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PROFESSOR: In the next
set of demonstrations,

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we're going to illustrate
some of the very

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classic phenomena associated
with the diffraction of light.

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I hope that there will be
educational but also a lot

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of fun to see these effects.

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We'll start first with the
Fraunhofer diffraction,

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both in one dimension
and two dimensions.

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And then we'll go to
Fresnel diffraction.

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So let's start first with
Fraunhofer diffraction

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associated with a single slit.

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The setup is, over here
we have helium-neon laser,

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and here is the
beam from the laser.

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We reflect it by
this mirror here,

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and then we reflect it again by
this mirror here onto the slit.

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This slit is an adjustable slit.

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By turning this knob here,
I can adjust the slit.

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But in order to see that
indeed this is a slit,

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I'm going to put a
screen over here,

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so that when I adjust the
jaws of the slit here,

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you can see that,
indeed, I am varying

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the spacing between the jaws.

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Of course, when it gets very
small, it's difficult to see.

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So here's our adjustable slit,
and here's the laser beam

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going through the slit.

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And then the
diffracted light then

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falls on the screen
over there, which

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is about 200 centimeters
from the slit.

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So, now, we're ready to look
at the diffraction pattern.

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So if we have a close-up
then of the screen,

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we see the diffraction pattern
for the slit separation

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that I have over here.

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Now, the wavelength
of the light,

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that of the helium-neon
laser 6328 angstroms--

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so I gave you the
separation between the slit

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and the screen, which
is 200 centimeters,

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so now you should be able
to calculate the slit width

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from the diffraction pattern.

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Now, to help you do
this calculation,

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we have, on the screen, markers.

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And the separation between
these markers is 5 centimeters.

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So there are 5 centimeters
markers on the screen.

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So now you have all the tools
to calculate the slit width as I

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adjust the slit separation.

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So let me start with
a very narrow slit

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where the central lobe is--

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you can make it almost
5 centimeters or so.

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And you see the other
lobes, the weaker one.

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And maybe if we can
overexpose a little bit,

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we can see that, indeed,
there are lots of lobes.

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But if we expose
for all of them,

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then the central one is
going to get way overexposed.

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So, in general, we'll be
looking just at a few of them.

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So, now, let's go back
to where we were before.

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And then we'll start
again at a slit width

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so that the central lobe is
about a few centimeters wide.

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And then I will now
make the spacing--

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whoops-- wider.

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And you can see the
central lobe now

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will get smaller and smaller.

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And now, you can even see
the spacing between the jaws.

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And here we are.

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You can see it gets--

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the intensity is so high
in the central lobe,

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that we are saturating.

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But I think this is a
pretty good illustration

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of the Fraunhofer diffraction
pattern of a single slit.

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Now that we've seen Fraunhofer
diffraction pattern associated

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with a single slit, we're going
to make life a little bit more

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complicated.

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We're going to look
at the diffraction

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pattern of two slits.