1 00:00:00,690 --> 00:00:03,938 Welcome back everyone. I'm Charles Clark. 2 00:00:03,938 --> 00:00:08,789 And in the second art of this lecture, we're going to look at how these atomic 3 00:00:08,789 --> 00:00:14,961 spectra are acquired and what the sort of instrumentation used. 4 00:00:14,961 --> 00:00:19,697 And then the implications of that understand the observations and for other 5 00:00:19,697 --> 00:00:25,881 future experiments on quantum systems. Here's a nice simple picture, peaceful 6 00:00:25,881 --> 00:00:30,610 system, something I'm sure you've seen many times before. 7 00:00:30,610 --> 00:00:34,027 This I think is a very nice illustration due to Paul Doherty of the Exploratorium 8 00:00:34,027 --> 00:00:39,002 in San Francisco. it shows that evidently a pond, I think 9 00:00:39,002 --> 00:00:46,092 those are fish swimming about it. And you see two sort of distinctive wave, 10 00:00:46,092 --> 00:00:50,808 wave structures. One of which is, let's say, around here 11 00:00:50,808 --> 00:00:55,630 centered presumably at that point. Then there's another one centered around 12 00:00:55,630 --> 00:00:59,096 here. So, I want to note several things about 13 00:00:59,096 --> 00:01:02,876 this picture that are, that are evident to anyone who has looked at a wave 14 00:01:02,876 --> 00:01:07,861 pattern of this type. First of all,you see a nice regular 15 00:01:07,861 --> 00:01:12,533 motion here since it's still water and maybe some object was dropped here and 16 00:01:12,533 --> 00:01:19,195 another one dropped a different time there, and the waves spread out. 17 00:01:19,195 --> 00:01:23,955 And when they overlap, you, so they show these regular curve, regular wave fronts 18 00:01:23,955 --> 00:01:28,597 when they're isolated. When the wave motion is mixed, you see a 19 00:01:28,597 --> 00:01:33,744 more complex pattern. This is the phenom on of interference. 20 00:01:33,744 --> 00:01:38,368 But significantly, when the waves have passed through each other, they continue 21 00:01:38,368 --> 00:01:44,660 on as if they had not encountered any other anything other than clear water. 22 00:01:44,660 --> 00:01:49,779 So, this is, the, the, the problem. The, the phenomenon of interference is 23 00:01:49,779 --> 00:01:52,980 also related to the phenomenon of linearity. 24 00:01:52,980 --> 00:01:57,579 In that, the presence of a second wave doesn't really disturb the, the other 25 00:01:57,579 --> 00:02:02,090 wave. It's only that those two waves, for a 26 00:02:02,090 --> 00:02:08,616 while, operate together and interfere. Now, it must have been an observation 27 00:02:08,616 --> 00:02:14,192 like this that motivated Thomas Young to build a device that is one of the one of 28 00:02:14,192 --> 00:02:22,410 the most ingenious simple experimental devices every made, in my opinion. 29 00:02:27,590 --> 00:02:30,730 So, here's a schematic of the operation of Young's device. 30 00:02:30,730 --> 00:02:33,980 It's, it's called today a double slit interferometer, in fact, it was a double 31 00:02:33,980 --> 00:02:40,391 hole interferometer. He just took a thin piece of paper and 32 00:02:40,391 --> 00:02:49,210 put two holes in it, at a a short distance of separation. 33 00:02:49,210 --> 00:02:54,345 And so, the idea is at some light, light incident on the holes would produce the 34 00:02:54,345 --> 00:02:58,686 following effect. And let me say, first of all, that at the 35 00:02:58,686 --> 00:03:02,025 time that Young did this, there was considerable debate as to whether light 36 00:03:02,025 --> 00:03:08,275 was a wave motion or a type of particle. And by constructing this device, Young 37 00:03:08,275 --> 00:03:12,230 demonstrates quite conclusively that there was a wave. 38 00:03:12,230 --> 00:03:15,940 And he did something even more important which was to actually be able to measure 39 00:03:15,940 --> 00:03:20,646 the wavelength using this device. We will show how that works in a moment. 