WEBVTT 00:00:00.090 --> 00:00:02.490 The following content is provided under a Creative 00:00:02.490 --> 00:00:04.059 Commons license. 00:00:04.059 --> 00:00:06.330 Your support will help MIT OpenCourseWare 00:00:06.330 --> 00:00:10.690 continue to offer high-quality educational resources for free. 00:00:10.690 --> 00:00:13.320 To make a donation or view additional materials 00:00:13.320 --> 00:00:17.280 from hundreds of MIT courses, visit MIT OpenCourseWare 00:00:17.280 --> 00:00:20.610 at ocw.mit.edu. 00:00:20.610 --> 00:00:22.110 PROFESSOR: Now, we are ready to look 00:00:22.110 --> 00:00:24.750 at Fraunhofer diffraction associated 00:00:24.750 --> 00:00:27.100 with circular apertures. 00:00:27.100 --> 00:00:30.430 The setup, again, is the same as before. 00:00:30.430 --> 00:00:32.150 But I just want to remind you of it. 00:00:32.150 --> 00:00:33.900 We have a helium-neon laser. 00:00:33.900 --> 00:00:37.980 Beam from the laser is reflected by this mirror 00:00:37.980 --> 00:00:41.550 and then reflected again by this mirror into this lens. 00:00:41.550 --> 00:00:45.040 And now we have the lens focusing the light 00:00:45.040 --> 00:00:47.940 onto a set of apertures. 00:00:47.940 --> 00:00:51.300 And then the diffraction light from the aperture 00:00:51.300 --> 00:00:56.820 then goes onto the screen. 00:00:56.820 --> 00:01:02.850 Now, these apertures have different diameters. 00:01:02.850 --> 00:01:05.800 Actually, they're tiny pinholes. 00:01:05.800 --> 00:01:10.740 So the first one we have has a 100 micron diameter. 00:01:10.740 --> 00:01:14.880 So, now, if we look at the screen, 00:01:14.880 --> 00:01:20.310 we see the diffraction pattern associated with 100 micron 00:01:20.310 --> 00:01:22.200 diameter aperture. 00:01:22.200 --> 00:01:25.620 Again, let me give you some dimensions. 00:01:25.620 --> 00:01:30.270 The separation between the aperture and the screen 00:01:30.270 --> 00:01:33.300 is about 100 centimeters, the light 00:01:33.300 --> 00:01:37.590 is from the helium-neon laser at 6328 angstroms, 00:01:37.590 --> 00:01:44.380 and the scale on the screen from here to here, 00:01:44.380 --> 00:01:47.940 which is the diameter of the first dark ring, 00:01:47.940 --> 00:01:52.530 is about 1.6 centimeters. 00:01:52.530 --> 00:01:56.850 So, now, you can check the diffraction pattern, 00:01:56.850 --> 00:02:01.770 given the information that I've just presented to you. 00:02:01.770 --> 00:02:03.105 At the moment, you see-- 00:02:06.502 --> 00:02:08.169 first of all, let me do some adjustment, 00:02:08.169 --> 00:02:12.520 make sure that I'm picked up. 00:02:12.520 --> 00:02:15.730 And what you see, you see a bright ring in the middle 00:02:15.730 --> 00:02:24.170 and then some faint rings around the central bright ring. 00:02:24.170 --> 00:02:27.370 There are actually many rings, but because they're so faint, 00:02:27.370 --> 00:02:28.190 you can't see them. 00:02:28.190 --> 00:02:32.080 So what I'm going to do is pull away this screen 00:02:32.080 --> 00:02:35.740 and expose to you a more sensitive screen, 00:02:35.740 --> 00:02:37.900 as we can see. 00:02:37.900 --> 00:02:40.100 Now, you see that we're saturating in the middle, 00:02:40.100 --> 00:02:41.740 but we're beginning to see other rings. 00:02:41.740 --> 00:02:44.890 Now, if we open up the camera aperture some more, 00:02:44.890 --> 00:02:47.620 you can see even more rings. 00:02:47.620 --> 00:02:50.470 And, again, all these rings, and their intensities, 00:02:50.470 --> 00:02:52.240 and sizes, and what have you can be 00:02:52.240 --> 00:02:56.770 calculated from the information that I've given you. 00:02:56.770 --> 00:03:00.640 So, now, this is then the Fraunhofer diffraction pattern 00:03:00.640 --> 00:03:03.430 associated with 100 micron aperture. 00:03:03.430 --> 00:03:09.580 Now, I would like to move to the next aperture, which 00:03:09.580 --> 00:03:16.310 is 50 microns. 00:03:16.310 --> 00:03:22.310 And then I'm going to do a little tweaking 00:03:22.310 --> 00:03:24.600 so I have it nicely picked up. 00:03:24.600 --> 00:03:25.100 Good. 00:03:25.100 --> 00:03:28.790 So, now, if we can look at it on the whole screen, 00:03:28.790 --> 00:03:33.680 this is then the diffraction pattern of a 50 micron 00:03:33.680 --> 00:03:37.940 aperture, and you can see that the size of the central fringe 00:03:37.940 --> 00:03:38.912 is bigger. 00:03:38.912 --> 00:03:41.120 And, again, from the information that I've given you, 00:03:41.120 --> 00:03:44.810 you should be able to calculate what it should be. 00:03:44.810 --> 00:03:49.700 Again, if we open up the aperture in the camera, 00:03:49.700 --> 00:03:55.090 maybe we can start to see a few more rings. 00:03:55.090 --> 00:03:57.200 And, again, the light level is smaller 00:03:57.200 --> 00:04:00.450 than with the previous aperture, because, again, the aperture 00:04:00.450 --> 00:04:05.540 is only 50 microns this time. 00:04:05.540 --> 00:04:10.550 Now, we've seen the Fraunhofer diffraction pattern associated 00:04:10.550 --> 00:04:12.530 with a rectangular aperture. 00:04:12.530 --> 00:04:16.730 And then, just now, we saw the diffraction associated 00:04:16.730 --> 00:04:19.279 with a circular aperture. 00:04:19.279 --> 00:04:21.630 In the next demonstration, we're going 00:04:21.630 --> 00:04:24.590 to show the Fraunhofer diffraction pattern associated 00:04:24.590 --> 00:04:28.730 with multiple slits-- with a two-dimensional array 00:04:28.730 --> 00:04:29.900 of multiple slits. 00:04:29.900 --> 00:04:33.230 So when we come back, we'll have the setup 00:04:33.230 --> 00:04:37.930 arranged so we can demonstrate that effect for you.