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Optics: Fraunhofer and Fresnel Diffraction

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Optics: Fraunhofer and Fresnel Diffraction
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Fresnel diffraction - adjustable slit
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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.
VideoLaserOpticsDiffractionGround stationQuality (business)Material
Gaussian beamDiffractionApertureCamera lensShort circuitRear-view mirrorLightKardierenFocus (optics)Screen printingLaserFocal lengthLaserWeather front
DiffractionWeather frontVideoEffects unitScreen printingLaserPattern (sewing)LightCartridge (firearms)ForgingBrightness
VideoDiffractionSeparation processWeather frontFACTS (newspaper)BrightnessContrast (vision)Pattern (sewing)TheodoliteLightScreen printingVisibilityMonitorüberwachung
DiffractionApertureMeeting/Interview
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 Fresnel diffraction. We're going to look at Fresnel diffraction associated with a single slit. And then later, we'll look at Fresnel diffraction associated with circular apertures. The setup for observing Fresnel diffraction is here.
Here's the laser. Here's the beam from the laser again. We reflect it by this mirror. And then we reflect the beam again by this mirror into this lens. Now, this is a short focal length lens that focuses the light and passes it through a pinhole over here, which we center it
at the focus spot. The light coming out from the pinhole then is shown on the card here and then goes and falls onto the screen. So this is the setup. Now I'm going to bring in the slit.
So I'm going to put the slit in front of the laser beam without disturbing anything, I hope. So here is the single slit.
Now I'm going to adjust the spacing to be very small so that we start out with the Fraunhofer diffraction pattern. Now here it is. You can see on the screen, we see the Fraunhofer diffraction pattern because the slit width is small enough
so that we're in the Fraunhofer region. Now what I'm going to do is I'm going to start to widen the slit, so to bring in the Fresnel diffraction pattern. So here I am going to start widening. And you can see now you have to watch
what happens to the fringes. Now things are going to get pretty bright. And so what I'm going to do, I'm going to introduce another screen that is not so sensitive so that we can observe the bright Fresnel pattern on that one. Here we are. And you can see I get a dark line in the middle.
Now we get two. And others, you can see how the fringes behave in this case. You want to see what the fringes near the edges look like. And in the center.
Now I know that the effect is not observable very well under these conditions. So what I'm going to do is to turn down the room lights
and see if we can improve the visibility of the Fresnel diffraction pattern. Now with the room lights dim, we're going to start again with the Fraunhofer diffraction pattern on the sensitive screen. So again, you can see that at present we
have Fraunhofer diffraction. And I hope it's a little clearer than with the room lights on. So I'm going to start here. And then as I increase the slit width, we're going to have a transition to Fresnel diffraction.
But as we know, the brightness is going to get pretty large. So I will then bring in the other screen here so that we're not going to saturate the camera too much when we observe Fresnel diffraction. So here we go. This is Fraunhofer. Then I will now go make the transition to Fresnel.
And it's very, very interesting how the light going through the slit solves Maxwell's equations. And here we are. We've got that dark line in the middle. And then now we have, let me go back again.
We have the two lines and the three. Now it's very important to notice that the contrast is not 100% like it is in the Fraunhofer
diffraction. And those of you who will do the calculation will, of course, understand what I'm referring to. The spacing, you notice that the spacing between the fringes gets tighter and tighter as you approach the center, or the center between the two slits. In fact, the fringe spacing gets so fine
with large slit separation that you can't even resolve them by eye. So there's lots of information in that diffraction pattern. And I hope that you've adjusted your monitors so that you can see these patterns.
So here we are all the way to Fraunhofer. And there's a little transition, which is not that easy to calculate, and then go into Fresnel.
Here we are as we get larger and larger. You can see, you can't even see the fringes in the center. Now that we've seen Fresnel diffraction associated with a single slit, now we're ready to look
at Fresnel diffraction associated with circular apertures.