40 00:03:20,646 --> 00:03:24,560 But basically the idea is, it's like that ripple pattern in the pond. 41 00:03:24,560 --> 00:03:30,908 You create two independent sources of waves which will interfere and if you get 42 00:03:30,908 --> 00:03:36,980 far out away from the, away from the the plane where the, where the holes are, 43 00:03:36,980 --> 00:03:44,490 you'll see regular alternation of the, the light. 44 00:03:44,490 --> 00:03:48,550 So, here's the schematic of the Young, it's just a reduced size. 45 00:03:48,550 --> 00:03:52,581 You have these two holes. We'll say they're separated by a value of 46 00:03:52,581 --> 00:03:54,579 d. This is the separation. 47 00:03:57,690 --> 00:03:59,096 [SOUND]. That's something you can measure if 48 00:03:59,096 --> 00:04:03,942 you're making it yourself. And then if you the light comes out in 49 00:04:03,942 --> 00:04:09,150 all directions but it's in those directions where the, let's say, the path 50 00:04:09,150 --> 00:04:14,274 difference between two rays, the ray coming from one, one hole and the ray 51 00:04:14,274 --> 00:04:21,910 coming from the other. The path difference which is indicated 52 00:04:21,910 --> 00:04:27,418 here, which I'll, designated by delta, well, that path difference for this angle 53 00:04:27,418 --> 00:04:34,800 theta as indicated here that path difference is just d sine theta. 54 00:04:34,800 --> 00:04:37,937 I think you can see that by simple geometrical construction. 55 00:04:37,937 --> 00:04:40,970 And so, this is the equation for constructive interference. 56 00:04:40,970 --> 00:04:44,508 This is sort of the [UNKNOWN] equation that when the path difference is an 57 00:04:44,508 --> 00:04:48,510 integral number of wavelengths, you get constructive interference and so you see 58 00:04:48,510 --> 00:04:53,590 a bright, you see a bright spot on the screen. 59 00:04:53,590 --> 00:04:57,808 And you see those at different orders so that there's, there's here's, here, this 60 00:04:57,808 --> 00:05:02,680 would be the m equals 0 order that light could go straight through. 61 00:05:02,680 --> 00:05:06,112 You clearly get the same path from either slit if you're looking at the midpoint 62 00:05:06,112 --> 00:05:11,244 between the slits. And then you get by convention, here's 63 00:05:11,244 --> 00:05:18,318 the first-order diffraction. Down here, there'd be a minus 1th order 64 00:05:18,318 --> 00:05:22,690 and so on. Now, the really striking thing about this 65 00:05:22,690 --> 00:05:26,290 ex, experiment of Young's and I, I encourage you to, you read it in his own 66 00:05:26,290 --> 00:05:30,090 words. Here's an excerpt from it here. 67 00:05:30,090 --> 00:05:34,299 But you can find the original in our supplementary documents the one that's 68 00:05:34,299 --> 00:05:38,200 entitled Young's Slit Experiment in Diffraction. 69 00:05:38,200 --> 00:05:41,327 The really remarkable thing is, that he used this device to measure the 70 00:05:41,327 --> 00:05:45,311 wavelength of the light. Now, this again, at a, was a time, it 71 00:05:45,311 --> 00:05:48,060 wasn't even known that light was a wave motion. 72 00:05:48,060 --> 00:05:51,773 But he used this analysis of the interference pattern. 73 00:05:51,773 --> 00:05:56,957 And here, here's the results that he quote extreme red light must be in air 74 00:05:56,957 --> 00:06:04,208 about one 36 thousandth of an inch for those of you outside the USA. 75 00:06:04,208 --> 00:06:08,140 That's an inch is 2.54 centimeters, exactly. 76 00:06:08,140 --> 00:06:13,530 And those are the extreme violet about one 60 thousandth. 77 00:06:13,530 --> 00:06:19,272 That's a, in modern notation, 423 nanometers compared to our reference 78 00:06:19,272 --> 00:06:23,520 standard 405. So not, not far away from the blue ray 79 00:06:23,520 --> 00:06:27,997 pointer. And then here is the, the, the thing that 80 00:06:27,997 --> 00:06:32,514 is really striking. He says, from these dimensions it 81 00:06:32,514 --> 00:06:36,298 follows, calculating upon the known velocity of light. 