1 00:00:00,090 --> 00:00:02,490 The following content is provided under a Creative 2 00:00:02,490 --> 00:00:04,030 Commons license. 3 00:00:04,030 --> 00:00:06,330 Your support will help MIT OpenCourseWare 4 00:00:06,330 --> 00:00:10,690 continue to offer high-quality educational resources for free. 5 00:00:10,690 --> 00:00:13,320 To make a donation or view additional materials 6 00:00:13,320 --> 00:00:17,260 from hundreds of MIT courses, visit MIT OpenCourseWare 7 00:00:17,260 --> 00:00:18,200 at ocw.mit.edu. 8 00:00:21,642 --> 00:00:23,100 PROFESSOR: Now, we're ready to look 9 00:00:23,100 --> 00:00:32,250 at diffraction by thin fibers or wires, opposite to slits. 10 00:00:32,250 --> 00:00:34,740 The setup is essentially very simple. 11 00:00:34,740 --> 00:00:37,080 We even took out the lens that we had here 12 00:00:37,080 --> 00:00:43,770 so the laser beam can go directly onto these wires. 13 00:00:43,770 --> 00:00:46,380 So if we take a close look at what we have here, 14 00:00:46,380 --> 00:00:52,170 we have just four wires then I can just 15 00:00:52,170 --> 00:00:55,290 put in the way of the laser beam, 16 00:00:55,290 --> 00:00:59,745 and then we, on the screen, can see the associated diffraction 17 00:00:59,745 --> 00:01:00,780 pattern. 18 00:01:00,780 --> 00:01:05,069 So let's start with the thickest wire. 19 00:01:05,069 --> 00:01:07,680 So you can see on the screen, here 20 00:01:07,680 --> 00:01:12,370 is the diffraction pattern associated with this wire. 21 00:01:12,370 --> 00:01:14,508 And what you can see-- 22 00:01:14,508 --> 00:01:16,050 well, first of all, let me remind you 23 00:01:16,050 --> 00:01:18,600 that we have the little circles there, which 24 00:01:18,600 --> 00:01:20,750 are the 5 centimeter markers. 25 00:01:20,750 --> 00:01:23,060 So you get a feel for the spacing. 26 00:01:23,060 --> 00:01:27,270 Again, the distance between the wire and the screen 27 00:01:27,270 --> 00:01:30,720 is 200 centimeters, and the wavelength of the laser 28 00:01:30,720 --> 00:01:36,007 is 6328 angstroms. 29 00:01:36,007 --> 00:01:37,590 I'm not going to tell you the diameter 30 00:01:37,590 --> 00:01:39,660 of the wire, because I'm going to let 31 00:01:39,660 --> 00:01:42,150 you work that out for yourself. 32 00:01:42,150 --> 00:01:44,700 But first you want to look at the diffraction pattern. 33 00:01:44,700 --> 00:01:48,300 And you can see it looks very similar to the single slit 34 00:01:48,300 --> 00:01:52,290 diffraction pattern, except for the blob in the center. 35 00:01:52,290 --> 00:01:55,800 And the blob in the center is a little complicated, 36 00:01:55,800 --> 00:02:00,120 but I can attenuate the laser beam. 37 00:02:00,120 --> 00:02:02,910 And you can see that it is pretty bright, 38 00:02:02,910 --> 00:02:04,800 because essentially it is the laser beam. 39 00:02:07,670 --> 00:02:12,000 And so it's very difficult to get information from it. 40 00:02:12,000 --> 00:02:16,710 The information is hidden in the lobes on the side 41 00:02:16,710 --> 00:02:18,520 and the spacing between them. 42 00:02:18,520 --> 00:02:20,730 Because that will be then related 43 00:02:20,730 --> 00:02:23,190 to the diameter of the wire. 44 00:02:23,190 --> 00:02:26,010 So then, for the thickest wire, we 45 00:02:26,010 --> 00:02:29,290 see this kind of pattern, this kind of spacing. 46 00:02:29,290 --> 00:02:35,960 Now, let me move on to wire number two, which is thinner. 47 00:02:35,960 --> 00:02:38,340 Here's wire number two. 48 00:02:38,340 --> 00:02:42,990 And we can see that, first of all, 49 00:02:42,990 --> 00:02:49,050 there's less light in the wings, because the wire is thinner. 50 00:02:49,050 --> 00:02:52,450 So we don't see as many side lobes. 51 00:02:52,450 --> 00:02:57,270 And then, again, you'll see this ugly blob in the center. 52 00:02:57,270 --> 00:03:00,653 But the information is in the lobes on the side. 53 00:03:00,653 --> 00:03:02,070 And you can see them very clearly, 54 00:03:02,070 --> 00:03:04,520 and you can see their spacing very clearly. 55 00:03:04,520 --> 00:03:10,200 Now, let me go on to wire number three, which 56 00:03:10,200 --> 00:03:12,810 is, again, thinner still. 57 00:03:12,810 --> 00:03:18,990 And maybe here we can zoom in a little bit 58 00:03:18,990 --> 00:03:23,410 so that we can see it a little bit better. 59 00:03:23,410 --> 00:03:27,930 And again, you can see the central spot is a little messy, 60 00:03:27,930 --> 00:03:31,890 but the information, as I said before, 61 00:03:31,890 --> 00:03:35,790 is in the fringes on the side. 62 00:03:35,790 --> 00:03:41,182 Now, here, again, if I reduce the laser intensity, 63 00:03:41,182 --> 00:03:44,580 you see what the central fringe looks like. 64 00:03:44,580 --> 00:03:47,710 And as I bring it up, we'll see the rest of the fringe. 65 00:03:47,710 --> 00:03:48,210 Now, 66 00:03:48,210 --> 00:03:50,760 Finally, I'm going to go to my thinnest 67 00:03:50,760 --> 00:03:57,600 wire, which is over here. 68 00:03:57,600 --> 00:04:03,030 And now we may have to zoom out a little bit 69 00:04:03,030 --> 00:04:06,438 so that we can see the spacing. 70 00:04:06,438 --> 00:04:08,730 And maybe we can increase the sensitivity a little bit, 71 00:04:08,730 --> 00:04:11,710 because this isn't all that much light. 72 00:04:11,710 --> 00:04:12,660 So we can see good. 73 00:04:12,660 --> 00:04:18,810 Now, we can see the separation between the fringes 74 00:04:18,810 --> 00:04:20,529 in the wings. 75 00:04:20,529 --> 00:04:26,040 So I hope you've been watching carefully the spacing 76 00:04:26,040 --> 00:04:30,780 between the fringes, so that you can come up 77 00:04:30,780 --> 00:04:36,500 with the diameter of the individual wires. 78 00:04:36,500 --> 00:04:39,530 Now, this brings me to the end of 79 00:04:39,530 --> 00:04:42,890 the one-dimensional Fraunhofer diffraction patterns. 80 00:04:42,890 --> 00:04:44,930 Next, what we're going to do is look 81 00:04:44,930 --> 00:04:48,560 at two-dimensional Fraunhofer diffraction patterns. 82 00:04:48,560 --> 00:04:50,300 And I assure you, they're even more fun 83 00:04:50,300 --> 00:04:53,150 than the one-dimensional.