16,940 of titanium in the path of an electron beam and I don't cool 803 00:42:16,940 --> 00:42:17,710 the titanium? 804 00:42:17,710 --> 00:42:20,320 Eventually, I get the temperature so high that I can 805 00:42:20,320 --> 00:42:22,010 weld titanium. 806 00:42:22,010 --> 00:42:25,060 This is the birth of electron beam welding. 807 00:42:25,060 --> 00:42:27,130 And that's how you weld refracting metals. 808 00:42:27,130 --> 00:42:30,260 You get one of those titanium bicycle frames 809 00:42:30,260 --> 00:42:32,210 that cost about $4,000. 810 00:42:32,210 --> 00:42:34,030 It might be tungsten inert gas. 811 00:42:34,030 --> 00:42:36,910 It might be electron beam, depending. 812 00:42:36,910 --> 00:42:39,430 So that's the flip side of the technology. 813 00:42:39,430 --> 00:42:44,830 By the way, if you were in attendance observing 814 00:42:44,830 --> 00:42:47,450 electronic beam welding, what do you think is being 815 00:42:47,450 --> 00:42:51,460 generated in the electron beam welding apparatus? 816 00:42:51,460 --> 00:42:56,320 X-rays, by the boatload 817 00:42:56,320 --> 00:42:59,240 So there's a fourth thing-- maybe the most important thing 818 00:42:59,240 --> 00:43:01,660 that Coolidge came up with-- shielding. 819 00:43:01,660 --> 00:43:03,050 You see the yellow here? 820 00:43:03,050 --> 00:43:04,350 That's lead shielding. 821 00:43:14,840 --> 00:43:18,200 Why did he choose lead? 822 00:43:18,200 --> 00:43:24,620 Well, it's got a very high z. 823 00:43:24,620 --> 00:43:26,900 That means it's got many energy levels. 824 00:43:31,820 --> 00:43:35,300 So that if you start looking at the energy level diagram of 825 00:43:35,300 --> 00:43:41,640 lead, you've got lots of action up in here. 826 00:43:41,640 --> 00:43:42,930 So what can happen? 827 00:43:42,930 --> 00:43:50,470 When the x-rays come with their high energy, the x-rays 828 00:43:50,470 --> 00:43:54,720 from the tube on their way out to get you, and everybody 829 00:43:54,720 --> 00:43:57,160 standing observing this marvel. 830 00:43:57,160 --> 00:43:59,950 They excite electrons inside. 831 00:43:59,950 --> 00:44:02,970 So these x-rays are absorbed. 832 00:44:02,970 --> 00:44:07,410 The electrons inside the lead rise, cascade down and now 833 00:44:07,410 --> 00:44:08,380 they come out. 834 00:44:08,380 --> 00:44:10,240 And what's the difference in energy between 835 00:44:10,240 --> 00:44:12,590 the x-ray and these? 836 00:44:12,590 --> 00:44:16,250 The ones up here are much lower energy. 837 00:44:16,250 --> 00:44:18,870 So this is, if you like, a frequency shifter 838 00:44:18,870 --> 00:44:20,260 or an energy shifter. 839 00:44:20,260 --> 00:44:23,220 So now we're using photons to excite 840 00:44:23,220 --> 00:44:25,650 electrons to generate photons. 841 00:44:25,650 --> 00:44:30,000 And that's how the shielding works. 842 00:44:30,000 --> 00:44:32,570 So you want to absorb and re-emit. 843 00:44:38,720 --> 00:44:41,690 And while we're on the topic, he gave us lead shielding and 844 00:44:41,690 --> 00:44:44,030 he gave us beryllium windows because they were using 845 00:44:44,030 --> 00:44:47,690 silicate glass, just as you use in your home, but the 846 00:44:47,690 --> 00:44:50,930 silicate glass was absorbing some of the x-rays. 847 00:44:50,930 --> 00:44:53,440 So to get higher efficiency, he chose beryllium. 848 00:44:53,440 --> 00:44:54,820 Why did he choose beryllium? 