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Optics: Fraunhofer and Fresnel Diffraction

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Optics: Fraunhofer and Fresnel Diffraction
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Fraunhofer diffraction - thin wires
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49
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This resource contains demonstrations used to illustrate the theory and applications of lasers and optics. A detailed listing of the topics can be found below. Lasers today are being used in an ever-increasing number of applications. In fact, there is hardly a field that has not been touched by the laser. Lasers are playing key roles in the home, office, hospital, factory, outdoors, and theater, as well as in the laboratory. To learn about lasers and related optics, one usually takes a course or two, or acquires the necessary information from books and journal articles. To make this learning more vivid and more exciting, and, one hopes, more understandable, one needs to see some of the basic phenomena involved. To fill this need, Professor Ezekiel has videotaped 48 demonstrations that illustrate most of the fundamental phenomena relating to lasers and physical optics. By using split-screen inserts and a wide range of video-recording capabilities, it is possible to show real-time effects in lasers and optics with the simultaneous manipulation of the components that cause these effects. In this way, one can see effects in close up that would be difficult, if not impossible, to display in front of an audience or in the classroom. These video demonstrations are designed for: The individual student of lasers and optics who wants to observe the various phenomena covered in theoretical treatments in courses, books, and technical papers. The Instructor in lasers and optics in a company, university, college, or high school who wants to illustrate, in class, many of the fundamental phenomena in optics and lasers.
LaserVideoOpticsDiffractionThin filmWireGround stationMaterialQuality (business)
LaserDiffractionWireThin filmFiberCamera lens
WireLaserScreen printingPhotodissoziationDiffractionKopfstützePattern (sewing)SizingLaserWavelengthIntensity (physics)ButtonMarker penCosmic distance ladderFender (vehicle)LightSeparation processComputer animation
Transcript: English(auto-generated)
The following content is provided under a Creative Commons license. Your support will help MIT OpenCourseWare continue to offer high-quality educational resources for free. To make a donation or view additional materials from hundreds of MIT courses, visit MIT OpenCourseWare at ocw.mit.edu.
Now we're ready to look at diffraction by thin fibers or wires opposite to slits. The setup is essentially very simple. We even took out the lens that we had here. So the laser beam can go directly onto these wires.
So if we take a close look at what we have here, we have just four wires that I can just put in the way of the laser beam. And then on the screen, we can see the associated diffraction pattern.
So let's start with the thickest wire. So here is, as you can see on the screen, here is the diffraction pattern associated with this wire. And what you can see, well, first of all, let me remind you that we have the little circles there, which are the 5-centimeter markers so you
get a feel for the spacing. Again, the distance between the wire and the screen is 200 centimeters. And the wavelength of the laser is 6328 angstroms. I'm not going to tell you the diameter of the wire, because I'm going to let you work that out for yourself.
But first, you want to look at the diffraction pattern. When you can see, it looks very similar to the single slit diffraction pattern, except for the blob in the center. And the blob in the center is a little complicated, but I can attenuate the laser beam.
And you can see that it is pretty bright, because essentially, it is the laser beam. And so it's very difficult to get information from it. The information is hidden in the lobes on the sides and the spacing between them, because that will be then
related to the diameter of the wire. So then for the thickest wire, we see this kind of pattern, this kind of spacing. Now let me move on to wire number two, which is thinner. Here's wire number two. And we can see that, first of all,
there's less light in the wings, because the wire is thinner. So we don't see as many side lobes. And then again, you'll see this blob in the ugly blob in the center, but the information is in the lobes on the side.
And you can see them very clearly, and you can see their spacing very clearly. Now let me go on to wire number three, which is again thinner still. And maybe here we can zoom in a little bit so that we can see it a little bit better.
And again, you can see the central spot is a little messy, but the information, as I said before, is in the fringes on the side. Now here again, if I reduce the laser intensity,
see what the central fringe looks like. And as I bring it up, we'll see the rest of the fringes. Now finally, I'm going to go to my thinnest wire, which is over here. And now we may have to zoom out a little bit
so that we can see the spacing. And maybe we can increase the sensitivity a little bit because it isn't all that much light. So we can see good. Now we can see the separation between the fringes in the wings.
So I hope you've been watching carefully the spacing between the fringes so that you can come up with the diameter of the individual wires. Now this is bringing me to the end of the one-dimensional Fraunhofer diffraction patterns.
Next, what we're going to do is look at two-dimensional Fraunhofer diffraction patterns. And I assure you, they're even more fun than the one-dimension.