82 00:06:36,298 --> 00:06:40,015 So, in other words, he used this fundamental equation that you've just 83 00:06:40,015 --> 00:06:44,299 been looking at in the previous lecture did an exercise on if I'm not mistaken, 84 00:06:44,299 --> 00:06:50,390 the wavelength times the frequency is equal to the speed of light. 85 00:06:50,390 --> 00:06:54,075 So there, the speed of light was known at the beginning of the 19th century, known 86 00:06:54,075 --> 00:07:01,208 to some accuracy. He says, there are 500 millions of 87 00:07:01,208 --> 00:07:06,392 millions of the slowest of those waves, meaning, I think, he means the lowest 88 00:07:06,392 --> 00:07:14,956 frequency in the optical do, domain. So, here he has carried out an indirect 89 00:07:14,956 --> 00:07:21,342 measurement of a frequency of, well, how do we say that, that's like 500 90 00:07:21,342 --> 00:07:30,155 terahertz, I guess, 5 times 10 to the 14th cycles per second. 91 00:07:30,155 --> 00:07:37,262 Now, that was a frequency completely impossible to measure at the time that 92 00:07:37,262 --> 00:07:44,206 Young did his experiment. There were no electronics then, so the, 93 00:07:44,206 --> 00:07:48,110 the fastest the fastest frequency that could be measured accurately was 94 00:07:48,110 --> 00:07:53,670 presumably due to some mechanical contrivance or tuning fork perhaps. 95 00:07:53,670 --> 00:07:57,198 and even, I have to say, even today, the direct measurement of an optical 96 00:07:57,198 --> 00:08:00,465 frequency is by no means a routine matter. 97 00:08:00,465 --> 00:08:06,562 this, this is a this is 10,000 times higher frequency than that of, of a 98 00:08:06,562 --> 00:08:12,582 microwave oven, for example. It's a million times higher frequency 99 00:08:12,582 --> 00:08:16,184 than FM radio. It's a frequency that can't be handled by 100 00:08:16,184 --> 00:08:20,586 conventional electronics. And, you know, just as an indicator of 101 00:08:20,586 --> 00:08:28,640 this in 2005, the Nobel Prize in Physics was awarded to John Hall and Ted Hansch. 102 00:08:28,640 --> 00:08:33,536 as you can read for and, and especially noting this optical frequency comb 103 00:08:33,536 --> 00:08:38,144 technique, which is, right now, the main way in which people are able to, to do 104 00:08:38,144 --> 00:08:45,357 direct optical frequency measurement. So, it has come to pass, but Young's work 105 00:08:45,357 --> 00:08:54,140 in this sense was really visionary. Now, there's been, so the interferometry, 106 00:08:54,140 --> 00:08:59,280 optical interferometry has been a great tool for Science and Engineering. 107 00:09:00,570 --> 00:09:05,720 And the techniques have been extended to other systems. 108 00:09:05,720 --> 00:09:10,530 So, in particular, to things we would ordinarily characterize as particles, 109 00:09:10,530 --> 00:09:14,480 electrons, neutrons, atoms, and molecules. 110 00:09:14,480 --> 00:09:20,971 I'm just going to show you two examples. this is, this is an example of neutron 111 00:09:20,971 --> 00:09:24,616 interferometry. It uses, okay, it's not a two slit 112 00:09:24,616 --> 00:09:28,030 experiment per say, but it does something similar. 113 00:09:28,030 --> 00:09:31,800 So, what happens is you have a beam, a neutron beam, that's incident of, this is 114 00:09:31,800 --> 00:09:35,164 a, this is a solid block of silicon that's just been milled down to form 115 00:09:35,164 --> 00:09:41,445 these three blades. and so the neutron beam comes in and it, 116 00:09:41,445 --> 00:09:44,996 it enters this part of, part of the, sorry, part of the beam is, is 117 00:09:44,996 --> 00:09:49,351 transmitted, then and part of it undergoes Bragg reflection to the second 118 00:09:49,351 --> 00:09:54,542 plate. Then, part of that goes, goes through, 119 00:09:54,542 --> 00:09:59,874 part of those two beams go through. But then, there's also Bragg reflection 120 00:09:59,874 --> 00:10:04,659 from the internal blade. Sorry, did I make that clear? 