1 00:00:00,350 --> 00:00:05,643 Well the quad electric effect clearly suggest that, that the light sometimes can 2 00:00:05,643 --> 00:00:10,778 behave as a beam of particles, the other players in the effect at the electrons 3 00:00:10,778 --> 00:00:16,107 will not really suspect it to represent anything but particle like entities. 4 00:00:16,108 --> 00:00:20,280 However this lecture picture was two questions by number of theories back in 5 00:00:20,280 --> 00:00:26,161 the beginning of the last century. Another by Prince Louis de Broglie, who 6 00:00:26,161 --> 00:00:31,677 suggest that massive particles can sometimes behave as waves. 7 00:00:31,678 --> 00:00:36,415 He was also bit often thinking that the photon also has a mass, but from apart 8 00:00:36,415 --> 00:00:41,515 from this idea that didn't quite work out his other hypothesis turned out to be 9 00:00:41,515 --> 00:00:45,762 correct. But it was actually serendipity that plays 10 00:00:45,762 --> 00:00:50,082 such an important role in Physics sometimes, that produce the clearest 11 00:00:50,082 --> 00:00:55,266 experimental results that confirmed de Broglie's hypothesis and put a nail in the 12 00:00:55,266 --> 00:00:59,314 coffin of classical Physics. So let me tell you about his experiment 13 00:00:59,314 --> 00:01:01,789 and actually a rather curious story behind it. 14 00:01:03,700 --> 00:01:09,358 In April of 1925, these two gentlemen Clinton Davisson and Lester Germer, were 15 00:01:09,358 --> 00:01:15,180 working in the physics lab at the Bell Laboratories and they were studying how 16 00:01:15,180 --> 00:01:20,875 electrons skitter off of a nuclear target. They probably didn't expect any 17 00:01:20,875 --> 00:01:26,205 groundbreaking results when they started this experiment and their initial results, 18 00:01:26,205 --> 00:01:30,820 as a matter of fact, were consistent with their sort of boring expectation of a 19 00:01:30,820 --> 00:01:34,079 diffusive scattering of electrons off of the target. 20 00:01:35,720 --> 00:01:41,542 So here's the diagram of the paper which shows the from the original paper they 21 00:01:41,542 --> 00:01:46,380 eventually published in 1927 which shows the apparatus they used. 22 00:01:46,380 --> 00:01:51,271 So here we have the, this part which is a gun, electron gun which shoots electron, 23 00:01:51,271 --> 00:01:55,943 electrons towards the target and so this part is a nickel target they were 24 00:01:55,943 --> 00:02:00,519 studying. So another part of the apparatus is this 25 00:02:00,520 --> 00:02:06,844 dectector which detects electrons and they can move it around and by doing so they 26 00:02:06,844 --> 00:02:11,894 can study the angular distribution of the scattered electrons. 27 00:02:11,894 --> 00:02:16,850 However, what this diagram on the paper don't discuss and don't show is the full 28 00:02:16,850 --> 00:02:21,214 history of the event. Namely, the diagram doesn't show a bottle 29 00:02:21,214 --> 00:02:27,079 of liquid air that was sitting nearby and which exploded due to the heat coming from 30 00:02:27,079 --> 00:02:31,798 this target presumably. And so, this error and this smashed the 31 00:02:31,798 --> 00:02:37,407 vacuum chambers surrounding this apparatus and the air rushed into the chamber, 32 00:02:37,407 --> 00:02:40,469 oxiding him and oxidizing the nickel target. 33 00:02:41,970 --> 00:02:49,000 So this was unpleasant by itself, perhaps but it also made the target they were 34 00:02:49,000 --> 00:02:55,745 using completely useless for the experiments, because it was completely 35 00:02:55,745 --> 00:03:00,799 oxidized. So Davisson and Germer decided to save the 36 00:03:00,799 --> 00:03:05,994 target by heating it up and by nailing it to get rid of the oxide. 37 00:03:05,994 --> 00:03:09,991 So, they did so and put it back into the chamber. 38 00:03:09,992 --> 00:03:14,754 And so, they repeated their experiment the new target, well the old target but which 39 00:03:14,754 --> 00:03:19,467 was should be nailed. But to their amazement, what they saw the 40 00:03:19,467 --> 00:03:24,224 picture of scattered electrons that they saw was completely different from the 41 00:03:24,224 --> 00:03:29,020 picture they had seen before the accident. But why did the results change? 42 00:03:29,020 --> 00:03:32,631 What was different? In trying, in trying to resolve this 43 00:03:32,631 --> 00:03:38,267 mystery data sent in Germer examine the nickel target with a little closer. 44 00:03:38,267 --> 00:03:44,280 And concluded that heating it up and nailing it resulted in this transformation 45 00:03:44,280 --> 00:03:50,598 from a fully crystalline form which basically implies a number of small, small 46 00:03:50,598 --> 00:03:56,384 crystals put together in random fashion. To a few single crystals with a perfect 47 00:03:56,384 --> 00:04:01,033 crystalline order inside. So and these eventually explained the new 48 00:04:01,033 --> 00:04:04,022 results. But what are those new results? 49 00:04:04,022 --> 00:04:08,672 So instead of random blob of scattered electrons, they now saw clear peaks 50 00:04:08,672 --> 00:04:14,147 corresponding to the electrons beams reflected from the crystal at certain 51 00:04:14,147 --> 00:04:17,428 specific angles. And this [inaudible] were strongly 52 00:04:17,428 --> 00:04:20,140 dependent on the energy of incoming electrics. 