Optics: Fraunhofer and Fresnel Diffraction
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VideoLaserOpticsDiffractionGround stationQuality (business)Material
00:20
Gaussian beamDiffractionApertureCamera lensShort circuitRear-view mirrorLightKardierenFocus (optics)Screen printingLaserFocal lengthLaserWeather front
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DiffractionWeather frontVideoEffects unitScreen printingLaserPattern (sewing)LightCartridge (firearms)ForgingBrightness
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VideoDiffractionSeparation processWeather frontFACTS (newspaper)BrightnessContrast (vision)Pattern (sewing)TheodoliteLightScreen printingVisibilityMonitorüberwachung
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DiffractionApertureMeeting/Interview
Transcript: English(auto-generated)
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00:21
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.
00:42
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
01:05
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.
01:25
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.
01:41
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
02:00
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
02:20
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.
02:41
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.
03:07
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
03:22
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
03:41
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.
04:01
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.
04:23
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.
04:45
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
05:01
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
05:23
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.
05:42
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.
06:05
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
06:21
at Fresnel diffraction associated with circular apertures.