121 00:10:04,659 --> 00:10:09,759 And so then, the beams recombine and then their, the summed beams are output in two 122 00:10:09,759 --> 00:10:14,670 directions here. So basically, what happens is when a 123 00:10:14,670 --> 00:10:20,097 neutron go in the amazing thing about this is that at the intensities that are 124 00:10:20,097 --> 00:10:24,957 used, there's never more than one neutrons in the interferometer at any 125 00:10:24,957 --> 00:10:30,804 given time. In fact, there's mostly no neutrons in 126 00:10:30,804 --> 00:10:34,502 the interferometer. But when you accumulate the, the pattern 127 00:10:34,502 --> 00:10:40,240 of signals, you get the same interference pattern that you do for light wave. 128 00:10:40,240 --> 00:10:44,878 So, this is the origin of this famous expression of Paul Dirac. 129 00:10:44,878 --> 00:10:51,381 A particle only interferes with itself. I think the modern version of that is a 130 00:10:51,381 --> 00:10:58,440 particle does interfere with itself. So, even on this macroscopic scale, which 131 00:10:58,440 --> 00:11:04,068 you can see is just about the size of a, a 12 ounce soft drink can even on this 132 00:11:04,068 --> 00:11:09,528 macroscopic scale, single neutrons actually exhibit the, the interference 133 00:11:09,528 --> 00:11:17,826 pattern that you see here. And this was a very important experiment. 134 00:11:17,826 --> 00:11:23,360 It was the first used to measure the effect of gravity on a quantum particle. 135 00:11:23,360 --> 00:11:27,447 So, what you're seeing here is basically a, it's a rotate, this [UNKNOWN], this, 136 00:11:27,447 --> 00:11:32,030 this interferometer was rotated in the gravitational field. 137 00:11:32,030 --> 00:11:34,814 So, one of these paths was a little bit, was a little bit higher up in the 138 00:11:34,814 --> 00:11:38,997 gravitational field than the other. And that gives this phase effect, this, 139 00:11:38,997 --> 00:11:44,626 this interference effect that you see. The the de Broglie wavelength here is 10 140 00:11:44,626 --> 00:11:51,642 to the minus 10 meters. just recall that for light the, the 141 00:11:51,642 --> 00:11:58,506 wavelength is like 500 nanometers which would be, you know, 5 times 10 to the 142 00:11:58,506 --> 00:12:05,445 minus 7 meters. So, this is, this is the, the de Broglie 143 00:12:05,445 --> 00:12:11,220 wavelength here is comparable to the radius of an atom. 144 00:12:11,220 --> 00:12:16,600 And you see this nice interference pattern. 145 00:12:16,600 --> 00:12:21,893 Now, here's another object on which a quantum interference experiment has been 146 00:12:21,893 --> 00:12:26,082 done. not exactly like the Young's double slit, 147 00:12:26,082 --> 00:12:31,040 but again, with different paths, paths in space to a destination, and seeing 148 00:12:31,040 --> 00:12:35,972 interference fringes. And this thing, gosh, I don't even know 149 00:12:35,972 --> 00:12:39,253 if I can pronounce its name. I'll just let you read it. 150 00:12:39,253 --> 00:12:45,310 TPP, it says, contains 152 fluorine atoms. 151 00:12:45,310 --> 00:12:48,770 Actually, it kind of looks like the logo for the burning man festival. 152 00:12:48,770 --> 00:12:53,220 Anyway, this is, this is okay, this you can't see with your eye. 153 00:12:53,220 --> 00:12:56,642 It's 1 nanometer in size. But it's, it's a pretty good size for a 154 00:12:56,642 --> 00:13:01,430 molecule. And it has a de Broglie wavelength of 10 155 00:13:01,430 --> 00:13:08,252 to the minus 12 meters. Now, that is you know a factor of a 100 156 00:13:08,252 --> 00:13:16,680 smaller than an atomic diameter. So it's really quite extraordinary that, 157 00:13:16,680 --> 00:13:23,016 that people can now produce and control such large molecules to the degree that 158 00:13:23,016 --> 00:13:29,820 makes quantum interference possible. But it can be done. 159 00:13:29,820 --> 00:13:35,190 And I point you to this recent review article. 160 00:13:35,190 --> 00:13:38,374 It's a colloquium style article. So I mean, if, if you have no physics 161 00:13:38,374 --> 00:13:42,690 background whatever, you'll probably find this quite hard to understand. 