53 00:04:20,140 --> 00:04:25,954 So, this lot here is actually the plot from the original paper by Davisson and 54 00:04:25,954 --> 00:04:31,515 Gerner, and it shows circles scattering curves for what they call the 54 wall and 55 00:04:31,515 --> 00:04:36,646 65 wall electron beam. That was not once referred to the energy 56 00:04:36,646 --> 00:04:40,282 of electrons. Perhaps a more illuminating modern picture 57 00:04:40,282 --> 00:04:44,940 of the same phenomenon they discovered is of course, a different experiment but 58 00:04:44,940 --> 00:04:49,258 maybe different material, but the same phenomenon, now known as electron 59 00:04:49,258 --> 00:04:53,292 diffraction is presented here. And so what is remarkable here is that 60 00:04:53,292 --> 00:04:56,954 this found phenomenon is something that only waves can exhibit. 61 00:04:56,955 --> 00:04:59,740 Upon propagation through irregular ray of scattering. 62 00:04:59,740 --> 00:05:06,865 So these bright spots here correspond to the directions along which the intensity 63 00:05:06,865 --> 00:05:13,376 of the scattered electron beam is high and so this phenomenon is known. 64 00:05:13,376 --> 00:05:18,688 So here is a reminder or of this picture so if we sent away towards the crystal, 65 00:05:18,688 --> 00:05:24,000 depending on the handle the reflective waves from different layers they can 66 00:05:24,000 --> 00:05:29,478 either enhance each other if they appear in phase or they can appear out of phase 67 00:05:29,478 --> 00:05:35,205 in which case they will cancel each other out, and this will correspond sort of dark 68 00:05:35,205 --> 00:05:40,716 regions in this block. And well, this had been known before 69 00:05:40,716 --> 00:05:46,374 Davisson and Germer's experiment, and they readily recognized it, and found that 70 00:05:46,374 --> 00:05:51,647 their data made perfect sense if the liberal hypothesis were to be accepted. 71 00:05:51,648 --> 00:05:56,484 And so, they actually, say in their paper, explicitly [inaudible] here's a quote from 72 00:05:56,484 --> 00:06:00,370 the paper, the most striking characteristic of these beams Is a one to 73 00:06:00,370 --> 00:06:05,127 one correspondence, which the strongest of them bear to the beams that would be found 74 00:06:05,127 --> 00:06:09,180 issuing from the same crystal if the incident beam were a beam of x-rays. 75 00:06:09,181 --> 00:06:13,192 So they're saying if they were to scatter x-rays over the same crystals, they would 76 00:06:13,192 --> 00:06:17,984 show the same picture. And finally, they say this so their data 77 00:06:17,984 --> 00:06:22,969 made sense, if, if it involves association of a wavelength. 78 00:06:22,970 --> 00:06:28,474 With the incident electron beam and this wave length turns out to be in acceptable 79 00:06:28,474 --> 00:06:33,544 driven with the value of h over nb, h being the blanks actions constant divided 80 00:06:33,544 --> 00:06:38,027 by the momentum of the electron. So this was a rather remarkable 81 00:06:38,028 --> 00:06:43,900 verification of Prince de Broglie idea and the main equation, which again is this 82 00:06:43,900 --> 00:06:49,462 equation, which relates. The properties of a particle with a mass 83 00:06:49,462 --> 00:06:55,510 moving with a certain velocity to a wave that would be associated with this with 84 00:06:55,510 --> 00:07:00,824 this particle. And these ideas brought him a Nobel Prize 85 00:07:00,824 --> 00:07:05,986 just for years later. So to summarize, let me just tell you what 86 00:07:05,986 --> 00:07:08,750 we've learned so far in the last two segments. 87 00:07:08,750 --> 00:07:14,637 So far in the first lecture, we have talked about two Nobel Prize winning works 88 00:07:14,638 --> 00:07:19,700 the photoelectric effect and the electron diffraction, and all in all we met in 89 00:07:19,700 --> 00:07:24,650 passing at least at least six Nobel laureates in our first lecture Feynman, 90 00:07:24,650 --> 00:07:29,148 Michelson, Lenard, Einstein, Davisson, de Broglie, so not too bad. 91 00:07:29,148 --> 00:07:35,042 And we have seen, and these are basically the main sort of conclusions from the two 92 00:07:35,042 --> 00:07:38,925 experiments we just discussed. We have seen that there is a clear 93 00:07:38,925 --> 00:07:43,540 experimental evidence that light behaves sometimes as a beam of particles carrying 94 00:07:43,540 --> 00:07:46,800 energy quanta. And the frequency relates to the energy as 95 00:07:46,800 --> 00:07:50,918 so, where the wavelength of light relates to the energy according to this expression 96 00:07:50,918 --> 00:07:54,290 where the coefficient of proportionality is the Planck constant. 97 00:07:54,290 --> 00:07:59,879 So here, I mean, I see the frequency of light C is the speed of light and lambda 98 00:07:59,879 --> 00:08:02,921 is the wavelength. So on the other hand there is also 99 00:08:02,921 --> 00:08:06,854 evidence that the electrons message particles, sometimes behave as waves. 100 00:08:06,855 --> 00:08:10,780 And the relation we just discussed with these liberal relations. 101 00:08:10,780 --> 00:08:14,824 And in the next lecture, we'll try to figure out what is going on here.