849 00:44:54,820 --> 00:44:58,350 Well, it's got a low z. 850 00:44:58,350 --> 00:45:01,840 So that means it has few energy levels. 851 00:45:06,640 --> 00:45:10,880 So therefore, there's less absorption and re-emission-- 852 00:45:10,880 --> 00:45:12,130 again, higher efficiency. 853 00:45:18,050 --> 00:45:20,400 I think this was taken from one of the other readings. 854 00:45:20,400 --> 00:45:21,700 So they flipped it around. 855 00:45:21,700 --> 00:45:26,250 See, here you can see the cathode here, the anode. 856 00:45:26,250 --> 00:45:28,390 That's one of these things. 857 00:45:28,390 --> 00:45:28,720 All right. 858 00:45:28,720 --> 00:45:29,820 So the window is up here. 859 00:45:29,820 --> 00:45:33,860 You don't use lithium because lithium is unstable in the air 860 00:45:33,860 --> 00:45:38,950 unless you had a giant room of humidity set at a dew point of 861 00:45:38,950 --> 00:45:40,560 minus 38 degrees, see, and then you can 862 00:45:40,560 --> 00:45:42,090 use a lithium window. 863 00:45:42,090 --> 00:45:44,580 Otherwise, you use beryllium and lead down here. 864 00:45:44,580 --> 00:45:45,600 Why'd he choose lead? 865 00:45:45,600 --> 00:45:48,690 Well, you're pretty much at the bottom of the naturally 866 00:45:48,690 --> 00:45:50,390 occurring elements in the Periodic Table. 867 00:45:50,390 --> 00:45:52,340 So it's the cheapest of all of these heavies. 868 00:45:54,940 --> 00:45:55,410 All right. 869 00:45:55,410 --> 00:45:58,240 So now let's take a few minutes and talk about the use 870 00:45:58,240 --> 00:46:02,820 of x-rays in characterizing art. 871 00:46:02,820 --> 00:46:06,570 So this painting here at once time was arguably one of the 872 00:46:06,570 --> 00:46:08,950 most recognizable paintings in the western world. 873 00:46:08,950 --> 00:46:11,270 It's The Angelus by Jean-Francois Millet. 874 00:46:11,270 --> 00:46:16,430 It was painted at 1857 to 1859 on commission for an insurance 875 00:46:16,430 --> 00:46:18,870 company here in Boston. 876 00:46:18,870 --> 00:46:24,490 The Boston Brahmins loved BA's paintings of rural life in 877 00:46:24,490 --> 00:46:26,800 19th century France. 878 00:46:26,800 --> 00:46:31,680 And this is couple of peasants who are giving thanks to God. 879 00:46:31,680 --> 00:46:35,160 The Angelus is a prayer, and they're thanking God for this 880 00:46:35,160 --> 00:46:38,790 pitiful bounty of potatoes evident. 881 00:46:38,790 --> 00:46:42,300 Salvador Dali, as an art student, was required to paint 882 00:46:42,300 --> 00:46:43,600 this as part of training. 883 00:46:43,600 --> 00:46:47,340 You know how when you learn to play the piano, you reproduce 884 00:46:47,340 --> 00:46:48,660 the works of the Grand Masters. 885 00:46:48,660 --> 00:46:50,390 So when you go to art school, you have to paint. 886 00:46:50,390 --> 00:46:53,170 He hated this painting. 887 00:46:53,170 --> 00:46:55,460 For many years, it was here in Boston, and then it was 888 00:46:55,460 --> 00:46:59,160 repatriated around World War I, it hung in the Louvre, and 889 00:46:59,160 --> 00:47:01,305 then ultimately, now it's in the Musee d'Orsay. 890 00:47:01,305 --> 00:47:04,880 In 1963 when Dali was at the peak of his career, he asked 891 00:47:04,880 --> 00:47:07,060 the curator of the Louvre if she would have 892 00:47:07,060 --> 00:47:08,460 this painting x-rayed. 