162 00:13:42,690 --> 00:13:46,968 But I'd say a a person with a Bachelor's degree in Physics can probably get quite 163 00:13:46,968 --> 00:13:51,058 a bit out of reading this article. So, it's again, it's written at a, a 164 00:13:51,058 --> 00:13:54,810 level that's intended to be accessible. In any event, the pictures are quite 165 00:13:54,810 --> 00:13:58,902 remarkable and the idea that you can do quantum interference on things that are 166 00:13:58,902 --> 00:14:04,200 so massive and, and so large is really, to me, quite inspiring. 167 00:14:07,060 --> 00:14:09,314 So now, we're going to talk about diffraction, which is just a 168 00:14:09,314 --> 00:14:12,120 generalization, you know, in the context that we're using it here, just a 169 00:14:12,120 --> 00:14:18,039 generalization, the idea of interference. So this, the, the earliest really, 170 00:14:18,039 --> 00:14:23,332 productive lasting work was done on this was again, by the great man Fraunhofer, 171 00:14:23,332 --> 00:14:28,862 he developed a diffraction grating which you can just think of as a multiple slit 172 00:14:28,862 --> 00:14:36,156 interferometer. So, you just have, you have waves coming 173 00:14:36,156 --> 00:14:42,069 out from different slits or in the case that's shown in this picture, you have 174 00:14:42,069 --> 00:14:47,172 rulings that are made on a mirror so that you get a, you get a reflection from a 175 00:14:47,172 --> 00:14:55,758 periodic array of objects. And so, this means that instead of having 176 00:14:55,758 --> 00:15:01,334 two light beams interfere, you can have a large number, literally thousands, and 177 00:15:01,334 --> 00:15:07,402 the the intensity signal, of the signal goes with the square of the number of of 178 00:15:07,402 --> 00:15:13,323 of scatters. Indeed Victor showed in his early 179 00:15:13,323 --> 00:15:16,450 lectures, a, a picture of X-ray diffraction which is very much the same 180 00:15:16,450 --> 00:15:19,245 idea. You have a periodic array. 181 00:15:19,245 --> 00:15:22,725 It, it, you can, you can see that with great clarity using, in the, in that 182 00:15:22,725 --> 00:15:28,603 case, X-rays in this case, light. The diffraction grating has to be one of 183 00:15:28,603 --> 00:15:33,949 the most powerful passive devices that was ever thought of because you can make 184 00:15:33,949 --> 00:15:39,619 it, just if you can do very accurate rule, make accurate rule, ruling marks at 185 00:15:39,619 --> 00:15:46,690 regular spacing. So, this, that, that was a technology, 186 00:15:46,690 --> 00:15:51,817 that was developed to quite a high degree in the 19th century. 187 00:15:51,817 --> 00:15:56,830 And Fraunhofer indeed used it to make direct measurement of wavelengths. 188 00:15:56,830 --> 00:16:01,185 So, the, his early work with the prism it only describes the deflection of the 189 00:16:01,185 --> 00:16:04,760 light in terms of the angles of deflection because we have no way of 190 00:16:04,760 --> 00:16:10,888 knowing what the wavelength was. But he repeated his studies later on 191 00:16:10,888 --> 00:16:15,048 diffraction grating and that made it possible to get quantitative data on the 192 00:16:15,048 --> 00:16:19,840 actual wavelengths of those dark lines in the sun. 193 00:16:19,840 --> 00:16:23,690 And in about two or three slides, we'll see why that's important. 194 00:16:23,690 --> 00:16:30,190 I'd like to just mention that if you're viewing this lecture right now, you are 195 00:16:30,190 --> 00:16:36,490 being helped by untold thousands of the diffraction gratings, which are an 196 00:16:36,490 --> 00:16:42,490 essential piece of the architecture of the the, modern fiber-optic 197 00:16:42,490 --> 00:16:52,876 telecommunications infrastructure. So, a wavelength division multiplexing is 198 00:16:52,876 --> 00:17:00,050 a procedure of putting a high data stream on a beam, on a beam of light. 