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.
SHAOUL EZEKIEL: 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,
is the laser, is the beam from the laser again reflected
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
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.
So I'm going to put this slit in front of the laser beam
without disturbing anything, I hope.
So here is the single slit.
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,
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.
I start widening.
And you can see now you have to watch
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.
Now we get two.
And others, you can see how the fringes behave in this case.
I want to see what the fringes near the edges
look like and in the center.
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
and see if we can improve the visibility of the Fresnel
diffraction pattern.
Now with the room lights dimmed, we're
going to start again with the Fraunhofer diffraction pattern
on the sensitive screen.
So again, you can see that, at present, we
have Fraunhofer diffraction.
And I hope it's a little clearer than with the room lights on.
OK?
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.
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.
And it's very, very interesting how
the light going through the slit solves Maxwell's equations.
And here we are.
We got that dark line in the middle.
And then we have to go back again and have
the two lines, then the three.
Now it's very important to notice
that the contrast is not 100% like it is in the Fraunhofer
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 with large slit
separation that you can't even resolve them by eye.
So there's lot of information in that diffraction pattern.
And I hope that you've adjusted your monitors,
so that you can see these patterns.
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.
Here we are.
Make it larger and larger.
You can 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
at Fresnel diffraction associated
with circular apertures.