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PROFESSOR: Now, we are ready to look
at Fraunhofer diffraction associated
with circular apertures.
The setup, again, is the same as before.
But I just want to remind you of it.
We have a helium-neon laser.
Beam from the laser is reflected by this mirror
and then reflected again by this mirror into this lens.
And now we have the lens focusing the light
onto a set of apertures.
And then the diffraction light from the aperture
then goes onto the screen.
Now, these apertures have different diameters.
Actually, they're tiny pinholes.
So the first one we have has a 100 micron diameter.
So, now, if we look at the screen,
we see the diffraction pattern associated with 100 micron
diameter aperture.
Again, let me give you some dimensions.
The separation between the aperture and the screen
is about 100 centimeters, the light
is from the helium-neon laser at 6328 angstroms,
and the scale on the screen from here to here,
which is the diameter of the first dark ring,
is about 1.6 centimeters.
So, now, you can check the diffraction pattern,
given the information that I've just presented to you.
At the moment, you see--
first of all, let me do some adjustment,
make sure that I'm picked up.
And what you see, you see a bright ring in the middle
and then some faint rings around the central bright ring.
There are actually many rings, but because they're so faint,
you can't see them.
So what I'm going to do is pull away this screen
and expose to you a more sensitive screen,
as we can see.
Now, you see that we're saturating in the middle,
but we're beginning to see other rings.
Now, if we open up the camera aperture some more,
you can see even more rings.
And, again, all these rings, and their intensities,
and sizes, and what have you can be
calculated from the information that I've given you.
So, now, this is then the Fraunhofer diffraction pattern
associated with 100 micron aperture.
Now, I would like to move to the next aperture, which
is 50 microns.
And then I'm going to do a little tweaking
so I have it nicely picked up.
Good.
So, now, if we can look at it on the whole screen,
this is then the diffraction pattern of a 50 micron
aperture, and you can see that the size of the central fringe
is bigger.
And, again, from the information that I've given you,
you should be able to calculate what it should be.
Again, if we open up the aperture in the camera,
maybe we can start to see a few more rings.
And, again, the light level is smaller
than with the previous aperture, because, again, the aperture
is only 50 microns this time.
Now, we've seen the Fraunhofer diffraction pattern associated
with a rectangular aperture.
And then, just now, we saw the diffraction associated
with a circular aperture.
In the next demonstration, we're going
to show the Fraunhofer diffraction pattern associated
with multiple slits-- with a two-dimensional array
of multiple slits.
So when we come back, we'll have the setup
arranged so we can demonstrate that effect for you.