199 00:17:00,050 --> 00:17:04,470 It basically, it doesn't use white light, it uses some, it uses some band of 200 00:17:04,470 --> 00:17:08,825 wavelengths that you can't see here, they're in the infrared, typically around 201 00:17:08,825 --> 00:17:14,718 1,500 nanometers. but it you, if you have a broadband if 202 00:17:14,718 --> 00:17:18,738 you have a optical, optical signal with a finite bandwidth, you can use a 203 00:17:18,738 --> 00:17:26,092 diffraction grating to, so you have, you input the light to a diffraction grating. 204 00:17:26,092 --> 00:17:30,646 It spreads it out, and then you can do encoding on the separate channels then 205 00:17:30,646 --> 00:17:36,875 recombine them into another grating to put them back into the common being. 206 00:17:36,875 --> 00:17:40,887 So, this wavelength division multiplexing is what makes it possibly to put such a 207 00:17:40,887 --> 00:17:45,916 high transmission rate on the Internet. There are many, many other applications 208 00:17:45,916 --> 00:17:49,101 of this technology as well, and again, it's governed by the simple grating 209 00:17:49,101 --> 00:17:55,040 equation. So now, we're going to have a a little 210 00:17:55,040 --> 00:18:00,080 online quiz that's sort, it's a non-mathematical one help you to retain 211 00:18:00,080 --> 00:18:05,520 some knowledge of the, of the things that I've just mentioned in these past few 212 00:18:05,520 --> 00:18:14,500 slides. Okay, I hope that was useful and let's 213 00:18:14,500 --> 00:18:19,131 now take a look at the optical spectrum of the sun again. 214 00:18:19,131 --> 00:18:23,163 I've taken a, I've taken and, I just, I just made an enlargement of a region of 215 00:18:23,163 --> 00:18:27,195 that picture of the high resolution spectrum of the sun that we were looking 216 00:18:27,195 --> 00:18:32,313 at previously. Just to show you, you know, in depth what 217 00:18:32,313 --> 00:18:36,780 the, the complexity of the line structure that's there. 218 00:18:36,780 --> 00:18:40,080 So, this is, this is acquired by a Fourier transform spectrometer with a 219 00:18:40,080 --> 00:18:44,570 resolving power of 500,000. That means that the, it can see a 220 00:18:44,570 --> 00:18:50,180 variation of the a significant variation of the optical signal over a, a 221 00:18:50,180 --> 00:18:55,705 wavelength region that is one over 500,000, about two parts in 10 to the 6th 222 00:18:55,705 --> 00:19:04,835 of the, the wavelength of importance. So, as you can see, there's very many 223 00:19:04,835 --> 00:19:09,589 narrow lines. This is, it's a dense dark packet 224 00:19:09,589 --> 00:19:16,670 communication. The sun is sending us a message, okay? 225 00:19:16,670 --> 00:19:20,266 And we didn't even know that until Fraunhofer's work but it looks like it's 226 00:19:20,266 --> 00:19:24,094 been sending the same message forever because these, these dark lines are very 227 00:19:24,094 --> 00:19:28,156 stable in time. Their intensities may very a little bit 228 00:19:28,156 --> 00:19:31,740 but they're always in the same spot. They don't wander around. 229 00:19:31,740 --> 00:19:38,960 So, so, what are these things anyway? A key clue to the nature of the 230 00:19:38,960 --> 00:19:43,835 Fraunhofer lines came from the study of emission of light by gasses subject to 231 00:19:43,835 --> 00:19:49,134 electric discharges. Now that's a mouthful, but you're 232 00:19:49,134 --> 00:19:54,908 familiar with this sort of phenomenon. Here's a, a famous neon sign in a movie 233 00:19:54,908 --> 00:20:00,164 theater in California, and if you go to entertainment districts around the world, 234 00:20:00,164 --> 00:20:07,580 you'll see these vibrant lights that are produced by typically rare gases. 235 00:20:07,580 --> 00:20:11,524 they, they have a very striking visual appearance, which is why they're so 236 00:20:11,524 --> 00:20:19,263 popular for display purposes. Now, here's a demonstration of why the, 237 00:20:19,263 --> 00:20:24,484 the visual appearance of these neon signs is so striking. 238 00:20:24,484 --> 00:20:29,730 And that is because they're not putting out a broad band of white light but 239 00:20:29,730 --> 00:20:35,590 rather a series of, of, of quite localized features. 