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SHAOUL EZEKIEL: Now we're ready
to look at Fresnel diffraction.

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We're going to look at
Fresnel diffraction associated

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with a single slit.

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And then later, we'll look at
Fresnel diffraction associated

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with circular apertures.

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The setup for observing
Fresnel diffraction is here,

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is the laser, is the beam
from the laser again reflected

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by this mirror.

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And then we reflect the
beam again by this mirror

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into this lens.

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Now this is a short focal length
lens that focuses the light

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and passes it through a pinhole
over here, which we center it

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at the focus spot.

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The light coming
out from the pinhole

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then is shown on the
card here and then goes

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and falls onto the screen.

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So this is the setup.

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Now I'm going to
bring in the slit.

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So I'm going to put this slit
in front of the laser beam

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without disturbing
anything, I hope.

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So here is the single slit.

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Now I'm going to
adjust the spacing

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to be very small, so that we
start out with the Fraunhofer

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diffraction pattern.

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Now here it is.

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You can see on the screen, we
see the Fraunhofer diffraction

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pattern because the slit
width is small enough,

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so that we're in the
Fraunhofer region.

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Now what I'm going
to do is I'm going

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to start to widen the slit,
so to bring in the Fresnel

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diffraction pattern.

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So here I am.

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I start widening.

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And you can see now
you have to watch

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what happens to the fringes.

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Now things are going
to get pretty bright.

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And so what I'm going
to do, I'm going

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to introduce another screen
that is not so sensitive, so

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that we can observe the bright
Fresnel pattern on that one.

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Here we are.

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And you can see, I get a
dark line in the middle.

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Now we get two.

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And others, you can see how the
fringes behave in this case.

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I want to see what the
fringes near the edges

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look like and in the center.

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Now I know that the
effect is not observable

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very well under
these conditions.

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So what I'm going to do is
to turn down the room lights

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and see if we can improve
the visibility of the Fresnel

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diffraction pattern.

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Now with the room
lights dimmed, we're

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going to start again with the
Fraunhofer diffraction pattern

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on the sensitive screen.

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So again, you can see
that, at present, we

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have Fraunhofer diffraction.

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And I hope it's a little clearer
than with the room lights on.

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OK?

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So I'm going to start here.

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And then as I increase
the slit width,

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we're going to have a transition
to Fresnel diffraction.

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But as we know, the brightness
is going to get pretty large.

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So I will then bring in
the other screen here,

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so that we're not
going to saturate

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the camera too much when we
observe Fresnel diffraction.

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So here we go.

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This is Fraunhofer.

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Then I will now go make
the transition to Fresnel.

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And it's very, very
interesting how

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the light going through the
slit solves Maxwell's equations.

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And here we are.

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We got that dark
line in the middle.

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And then we have to
go back again and have

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the two lines, then the three.

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Now it's very
important to notice

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that the contrast is not 100%
like it is in the Fraunhofer

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diffraction.

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And those of you who
will do the calculation

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will, of course, understand
what I'm referring to.

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The spacing, you notice that
the spacing between the fringes

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gets tighter and
tighter as you approach

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the center or the center
between the two slits.

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In fact, the fringe spacing
gets so fine with large slit

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separation that you can't
even resolve them by eye.

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So there's lot of information
in that diffraction pattern.

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And I hope that you've
adjusted your monitors,

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so that you can
see these patterns.

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So here we are all
the way to Fraunhofer.

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And there's a little
transition, which

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is not that easy to calculate,
and then go into Fresnel.

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Here we are.

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Make it larger and larger.

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You can even see the
fringes in the center.

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Now that we've seen Fresnel
diffraction associated

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with a single slit,
now we're ready to look

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at Fresnel
diffraction associated

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with circular apertures.