893 00:47:08,460 --> 00:47:10,160 He said this painting spooks him. 894 00:47:10,160 --> 00:47:12,480 He doesn't like this painting. 895 00:47:12,480 --> 00:47:13,820 And they x-rayed the painting. 896 00:47:13,820 --> 00:47:14,600 What did they find? 897 00:47:14,600 --> 00:47:17,640 They found that it had been painted over down here. 898 00:47:17,640 --> 00:47:21,870 Wasn't an art forgery, Millet himself had painted it over. 899 00:47:21,870 --> 00:47:24,630 You may have heard two years ago there was this art find. 900 00:47:24,630 --> 00:47:26,560 It was a van Gogh painting. 901 00:47:26,560 --> 00:47:28,930 They found that van Gogh had painted over one of his own 902 00:47:28,930 --> 00:47:31,770 paintings that had never been seen before even though the 903 00:47:31,770 --> 00:47:34,370 pencil sketches survived, so they assumed that maybe the 904 00:47:34,370 --> 00:47:36,290 painting was destroyed in World War II 905 00:47:36,290 --> 00:47:37,160 or some such thing. 906 00:47:37,160 --> 00:47:41,070 It turns out that van Gogh was so poor at one point that he 907 00:47:41,070 --> 00:47:44,580 valued the canvas over the art, and he took his previous 908 00:47:44,580 --> 00:47:46,570 painting, and he painted over it. 909 00:47:46,570 --> 00:47:46,830 OK. 910 00:47:46,830 --> 00:47:48,140 So this was going on here. 911 00:47:48,140 --> 00:47:49,420 This is the truth. 912 00:47:49,420 --> 00:47:51,620 Now my speculation begins. 913 00:47:51,620 --> 00:47:56,800 So what was underneath here initially? 914 00:47:56,800 --> 00:48:01,230 It was the casket of a baby. 915 00:48:01,230 --> 00:48:02,290 Look at the pose. 916 00:48:02,290 --> 00:48:05,140 These people don't look like they're happy with their 917 00:48:05,140 --> 00:48:08,620 bounty of harvest. They're grieving. 918 00:48:08,620 --> 00:48:11,200 This was this futility of peasant life. 919 00:48:11,200 --> 00:48:14,530 It was a hard life and they lost their baby. 920 00:48:14,530 --> 00:48:18,000 So imagine Millet paints this, puts it on a boat, it comes 921 00:48:18,000 --> 00:48:21,820 over to Boston, they unveil it, and the directors of the 922 00:48:21,820 --> 00:48:25,090 insurance company go, we can't hang this in the lobby of an 923 00:48:25,090 --> 00:48:28,010 insurance company. 924 00:48:28,010 --> 00:48:29,550 That's my speculation. 925 00:48:29,550 --> 00:48:31,990 It didn't take him three years to paint this thing. 926 00:48:31,990 --> 00:48:33,920 I think it spent most of its time on the boat. 927 00:48:33,920 --> 00:48:35,680 They sent it back and said fix it. 928 00:48:35,680 --> 00:48:38,080 So he put the basket of potatoes. 929 00:48:38,080 --> 00:48:39,200 That's my theory. 930 00:48:39,200 --> 00:48:40,490 All right, look at the pose. 931 00:48:40,490 --> 00:48:43,990 This is from the Galleria Borghese in Rome. 932 00:48:43,990 --> 00:48:46,810 I was there about two years ago and saw this and went, 933 00:48:46,810 --> 00:48:48,010 wow, referential. 934 00:48:48,010 --> 00:48:51,460 You see, everything's been said. 935 00:48:51,460 --> 00:48:52,720 All of art has been said. 936 00:48:52,720 --> 00:48:55,940 Now we're just recasting it. 937 00:48:55,940 --> 00:48:57,560 So now here's Dali's revenge. 938 00:48:57,560 --> 00:49:00,160 Now this is what he paints. 939 00:49:00,160 --> 00:49:02,090 You see the man is shorter than the woman. 940 00:49:02,090 --> 00:49:05,070 His hat is down a little bit below the waistline. 