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 --> 00:00:18,210 at ocw.mit.edu. 8 00:00:21,048 --> 00:00:24,330 PROFESSOR: Now, we're ready to look at Fresnel diffraction 9 00:00:24,330 --> 00:00:26,250 from circular apertures. 10 00:00:26,250 --> 00:00:28,630 The setup is again the same as before. 11 00:00:28,630 --> 00:00:30,580 But let me go over it again. 12 00:00:30,580 --> 00:00:32,550 Here, we have a helium neon laser. 13 00:00:32,550 --> 00:00:36,760 The beam from the laser gets reflected by this mirror 14 00:00:36,760 --> 00:00:41,560 and then gets reflected again by this mirror into this lens. 15 00:00:41,560 --> 00:00:43,500 This is a short focal length lens, which 16 00:00:43,500 --> 00:00:46,140 is focused into a pinhole-- 17 00:00:46,140 --> 00:00:50,070 an adjustable pinhole-- and the light from the pinhole 18 00:00:50,070 --> 00:00:52,395 is a spherical wave. 19 00:00:52,395 --> 00:00:55,380 And you can see it here on the card, it's a spherical wave. 20 00:00:55,380 --> 00:00:58,300 And then we let it hit the screen. 21 00:00:58,300 --> 00:01:03,600 Now, we're ready look at Fresnel diffraction from apertures. 22 00:01:03,600 --> 00:01:05,580 We have two apertures for you. 23 00:01:05,580 --> 00:01:10,030 And I'm going to place the first one in the beam. 24 00:01:16,220 --> 00:01:23,250 So now, if we can look at the screen while 25 00:01:23,250 --> 00:01:31,160 I'm adjusting the aperture. 26 00:01:31,160 --> 00:01:33,480 Now, here we are. 27 00:01:33,480 --> 00:01:34,640 It's a fixed aperture. 28 00:01:34,640 --> 00:01:38,600 It's 1,000 microns in diameter. 29 00:01:38,600 --> 00:01:42,950 And as we can see, we see the same kind 30 00:01:42,950 --> 00:01:48,800 of Fresnel diffraction pattern as we saw with the slit. 31 00:01:48,800 --> 00:01:51,080 We see lots of fringes. 32 00:01:51,080 --> 00:01:54,590 They get finer and finer as you approach the center. 33 00:01:54,590 --> 00:01:57,560 And, also, the contrast is less and less 34 00:01:57,560 --> 00:02:00,210 as you approach the center. 35 00:02:00,210 --> 00:02:04,220 But what I'm going to do now, instead of keeping the distance 36 00:02:04,220 --> 00:02:10,039 fix, is vary the distance between the aperture 37 00:02:10,039 --> 00:02:17,000 and the light, or the aperture from the pinhole. 38 00:02:17,000 --> 00:02:21,780 And I would like you to watch what happens. 39 00:02:21,780 --> 00:02:26,210 So now, I'm going to move the aperture very 40 00:02:26,210 --> 00:02:32,402 close to the pinhole, and then I'll move away. 41 00:02:32,402 --> 00:02:34,610 And then you can see, first, you see lots of fringes. 42 00:02:34,610 --> 00:02:39,830 And then, as I move away, you're seeing fewer and fewer fringes. 43 00:02:39,830 --> 00:02:42,320 And if you look in the center, I hope 44 00:02:42,320 --> 00:02:46,140 you can see that is a white spot in the center. 45 00:02:46,140 --> 00:02:49,460 Now we have a dark spot in the center. 46 00:02:49,460 --> 00:02:54,750 And then, now we're getting fewer and fewer fringes 47 00:02:54,750 --> 00:02:57,300 until we have only two or three. 48 00:02:57,300 --> 00:03:02,250 Now, let me go back towards the pinhole or towards the lens. 49 00:03:02,250 --> 00:03:05,630 You see the increase in the number of fringes. 