240 00:20:35,590 --> 00:20:41,981 Here you see, this is the lower the lower curve here, lower band here. 241 00:20:41,981 --> 00:20:47,880 It shows a photograph of a spectrum of neon dispersed by wavelength. 242 00:20:47,880 --> 00:20:54,606 So again, here's our common 650 nanometer 650 nanometer referenced laser. 243 00:20:54,606 --> 00:20:58,024 Here is the 530. You see, there's a couple green lines 244 00:20:58,024 --> 00:20:59,980 there. Here's our blue ray. 245 00:20:59,980 --> 00:21:05,190 And so here, is the neon spectrum is concentrated in color in one region. 246 00:21:05,190 --> 00:21:09,254 That's why it's so vivid. Now, the same is true for hydrogen though 247 00:21:09,254 --> 00:21:13,406 it has even a simpler spectrum. As you can see, you can see four lines 248 00:21:13,406 --> 00:21:17,284 here. One at 656, very near to our our 249 00:21:17,284 --> 00:21:23,872 reference laser at 650. and then another there's two more down 250 00:21:23,872 --> 00:21:26,300 here. This is sort of in a cyan color. 251 00:21:26,300 --> 00:21:30,186 And then, one at 410 nanometers, very close to our reference laser 405 252 00:21:30,186 --> 00:21:36,246 nanometers. So this, this study of emission of light 253 00:21:36,246 --> 00:21:44,020 by gasses developed independently in the 19th century. 254 00:21:44,020 --> 00:21:48,400 and it was facilitated by the ability to make precise measurements. 255 00:21:48,400 --> 00:21:55,460 And so, this lead to a major breakthrough. 256 00:21:55,460 --> 00:22:00,374 This breakthrough was due to Robert Bunsen and Gustav Kirchhoff, who found 257 00:22:00,374 --> 00:22:05,600 that emission and absorption lines of gases occur at the same wavelengths, that 258 00:22:05,600 --> 00:22:13,884 is, at least in many circumstances. so, it was natural to attribute the dark 259 00:22:13,884 --> 00:22:20,674 lines seen by Fraunhofer to absorption of sunlight by gases in the solar atmosphere 260 00:22:20,674 --> 00:22:28,930 in cases where the gases could be could be found on earth. 261 00:22:28,930 --> 00:22:33,553 And indeed, it was quickly ascertained that the in particular, this famous 262 00:22:33,553 --> 00:22:40,974 hydrogen line at 656 nanometers. I'm going to use red to illustrate it, 263 00:22:40,974 --> 00:22:52,459 which is called H alpha or Balmer alpha after the Swiss school teacher, Balmer. 264 00:22:54,070 --> 00:22:59,506 It's quickly ascertained that that was a line associated with a hydrogen gas. 265 00:22:59,506 --> 00:23:04,288 and here, here, here it is. Here's this little patch taken out of the 266 00:23:04,288 --> 00:23:08,230 high-resolution solar spectrum of the sun. 267 00:23:08,230 --> 00:23:16,710 So it is indeed seen in absorption. Now, why is that? 268 00:23:16,710 --> 00:23:21,312 So, it's clear, there's a, there's a way of making a common identification. 269 00:23:21,312 --> 00:23:25,218 One in emission, one in absorption, one is in the atmosphere of the sun, the 270 00:23:25,218 --> 00:23:31,404 other is in a laboratory on earth. why are atoms always in the same state in 271 00:23:31,404 --> 00:23:37,268 such different environments? And what is the mechanism that causes 272 00:23:37,268 --> 00:23:42,980 them to emit light only in these very specific regions of the spectrum, rather 273 00:23:42,980 --> 00:23:49,780 than in the continuum of the sun? And to me, this is actually, this is a 274 00:23:49,780 --> 00:23:54,132 quantum phenomenon in the sense that we see these packets localized in in 275 00:23:54,132 --> 00:23:58,772 wavelength. And that suggests there's something 276 00:23:58,772 --> 00:24:03,663 highly specific that goes on. There's a sort of fundamental energy or 277 00:24:03,663 --> 00:24:08,688 vibration in the atom that occurs for a reason that's rather hard to understand, 278 00:24:08,688 --> 00:24:15,890 when we think of motions of particles under the usual types of forces. 279 00:24:15,890 --> 00:24:19,660 So, we're going to explore that in the forthcoming lectures.