941 00:49:05,070 --> 00:49:07,540 There's all sorts of psychosexual things going on 942 00:49:07,540 --> 00:49:09,020 here, but we're running out of time. 943 00:49:09,020 --> 00:49:13,790 So here's Dali and his father and his dad is saying, see, 944 00:49:13,790 --> 00:49:15,360 this is life, et cetera, et cetera. 945 00:49:15,360 --> 00:49:17,120 So you can see the reference. 946 00:49:17,120 --> 00:49:20,810 See, man taller here, woman taller, et cetera, et cetera. 947 00:49:20,810 --> 00:49:21,640 Have you seen this one? 948 00:49:21,640 --> 00:49:24,000 The Hallucinogenic Toreador. 949 00:49:24,000 --> 00:49:27,690 OK, he went outside one day in Manhattan to buy some pencils. 950 00:49:27,690 --> 00:49:30,870 There was a company called the Venus Pencil Company, you see. 951 00:49:30,870 --> 00:49:33,220 And so he came back and made this trompe l'oeil. 952 00:49:33,220 --> 00:49:34,360 So this is the toreador. 953 00:49:34,360 --> 00:49:38,900 Do you see the breast here is the nose? 954 00:49:38,900 --> 00:49:40,390 There is the face. 955 00:49:40,390 --> 00:49:42,940 There are the flies of the Thames and here is 956 00:49:42,940 --> 00:49:44,545 the cape and so on. 957 00:49:48,960 --> 00:49:51,860 Symmetry plane-- all of those Venuses facing back, these 958 00:49:51,860 --> 00:49:54,690 Venuses facing forward. 959 00:49:54,690 --> 00:49:55,830 There's the bull. 960 00:49:55,830 --> 00:49:58,910 The life force of the bull is a crystal. 961 00:49:58,910 --> 00:50:00,970 What is the crystal structure? 962 00:50:00,970 --> 00:50:03,560 It's one of the 14 Bravais lattices. 963 00:50:03,560 --> 00:50:07,230 If you ignore color, it's simple cubic, isn't it? 964 00:50:07,230 --> 00:50:09,080 And if you don't believe me, check. 965 00:50:09,080 --> 00:50:11,310 And if you don't appreciate it, study. 966 00:50:11,310 --> 00:50:14,360 Here's the symmetry plane. 967 00:50:14,360 --> 00:50:17,770 Atoms, atoms, fly, fly et cetera, et cetera. 968 00:50:17,770 --> 00:50:18,630 What else do we have here? 969 00:50:18,630 --> 00:50:22,900 Oh, this is the one he painted on the occasion of the 970 00:50:22,900 --> 00:50:24,610 revelation of the structure of DNA. 971 00:50:24,610 --> 00:50:25,990 It's hard to see, but over here is the 972 00:50:25,990 --> 00:50:28,230 double helix of DNA. 973 00:50:28,230 --> 00:50:35,770 This is his wife Gaia and here are people standing in cubic 974 00:50:35,770 --> 00:50:37,600 arrays with guns pointing at each other. 975 00:50:37,600 --> 00:50:40,540 And the point that he's making is that now that we've 976 00:50:40,540 --> 00:50:47,420 discovered the instruction set for reproduction of life, not 977 00:50:47,420 --> 00:50:51,160 just human life, but life, we are still at a point where we 978 00:50:51,160 --> 00:50:52,770 can't resist killing each other. 979 00:50:52,770 --> 00:50:56,820 So this was the point and he chose to use cubic arrays, and 980 00:50:56,820 --> 00:50:59,090 I would venture to say this is simple cubic. 981 00:50:59,090 --> 00:51:02,060 If you put a fifth person here, it would be-- are you 982 00:51:02,060 --> 00:51:02,570 ready for this? 983 00:51:02,570 --> 00:51:03,820 Body-centered cubic. 984 00:51:07,320 --> 00:51:08,750 Oh, yes! 985 00:51:08,750 --> 00:51:11,260 And there's more, but I think we're running out of time so I 986 00:51:11,260 --> 00:51:14,610 think at this point we will dismiss the class.