50 00:03:05,630 --> 00:03:09,120 Now, again, this is very interesting. 51 00:03:09,120 --> 00:03:13,530 And I'm going to leave it to you to figure out. 52 00:03:13,530 --> 00:03:17,920 Here we are, very close to the lens. 53 00:03:17,920 --> 00:03:24,360 And then when we move away [INAUDIBLE] lens. 54 00:03:24,360 --> 00:03:26,040 Now, what I'm going to do, I'm going 55 00:03:26,040 --> 00:03:31,200 to move to the second aperture, which 56 00:03:31,200 --> 00:03:35,250 is 400 microns in diameter. 57 00:03:38,190 --> 00:03:41,500 And here we are. 58 00:03:41,500 --> 00:03:44,040 Let me again pick it up. 59 00:03:46,870 --> 00:03:47,430 Here we are. 60 00:03:47,430 --> 00:03:55,090 Now, this diffraction pattern looks slightly different. 61 00:03:55,090 --> 00:04:00,230 And, again, I want you to explain what's going on here. 62 00:04:00,230 --> 00:04:04,850 Again, now, I'm going to move this new aperture, 400 micron 63 00:04:04,850 --> 00:04:08,810 diameter aperture, as close as possible to the lens. 64 00:04:08,810 --> 00:04:12,660 You can see one ring inside, and then 65 00:04:12,660 --> 00:04:15,920 now you see the bright dot in the middle, 66 00:04:15,920 --> 00:04:24,020 and the bright dot becomes a dark spot in the middle. 67 00:04:24,020 --> 00:04:34,320 And now we get a white dot and the brighter dot in the middle. 68 00:04:34,320 --> 00:04:36,020 And here it's almost beginning to look 69 00:04:36,020 --> 00:04:38,330 like Fraunhofer diffraction. 70 00:04:38,330 --> 00:04:41,600 So here, let me move towards the lens-- 71 00:04:41,600 --> 00:04:42,960 towards the focus of the lens. 72 00:04:42,960 --> 00:04:43,460 I mean. 73 00:04:45,990 --> 00:04:50,570 You can see how the pattern changes. 74 00:04:50,570 --> 00:04:58,980 And as we move away, the pattern changes again. 75 00:05:02,750 --> 00:05:05,440 So let me hold it over here. 76 00:05:05,440 --> 00:05:06,590 Let you look at it. 77 00:05:09,460 --> 00:05:13,190 And I hope you'll be able to figure all this out. 78 00:05:13,190 --> 00:05:16,970 Remember, the diameter of this aperture is 400 microns, 79 00:05:16,970 --> 00:05:22,130 and in the previous one it was 1,000 microns. 80 00:05:22,130 --> 00:05:27,560 And again, the light is, as I say, being focused by a lens 81 00:05:27,560 --> 00:05:30,200 onto a small pinhole, and then we 82 00:05:30,200 --> 00:05:34,460 had a diverging beam that is impinging 83 00:05:34,460 --> 00:05:39,360 on these two apertures. 84 00:05:39,360 --> 00:05:42,800 So, in summary, we've seen a variety 85 00:05:42,800 --> 00:05:45,230 of diffraction patterns. 86 00:05:45,230 --> 00:05:50,160 We've seen one-dimensional Fraunhofer diffraction pattern. 87 00:05:50,160 --> 00:05:52,760 And also, we've seen two dimensional Fraunhofer 88 00:05:52,760 --> 00:05:54,570 diffraction patterns. 89 00:05:54,570 --> 00:05:59,570 And, finally, we looked at Fresnel diffraction associated 90 00:05:59,570 --> 00:06:06,560 with a slit, or an adjustable slit, and also with apertures. 91 00:06:06,560 --> 00:06:09,410 And then we saw how the diffraction pattern changes 92 00:06:09,410 --> 00:06:13,040 as we move the aperture closer and closer 93 00:06:13,040 --> 00:06:17,110 to the focus of the lens.