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PROFESSOR: In this demo, we're going
to illustrate the basic use of a quarter-wave plate.
And, as we know, a quarter-wave plate
is used to change the state of polarization of light
from plane polarized, for example, to elliptical,
and, under certain conditions, to circular polarized light.
So we'd like to do that using this setup.
We have a laser--
a helium-neon laser.
And here's the beam from the laser.
We're going to reflect it through this mirror
and then reflect it again through a second mirror.
Then we pass it through this polarizer,
and the beam coming through the polarizer then hits the screen.
This polarizer, the transmission axis
of which is shown by the white arrow,
sets the state of polarization of the light
here in this region.
No matter what the state of polarization
is before the light reaches the polarizer,
the polarizer then will clean it up and set it according
to the transmission axis.
And right now, the transmission axis is along the vertical,
as shown by the white arrow.
But just to check this we'll use the analyzer,
which is another polarizer.
And we place it over here.
The white arrow is along the vertical direction and labels
the transmission axis of this polarizer.
So let's see, indeed, whether the light here
is plane polarized.
So I'm going to rotate the analyzer
and see if I can extinguish the light.
Now, what I would like to do is look at this extinguishing
of the light more closely in the bottom left-hand corner, which
is a close-up of what you see on the screen.
And, here, you can see that the extinguishing
is pretty complete, which shows that, indeed,
the light in this region is plane polarized.
So let me set the analyzer back so that we're transmitting
the maximum intensity, which means the transmission axis is
lined up with the transmission axis of this polarizer.
Now we're ready to bring in the quarter-wave plate.
Here's the quarter-wave plate.
I'm going to place it in this position, secure it.
Now, this quarter-wave plate has, as we know,
two principal axes.
One of them is labeled by the white arrow here.
And, again, we know that when this light is propagated
along the principal axis of the quarter-wave plate,
there's no change in the polarization.
It's just like a piece of glass.
Let's check that.
So we take the analyzer again, and we rotate the analyzer
to see, indeed, whether the light is plane polarized.
And, as you can see, maybe I didn't get the labeling here.
But, as you can see, the light is pretty
close to plane polarized.
Now, we know that there are two principal axes.
So let's turn the arrow to 90 degrees and show the fact that
when light propagates along the other principal axes,
the light here is again plane polarized,
which means it cannot get past this polarizer when
the transmission axis is orthogonal to the initial
polarization set by this polarizer.
So now we've established that, indeed,
when we propagate along principal axes
of the quarter-wave plate, nothing happens to the light.
It just stays plane polarized.
Now, we're going to demonstrate the actual use
of a quarter-wave plate, which changes,
for example, in this case, the state
of polarization from plane polarized light to elliptical,
depending on the angular position that we
set the quarter-wave plate at.
So let's start with a very small angle away
from the principal axis.
And, now, we want to look at the intensity on the screen
and also in the insert to see whether, first of all,
the light gets extinguished, and also watch the intensity as I
rotate my analyzer.
Now, you can see the minimum is around here.
It's not 0.
And the maximum is around here. which
shows that the light is elliptically polarized,
certainly not plane polarized as it was before.
Now, if you want circular polarization,
we have to rotate the quarter-wave plate
to about 45 degrees from one of the principal axes.
And if the light is indeed circularly polarized,
then we wouldn't see any change in intensity
as we rotate the transmission axis of the analyzer.
And let's see if we got this close.
Now, you can see, as I'm rotating the analyzer,
you can see that the intensity on the screen
doesn't change all that much, which shows that the light is
certainly polarized.
In summary, we've shown that a quarter-wave way
plate can change the state of polarization of light.
In particular, we've demonstrated
that if plane polarized light is incident
on the quarter-wave plate and the quarter-wave plate is
oriented appropriately, we can get
elliptically polarized light.
If the quarter-wave plate is oriented
at 45 degrees with respect to the state of polarization
of the light, then we can get circularly polarized light.