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Optics: Fraunhofer and Fresnel Diffraction

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Optics: Fraunhofer and Fresnel Diffraction
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Fraunhofer diffraction - multiple slits
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49
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Abstract
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.
MaterialErdefunkstelleVideotechnikOptisches InstrumentDiffraktometerMultiplizitätInfrarotlaser
GlasLaserVideotechnikDiffraktometerMikroskopobjektivSchnittmusterGauß-BündelInfrarotlaserLinealBesprechung/Interview
GlasVideotechnikPostkutscheSchwarzKlinge
GlasFilzschreiberDiffraktometerWellenlängeA6M Zero-SenSchnittmusterLinealIntensitätsverteilungSchmalspurlokomotiveAvro Arrow
LichtSiebdruckVideotechnikDiffraktometerWellenlängeAbstandsmessungBesprechung/Interview
GlasVideotechnik
LichtVideotechnikMikroskopobjektivGauß-BündelLinealIntensitätsverteilung
SiebdruckVideotechnikFilzschreiberDiffraktometerWellenlängeLinealIntensitätsverteilungKotflügel
VideotechnikDiffraktometerSteckkarte
VideotechnikDiffraktometerDrehung <Textiltechnik>KlangeffektBesprechung/Interview
Transkript: Englisch(automatisch erzeugt)
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 I'd like to demonstrate multi-slit diffraction patterns. Now normally, one would take a, let's say, piece of glass with a ruling on it, many, many lines, many dark lines on a piece of glass, and then one would shine the laser light through it
and look on the screen and see the multi-slit diffraction pattern. But we're going to do it a little differently. What we're going to do, we're going to introduce one slit at a time until we'll build up to the many slits. So you can see the contribution of each slit. And we're going to do it in this way.
We have the same setup as before. Here's our laser and the two mirrors and the lens to expand the beam. Here's the expanded beam. Now over here, we have two things. First of all, we have a pair of jaws here, which are two razor blades, which
I can adjust the spacing between the razor blades. I can adjust with this translation stage over here. And then right behind the razor blades, there is a piece of glass, which is a Ronchi ruling, just a piece of glass with black lines.
The thickness of each black line is about 75 microns. And the spacing between the black lines is about 125 microns, is about 250 lines per inch
if you want. So this is then this Ronchi ruling, this piece of glass with all these narrow slits on it. The screen, as before, is 200 centimeters away. And again, the wavelength of light is 6,328 angstroms.
So now, first of all, let's close down the jaws, look at the screen, and see if we can see the single slit diffraction pattern. Here we have the single slit diffraction pattern.
I have to apologize because when we close down the jaws, we don't have much light. But I hope you can still see the single slit diffraction pattern. Now, on the screen, we have the circles. The circles are the 5 centimeter markers. And the little arrow tips, as you can see,
they mark the zeros of the central lobe of the single slit diffraction pattern associated with the 50 micron or so slit on this Ronchi ruling. So now what I'm going to do, I'm going now to separate the jaws to admit
one more, the second slit. Now, those of you who understand this theory will quickly verify that, indeed, you'll see three lobes within the single slit principal lobe.
Now I'm going to add one more slit by, again, opening up the jaws. As I bring in one more slit, now we have three slits. And look at the pattern now. It generates some weaker lobes in between. And the lobes themselves are getting narrower.
Now we'll add one more. Here it is four. And here it is five, six, seven, and so on. I'm just going to keep enlarging the spacing. And you can see that, first of all, that the principal three lobes get narrower and narrower.
And then you get a lot more little side lobes in between. So as I widen the spacing between the jaws, you can see that those three lobes will get narrower and narrower. And of course, you'll see the ones from these other side
lobes also. There's little dots to the side. Now, the intensity is so bright that it's saturating our camera. So what we'd like to do is take a little close-up so we can resolve the lobe. So if you can go in and take a close look at the three
lobes in the center, here we are. Let me go back again to when I had only two slits in there.
Now I've added a third, a fourth, fifth, and so on. As you can see, the width of the three principal lobes is getting narrower and narrower.
Now I have lots of slits now. And again, the intensity is high, so I can't really tell how narrow. But I know it looks very narrow, so maybe we can cut down the sensitivity a little bit
on the camera. And let's see if we get a feel of how narrow these spots will be. Again, all I can say, they look pretty narrow.
And I'll leave it to you to calculate how narrow they become because I've given you all the data. I've given you the spacing between the slits. I've given you the width of the slits and the wavelength of the light and the distance between the slits and the screen. So in summary, this is a very, very cute experiment,
demonstration of how the addition of each slit contributes to the Fraunhofer diffraction pattern. Now we're going to look at multi-slit diffraction as a function of line spacing.
What we have here are Ronchi rulings, which are pieces of glass with lots of lines drawn on them. This one here has about 100 lines per inch. So we're going to put this Ronchi ruling in here
in our setup. And the setup is the same as before with this lens here to enlarge the beam so we can illuminate as many of the lines as possible.
We've also added this attenuator here so that we can adjust the intensity of the light when we need to. So now let's look at the screen and see what we can see with this Ronchi ruling of 100
lines per inch. As you can see, there are plenty of very narrow dots. And in fact, if you want to get a feel for the scale, the little circles just below the diffraction pattern are the 5-centimeter markers that we've had before.
Now if I attenuate the intensity a little bit, you can see that the ones in the center are the brightest, of course. And also, as I reduce intensity, you can see that the spots are really very small.
Now if I've given you the number of lines per inch and the spacing between the Ronchi ruling and the screen is, again, 200 centimeters and the wavelength is 6328 angstroms, you should be able to check on the spacing between the fringes
and also on their widths. So here they are when I overexpose them so we can see the ones way out in the wings. So this is then the diffraction pattern, the Fraunhofer diffraction pattern associated with the Ronchi ruling of 100 lines per inch.
Now let's look at 200 lines per inch. So here is the 200 lines per inch. And you can see that the spacing now is different, but I leave it to you to check on it. And again, if I reduce the intensity,
and you get at least a little bit of a feel for how narrow these dots are. They're indeed very bright because they're saturating our camera. So that's for then 200 lines per inch. Now we go to 300 lines per inch.
Again, the spacing is different. And also if I change the orientation of the lines, you can see that the diffraction pattern also
changes. So that's then for 300 lines per inch. The next one is 2,000 lines per inch. Now when I put it over here, you can see that the spacing between the fringes are about 10 centimeters.
And again, that gives you a check on the number of lines per centimeter or per inch as we have it. Now let me see. If we pull back a little bit, pull back on the camera to see the other dots, yes, here they are. But they're so widely spaced, it's
difficult to get them all on the camera at once. So if we go back to the original position, if we go in again, here we are. And now I'm going to again reduce the intensity, and you can see how narrow the spots are.
So this then sums up multi-slit diffraction pattern as a function of line spacing. In the next demonstration, we're going to show the opposite effect. Instead of slits, we're going to use thin wires.
And then when we come back, we'll show you what the Fraunhofer diffraction pattern for very thin wires looks like.